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FeaTures
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LTC3858-1 Low IQ, Dual 2-Phase Synchronous Step-Down Controller DescripTion
TheLTC(R)3858-1isahighperformancedualstep-down switching regulator controller that drives all N-channel synchronouspowerMOSFETstages.Aconstantfrequency current mode architecture allows a phase-lockable frequencyofupto850kHz.Powerlossandnoiseduetothe inputcapacitorESRareminimizedbyoperatingthetwo controlleroutputsoutofphase. The170Ano-loadquiescentcurrentextendsoperating lifeinbatterypoweredsystems.OPTI-LOOPcompensationallowsthetransientresponsetobeoptimizedover awiderangeofoutputcapacitanceandESRvalues.The LTC3858-1featuresaprecision0.8Vreferenceandapower goodoutputindicator.Awide4Vto38Vinputsupplyrange encompassesawiderangeofintermediatebusvoltages andbatterychemistries. Independentsoft-startpinsforeachcontrollerrampthe outputvoltagesduringstart-up.Theoutputlatch-offfeature protectsthecircuitinshort-circuitconditions. Foraleadless32-pinQFNpackagewiththeadditionalfeaturesofadjustablecurrentlimit,clockout,phasemodulationandtwoPGOODoutputs,seetheLTC3858datasheet.
L,LT,LTC,LTM,BurstMode,OPTI-LOOP ,Module,LinearTechnologyandtheLinearlogo areregisteredtrademarksandNoRSENSEandUltraFastaretrademarksofLinearTechnology Corporation.Allothertrademarksarethepropertyoftheirrespectiveowners.ProtectedbyU.S. Patents,including5481178,5705919,5929620,6100678,6144194,6177787,6304066,6580258.
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Low Operating IQ: 170A (One Channel On) Wide Output Voltage Range: 0.8V VOUT 24V Wide VIN Range: 4V to 38V RSENSE or DCR Current Sensing Out-of-PhaseControllersReduceRequiredInput CapacitanceandPowerSupplyInducedNoise OPTI-LOOP(R)CompensationMinimizesCOUT Phase-LockableFrequency(75kHz-850kHz) ProgrammableFixedFrequency(50kHz-900kHz) SelectableContinuous,Pulse-Skippingor BurstMode(R)OperationatLightLoads VeryLowDropoutOperation:99%DutyCycle AdjustableOutputVoltageSoft-Start PowerGoodOutputVoltageMonitor OutputOvervoltageProtection OutputLatch-OffProtectionDuringShortCircuit LowShutdownIQ:8A InternalLDOPowersGateDrivefromVINorEXTVCC NoCurrentFoldbackDuringStart-Up Tiny4mmx5mmQFNandNarrowSSOPPackages
applicaTions
AutomotiveSystems BatteryOperatedDigitalDevices n DistributedDCPowerSystems
n n
Typical applicaTion
High Efficiency Dual 8.5V/3.3V Step-Down Converter
4.7F TG1 3.3H 0.1F VIN INTVCC TG2 22F 50V VIN 9V TO 38V 100 90 7.2H EFFICIENCY (%) 80 70 60 50 40 30 20 10 0 0.0001 0.001 VIN = 12V VOUT = 3.3V FIGURE 12 CIRCUIT 0.01 0.1 1 OUTPUT CURRENT (A) 10
38581 TA01b
Efficiency and Power Loss vs Load Current
10000 1000 POWER LOSS (mW) EFFICIENCY POWER LOSS
BOOST1 SW1 BG1 LTC3858-1
BOOST2 SW2 BG2 PGND SENSE2+
0.1F
100 10
SENSE1+ 0.007 VOUT1 3.3V 5A SENSE1- VFB1 ITH1 SS1 0.1F SGND
0.01 SENSE2- VFB2 ITH2 SS2 0.1F VOUT2 8.5V 3.5A 150F
1
62.5k 150F 20k
193k 680pF 15k 20k
680pF 15k
0.1
38581 TA01
38581fb
LTC3858-1 absoluTe MaxiMuM raTings (Note 1)
InputSupplyVoltage(VIN)......................... -0.3Vto40V TopsideDriverVoltages BOOST1,BOOST2................................ -0.3Vto46V . SwitchVoltage(SW1,SW2)........................ -5Vto40V (BOOST1-SW1),(BOOST2-SW2)................ -0.3Vto6V RUN1,RUN2............................................... -0.3Vto8V MaximumCurrentSourcedIntoPin fromSource>8V...............................................100A SENSE1+,SENSE2+,SENSE1- SENSE2-Voltages..................................... -0.3Vto28V . PLLIN/MODE,FREQVoltages.............. -0.3VtoINTVCC EXTVCC ..................................................... -0.3Vto14V . ITH1,ITH2,VFB1,VFB2Voltages...................... -0.3Vto6V PGOOD1Voltage......................................... -0.3Vto6V SS1,SS2,INTVCCVoltages......................... -0.3Vto6V OperatingJunctionTemperatureRange (Note2)................................................. -40Cto125C . MaximumJunctionTemperature(Note3)............ 125C StorageTemperatureRange.................. -65Cto150C . LeadTemperature(Soldering,10sec) SSOP................................................................ 300C
pin conFiguraTion
TOP VIEW PGOOD1 SW1 ITH1 VFB1 SENSE1+ 22 BOOST1 21 BG1 20 VIN 29 SGND 19 PGND 18 EXTVCC 17 INTVCC 16 BG2 15 BOOST2 9 10 11 12 13 14 VFB2 SENSE2+ ITH2 SS2 TG2 SW2 SENSE1- FREQ PLLIN/MODE SGND RUN1 RUN2 SENSE2+ 1 2 3 4 5 6 7 8 9 11 VFB1 ITH1 SS1 TG1 TOP VIEW 28 SS1 27 PGOOD1 26 TG1 25 SW1 24 BOOST1 23 BG1 22 VIN 21 PGND 20 EXTVCC 19 INTVCC 18 BG2 17 BOOST2 16 SW2 15 TG2
28 27 26 25 24 23 SENSE1+ 1 SENSE1- 2 FREQ 3 PLLIN/MODE 4 SGND 5 RUN1 6 RUN2 7 SENSE2- 8
SENSE2- 10 VFB2 12 ITH2 13 SS2 14
TJMAX=125C,JA=43C/W EXPOSEDPAD(PIN29)ISSGND,MUSTBESOLDEREDTOPCB
UFD PACKAGE 28-LEAD (4mm 5mm) PLASTIC QFN
GN PACKAGE 28-LEAD PLASTIC SSOP TJMAX=125C,JA=90C/W
orDer inForMaTion
LEAD FREE FINISH LTC3858EUFD-1#PBF LTC3858IUFD-1#PBF LTC3858EGN-1#PBF LTC3858IGN-1#PBF TAPE AND REEL LTC3858EUFD-1#TRPBF LTC3858IUFD-1#TRPBF LTC3858EGN-1#TRPBF LTC3858IGN-1#TRPBF PART MARKING* 38581 38581 LTC3858GN-1 LTC3858GN-1 PACKAGE DESCRIPTION 28-Lead(4mmx5mm)PlasticQFN 28-Lead(4mmx5mm)PlasticQFN 28-LeadPlasticSSOP 28-LeadPlasticSSOP TEMPERATURE RANGE -40Cto125C -40Cto125C -40Cto125C -40Cto125C
ConsultLTCMarketingforpartsspecifiedwithwideroperatingtemperatureranges.*Thetemperaturegradeisidentifiedbyalabelontheshippingcontainer. ConsultLTCMarketingforinformationonnon-standardleadbasedfinishparts. Formoreinformationonleadfreepartmarking,goto:http://www.linear.com/leadfree/ Formoreinformationontapeandreelspecifications,goto:http://www.linear.com/tapeandreel/
38581fb
LTC3858-1 elecTrical characTerisTics
SYMBOL VIN VFB1,2 IFB1,2 VREFLNREG VLOADREG PARAMETER InputSupplyOperatingVoltageRange RegulatedFeedbackVoltage (Note4)ITH1,2=1.2V -40Cto125C -40Cto85C (Note4) (Note4)VIN=4.5Vto38V (Note4) MeasuredinServoLoop, ITHVoltage=1.2Vto0.7V (Note4) MeasuredinServoLoop, ITHVoltage=1.2Vto2V gm1,2 IQ TransconductanceAmplifiergm InputDCSupplyCurrent PulseSkiporForcedContinuousMode (OneChannelOn) PulseSkiporForcedContinuousMode (BothChannelsOn) SleepMode(OneChannelOn) (Note4)ITH1,2=1.2V,Sink/Source=5A (Note5) RUN1=5VandRUN2=0Vor RUN1=0VandRUN2=5V, VFB1=0.83V(NoLoad) RUN1,2=5V,VFB1,2=0.83V(NoLoad) RUN1=5VandRUN2=0Vor RUN1=0VandRUN2=5V, VFB1=0.83V(NoLoad) RUN1,2=5V,VFB1,2=0.83V(NoLoad) RUN1,2=0V INTVCCRampingUp INTVCCRampingDown MeasuredatVFB1,2,RelativetoRegulatedVFB1,2 EachChannel EachChannel VOUT1,2INTVCC+0.5 InDropout,FREQ=0V VSS1,2=0V VRUN1,VRUN2Rising VSS1,VSS2Risingfrom1V VSS1,VSS2Risingfrom2V Short-CircuitConditionVFB1,2=0.5V VSS1,2=4.5V VFB1,2=0.7V,VSENSE1-,2-=3.3V
l l l l l l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.
CONDITIONS MIN 4 0.788 0.792 0.800 0.800 5 0.002 0.01 -0.01 2 1.3 2 170 300 8 3.6 7 4.0 3.8 10 540 98 0.7 1.23 1.9 1.3 7 43 99.4 1.0 1.28 50 2 1.5 10 50 2.1 1.7 13 57 1.4 1.33 250 450 20 4.2 4.0 13 1 1 700 TYP MAX 38 0.812 0.808 50 0.02 0.1 -0.1 UNITS V V V nA %/V % % mmho mA mA A A A V V % A A A % A V mV V V A mV
FeedbackCurrent ReferenceVoltageLineRegulation OutputVoltageLoadRegulation
l
SleepMode(BothChannelsOn) Shutdown UVLO VOVL UndervoltageLockout FeedbackOvervoltageProtection SENSE+PinCurrent SENSE-PinsCurrent
ISENSE+ ISENSE- DFMAX ISS1,2 VRUN1,2On VSS1,2LA VSS1,2LT IDSC1,2LT VSENSE(MAX) Gate Driver TG1,2 BG1,2
MaximumDutyFactor Soft-StartChargeCurrent RUNPinOnThresholdVoltage SSPinLatch-OffArmingThreshold Voltage SSPinLatch-OffThresholdVoltage SSDischargeCurrent MaximumCurrentSenseThreshold Voltage Pull-UpOn-Resistance Pull-DownOn-Resistance Pull-UpOn-Resistance Pull-DownOn-Resistance
VRUN1,2Hyst RUNPinHysteresisVoltage
2.5 1.5 2.4 1.1

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LTC3858-1 elecTrical characTerisTics
SYMBOL TG1,2tr TG1,2tf BG1,2tr BG1,2tf TG/BGt1D BG/TGt1D tON(MIN) VINTVCCVIN VLDOVIN VINTVCCEXT VLDOEXT VEXTVCC VLDOHYS f25k f65k f105k fLOW fHIGH fSYNC VPGL IPGOOD VPG PARAMETER TGTransistionTime: RiseTime FallTime BGTransistionTime: RiseTime FallTime TopGateOfftoBottomGateOnDelay SynchronousSwitch-OnDelayTime BottomGateOfftoTopGateOnDelay TopSwitch-OnDelayTime MinimumOn-Time InternalVCCVoltage INTVCCLoadRegulation InternalVCCVoltage INTVCCLoadRegulation EXTVCCSwitchoverVoltage EXTVCCHysteresisVoltage ProgrammableFrequency ProgrammableFrequency ProgrammableFrequency LowFixedFrequency HighFixedFrequency SynchronizableFrequency PGOOD1VoltageLow PGOOD1LeakageCurrent PGOOD1TripLevel RFREQ=25k,PLLIN/MODE=DCVoltage RFREQ=65k,PLLIN/MODE=DCVoltage RFREQ=105k,PLLIN/MODE=DCVoltage VFREQ=0V,PLLIN/MODE=DCVoltage VFREQ=INTVCC,PLLIN/MODE=DCVoltage PLLIN/MODE=ExternalClock IPGOOD=2mA VPGOOD=5V VFBwithRespecttoSetRegulatedVoltage VFBRampingNegative Hysteresis VFBwithRespecttoSetRegulatedVoltage VFBRampingPositive Hysteresis tPG DelayforReportingaFault(PGOODLow) Note 1:StressesbeyondthoselistedunderAbsoluteMaximumRatings maycausepermanentdamagetothedevice.ExposuretoanyAbsolute MaximumRatingsforextendedperiodsmayaffectdevicereliabilityand lifetime. Note 2:TheLTC3858E-1isguaranteedtomeetperformancespecifications from0Cto85C.Specificationsoverthe-40Cto125Coperating junctiontemperaturerangeareassuredbydesign,characterizationand correlationwithstatisticalprocesscontrols.TheLTC3858I-1isguaranteed overthefull-40Cto125Coperatingjunctiontemperaturerange. Note 3:TJiscalculatedfromtheambienttemperatureTAandpower dissipationPDaccordingtothefollowingformula: TJ=TA+(PD*JA) whereJA=43C/WfortheQFNpackageandJA=90C/WfortheSSOP package. -13 7 -10 2.5 10 2.5 25
l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.
CONDITIONS (Note6) CLOAD=3300pF CLOAD=3300pF (Note6) CLOAD=3300pF CLOAD=3300pF CLOAD=3300pFEachDriver CLOAD=3300pFEachDriver (Note7) 6VINTVCC Linear Regulator
Oscillator and Phase-Locked Loop
PGOOD1 Output
Note 4:TheLTC3858-1istestedinafeedbackloopthatservosVITH1,2to aspecifiedvoltageandmeasurestheresultantVFB1,2.Thespecificationat 85Cisnottestedinproduction.Thisspecificationisassuredbydesign, characterizationandcorrelationtoproductiontestingat125C. Note 5:Dynamicsupplycurrentishigherduetothegatechargebeing deliveredattheswitchingfrequency.SeeApplicationsinformation. Note 6:Riseandfalltimesaremeasuredusing10%and90%levels.Delay timesaremeasuredusing50%levels Note 7:Theminimumon-timeconditionisspecifiedforaninductorpeakto-peakripplecurrentofIMAX(SeeMinimumOn-TimeConsiderationsin theApplicationsInformationsection).
38581fb
LTC3858-1 Typical perForMance characTerisTics
Efficiency and Power Loss vs Output Current
100 FIGURE 12 CIRCUIT 90 VIN = 12V VOUT = 3.3V 80 70 60 50 40 30 20 10 0 0.0001 100 Burst Mode OPERATION 10 PULSESKIPPING MODE 1 FORCED CONTINUOUS MODE 0.1 0.001 0.01 0.1 1 10 OUTPUT CURRENT (A)
3858 G01
Efficiency vs Load Current
10000 100 90 1000 POWER LOSS (mW) EFFICIENCY (%) 80 70 60 50 40 30 20 10 0 0.0001 0.001 VOUT = 3.3V FIGURE 12 CIRCUIT 0.01 0.1 1 OUTPUT CURRENT (A) 10
3858 G02
VIN = 5V VIN = 12V
EFFICIENCY (%)
Efficiency vs Input Voltage
98 96 94 EFFICIENCY (%) 92 90 88 86 84 82 80 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 IL 2A/DIV FIGURE 12 CIRCUIT VOUT = 3.3V IOUT = 4A VOUT 100mV/DIV ACCOUPLED
Load Step (Burst Mode Operation)
VOUT 100mV/DIV ACCOUPLED
Load Step (Forced Continuous Mode)
IL 2A/DIV
VOUT = 3.3V 20s/DIV FIGURE 12 CIRCUIT
3858 G04
20s/DIV VOUT = 3.3V FIGURE 12 CIRCUIT
3858 G05
3858 G03
Load Step (Pulse-Skipping Mode)
VOUT 100mV/DIV ACCOUPLED
Inductor Current at Light Load
Soft-Start
FORCED CONTINUOUS MODE Burst Mode OPERATION 2A/DIV PULSESKIPPING MODE VOUT = 3.3V 20s/DIV FIGURE 12 CIRCUIT
3858 G06
VOUT2 2V/DIV
VOUT1 2V/DIV
IL 2A/DIV
VOUT = 3.3V 2s/DIV ILOAD = 200A FIGURE 12 CIRCUIT
3858 G07
20ms/DIV FIGURE 12 CIRCUIT
3858 G08
38581fb
LTC3858-1 Typical perForMance characTerisTics
Total Input Supply Current vs Input Voltage
400 350 SUPPLY CURRENT (A) 300 250 200 150 100 50 0 5 10 15 30 25 20 INPUT VOLTAGE (V) 35 40
3858 G10
EXTVCC Switchover and INTVCC Voltages vs Temperature
5.6 EXTVCC AND INTVCC VOLTAGE (V) 5.4 INTVCC VOLTAGE (V) 5.2 5.0 4.8 4.6 4.4 4.2 4.0 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130 5.0 EXTVCC RISING EXTVCC FALLING INTVCC 5.2 5.2
INTVCC Line Regulation
FIGURE 12 CIRCUIT VOUT = 3.3V ONE CHANNEL ON 300A LOAD
5.1
NO LOAD
5.1
0
5
10
15 20 25 30 INPUT VOLTAGE (V)
35
40
3858 G11
3858 G12
CURRENT SENSE THRESHOLD (mV)
-50 -100 SENSE- CURRENT (A) -150 -200 -250 -300 -350 -400 -450 -500 -550 1.4 -600 0 25 10 15 20 VSENSE COMMON MODE VOLTAGE (V) 5
3858 G14
60 40 20 0 -20 -40
MAXIMUM CURRENT SENSE VOLTAGE (mV)
80
Maximum Current Sense Voltage vs ITH Voltage
PULSE-SKIPPING MODE FORCED CONTINUOUS MODE Burst Mode OPERATION (FALLING) Burst Mode OPERATION (RISING) 0
SENSE- Pin Input Bias Current
80
Maximum Current Sense Threshold vs Duty Cycle
60
40
20
5% DUTY CYCLE 0 0.2 0.4 0.6 0.8 1.0 ITH PIN VOLTAGE 1.2
0
0
10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%)
3858 G15
3858 G13
Foldback Current Limit
MAXIMUM CURRENT SENSE VOLTAGE (mV) 90 80 QUIESCENT CURRENT (A) 70 60 50 40 30 20 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 FEEDBACK VOLTAGE (V)
3858 G16
Quiescent Current vs Temperature
230 210 190 170 150 130 110 -45 PLLIN/MODE = 0 VIN = 12V VOUT = 3.3V ONE CHANNEL ON 10 9 8 7 6 5
Shutdown Current vs Temperature
SHUTDOWN CURRENT (A)
-20
80 5 55 30 TEMPERATURE (C)
105
130
4 -45 -20
55 30 80 5 TEMPERATURE (C)
105
130
3858 G17
3858 G18
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LTC3858-1 Typical perForMance characTerisTics
Soft-Start Pull-Up Current vs Temperature
1.20 1.15 SS PULL-UP CURRENT (A) RUN PIN VOLTAGE (V) 1.10 1.05 1.00 0.95 0.90 0.85 0.80 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130 1.40 REGULATED FEEDBACK VOLTAGE (mV) 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 0.90 -45 -20 55 30 5 80 TEMPERATURE (C) 105 130
Shutdown (RUN) Threshold vs Temperature
808 806 804 802 800 798 796 794
Regulated Feedback Voltage vs Temperature
792 -45 -20
5
80 55 30 TEMPERATURE (C)
105
130
3858 G19
3858 G20
22554 G21
SENSE- Pin Input Current vs Temperature
50 0 -50 -100 -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 -45 14 VOUT = 3.3V INPUT CURRENT (A) 12
Shutdown Input Current vs Input Voltage
800 700 600 FREQUENCY (kHz) 500 400 300 200 100 5 10 25 20 30 15 INPUT VOLTAGE (V) 35 40
3858 G23
Oscillator Frequency vs Temperature
SENSE- CURRENT (A)
10 8 6 4 2 0
FREQ = INTVCC
FREQ = GND
VOUT = 28V -20 80 55 5 30 TEMPERATURE (C) 105 130
0 -45 -20
5
80 55 30 TEMPERATURE (C)
105
130
3858 G22
3858 G24
Oscillator Frequency vs Input Voltage
356 OSCILLATOR FREQUENCY (kHz) 354 352 350 348 346 344 INTVCC VOLTAGE (V) FREQ = GND 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 5 10 25 20 30 15 INPUT VOLTAGE (V) 35 40
3858 G28
Undervoltage Lockout Threshold vs Temperature
3.4 -45
-20
55 5 80 30 TEMPERATURE (C)
105
130
3858 G25
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LTC3858-1 Typical perForMance characTerisTics
INTVCC vs Load Current
5.20 VIN = 12V 2.3 2.2 2.1 INTVCC VOLTAGE (V) 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 4.95 0 20 40 60 80 100 120 140 160 180 200 LOAD CURRENT (mA)
3858 G26
Latch-Off Threshold Voltage vs Temperature
5.15 INTVCC VOLTAGE (V) 5.10 EXTVCC = 0V 5.05 5.00 EXTVCC = 8V
ARMING THRESHOLD
LATCH-OFF THRESHOLD
1.2 -45
-20
55 30 5 80 TEMPERATURE (C)
105
130
3858 G27
pin FuncTions
(QFN/SSOP)
SENSE1-, SENSE2- (Pin 2, Pin 4/Pin 8, Pin 10):The(-) InputtotheDifferentialCurrentComparators.Whengreater thanINTVCC-0.5V,theSENSE-pinsuppliescurrentto thecurrentcomparator. FREQ (Pin 3/Pin 5):TheFrequencyControlPinforthe InternalVoltage-ContolledOscillator(VCO).Connecting thispintoGNDforcestheVCOtoafixedlowfrequency of350kHz.ConnectingthispintoINTVCCforcestheVCO toafixedhighfrequencyof535kHz.Otherfrequencies between50kHzand900kHzcanbeprogrammedusinga resistorbetweenFREQandGND.Aninternal20ApullupcurrentdevelopsthevoltagetobeusedbytheVCOto controlthefrequency PLLIN/MODE (Pin 4/Pin 6): External Synchronization Input to Phase Detector and Forced Continuous Mode Input.Whenanexternalclockisappliedtothispin,the phase-lockedloopwillforcetherisingTG1signaltobe synchronizedwiththerisingedgeoftheexternalclock. Whennotsynchronizingtoanexternalclock,thisinput, which acts on both controllers, determines how the
LTC3858-1 operates at light loads. Pulling this pin to groundselectsBurstModeoperation.Aninternal100k resistor to ground also invokes Burst Mode operation whenthepinisfloated.TyingthispintoINTVCCforces continuousinductorcurrentoperation.Tyingthispinto avoltagegreaterthan1.2VandlessthanINTVCC-1.3V selectspulse-skippingoperation. SGND (Pin 5, Exposed Pad Pin 29/Pin 7):Small-signal ground common to both controllers, must be routed separatelyfromhighcurrentgroundstothecommon(-) terminalsoftheCINcapacitors.Theexposedpad(QFN only) must be soldered to the PCB for rated thermal performance. RUN1, RUN2 (Pin 6, Pin 8/Pin 7, Pin 9): Digital Run ControlInputsforEachController.Forcingeitherofthese pinsbelow1.2Vshutsdownthatcontroller.Forcingbothof thesepinsbelow0.7VshutsdowntheentireLTC3858-1, reducingquiescentcurrenttoapproximately8A.DoNOT floatthesepins.
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LTC3858-1 pin FuncTions
(QFN/SSOP)
INTVCC (Pin 17/Pin 19):OutputoftheInternalLinearLow Dropout Regulator. The driver and control circuits are poweredfromthisvoltagesource.Mustbedecoupledto powergroundwithaminimumof4.7Fceramicorother low ESR capacitor. Do not use the INTVCC pin for any otherpurpose. EXTVCC (Pin 18/Pin 20): External Power Input to an Internal LDO Connected to INTVCC. This LDO supplies INTVCCpower,bypassingtheinternalLDOpoweredfrom VIN whenever EXTVCC is higher than 4.7V. See EXTVCC ConnectionintheApplicationsInformationsection.Do notexceed14Vonthispin. PGND (Pin 19/Pin 21):DriverPowerGround.Connectsto thesourcesofbottom(synchronous)N-channelMOSFETs andthe(-)terminal(s)ofCIN. VIN (Pin 20/Pin 22): Main Input Supply Pin. A bypass capacitorshouldbetiedbetweenthispinandthesignal groundpin. BG1, BG2 (Pin 21, Pin 23/Pin 16, Pin 18): High Current Gate Drives for Bottom (Synchronous) N-Channel MOSFETs. Voltage swing at these pins is from ground toINTVCC. BOOST1, BOOST2 (Pin 22, Pin 24/Pin 15, Pin 17):BootstrappedSuppliestotheTopsideFloatingDrivers.CapacitorsareconnectedbetweentheBOOSTandSWpinsand SchottkydiodesaretiedbetweentheBOOSTandINTVCC pins.VoltageswingattheBOOSTpinsisfromINTVCCto (VIN+INTVCC). SW1, SW2 (Pin 23, Pin 25/Pin 14, Pin 16):SwitchNode ConnectionstoInductors.
TG1, TG2 (Pin 24, Pin 26/Pin 13, Pin 15):HighCurrent GateDrivesforTopN-ChannelMOSFETs.Thesearethe outputsoffloatingdriverswithavoltageswingequalto INTVCC-0.5Vsuperimposedontheswitchnodevoltage SW. PGOOD1 (Pin 25/Pin 27): Open-Drain Logic Output. PGOOD1ispulledtogroundwhenthevoltageontheVFB1 pinisnotwithin10%ofitssetpoint. SS1, SS2 (Pin 26, Pin 28/Pin 12, Pin 14):ExternalSoftStartInput.TheLTC3858-1regulatestheVFB1,2voltage tothesmallerof0.8VorthevoltageontheSS1,2pin.An internal1Apull-upcurrentsourceisconnectedtothis pin.Acapacitortogroundatthispinsetstheramptime tofinalregulatedoutputvoltage.Thispinisalsousedas theshort-circuitlatchofftimer. ITH1, ITH2 (Pin 27, Pin 1/Pin 11, Pin 13):ErrorAmplifier OutputsandSwitchingRegulatorCompensationPoints. Eachassociatedchannel'scurrentcomparatortrippoint increaseswiththiscontrolvoltage. VFB1, VFB2 (Pin 28, Pin 2/Pin 10, Pin 12):Receivesthe remotelysensedfeedbackvoltageforeachcontrollerfrom anexternalresistivedivideracrosstheoutput. SENSE1+, SENSE2+ (Pin 1, Pin 3/Pin 9, Pin 11): The (+)inputtothedifferentialcurrentcomparatorsthatare normallyconnectedtoinductorDCRsensingnetworksor currentsensingresistors.TheITHpinvoltageandcontrolled offsetsbetweentheSENSE-andSENSE+pinsinconjunctionwithRSENSEsetthecurrenttripthreshold.
38581fb
LTC3858-1 FuncTional DiagraM
INTVCC DUPLICATE FOR SECOND CONTROLLER CHANNEL BOOST DB CB D SW INTVCC BG PGND VCO CLK2 CLK1 - + 0.425V SLEEP IR - L RSENSE CIN VIN
FREQ
20A
-+
+-
3mV
PLLIN/MODE 100k
SYNC DET
2(VFB) 0.45V
SLOPE COMP +
VIN EXTVCC OV 5.1V LDO EN + 5.1V LDO EN
0.5A SHDN RST 2(VFB) FOLDBACK 1A SS CSS CC2 RC
10V
operaTion
Main Control Loop TheLTC3858-1usesaconstantfrequency,currentmode step-downarchitecturewiththetwocontrollerchannels operating180degreesoutofphase.Duringnormaloperation,eachexternaltopMOSFETisturnedonwhenthe clockforthatchannelsetstheRSlatch,andisturnedoff whenthemaincurrentcomparator,ICMP ,resetstheRS latch.ThepeakinductorcurrentatwhichICMPtripsand resetsthelatchiscontrolledbythevoltageontheITHpin, whichistheoutputoftheerroramplifier,EA.Theerror amplifiercomparestheoutputvoltagefeedbacksignalat theVFBpin(whichisgeneratedwithanexternalresistor divider connected across the output voltage, VOUT, to ground)totheinternal0.800Vreferencevoltage.Whenthe loadcurrentincreases,itcausesaslightdecreaseinVFB relativetothereference,whichcausestheEAtoincrease theITHvoltageuntiltheaverageinductorcurrentmatches thenewloadcurrent. AfterthetopMOSFETisturnedoffeachcycle,thebottom MOSFETisturnedonuntileithertheinductorcurrentstarts toreverse,asindicatedbythecurrentcomparatorIR,or thebeginningofthenextclockcycle.
38581fb
0
-
4.7V
SGND
INTVCC
RUN
SHORT CKT LATCH-OFF
SHDN 10A
38581 FD
+
-
-
+
+
-
+ + - -
PGOOD1
0.88V VFB1 0.72V S R Q Q
DROP OUT DET
TOP BOT TOP ON SWITCH LOGIC BOT
TG
SHDN
COUT VOUT
PFD
ICMP
SENSE+
SENSE-
VFB EA 0.80V TRACK/SS RA
RB
0.88V
ITH
CC
LTC3858-1 operaTion (Refer to the Functional Diagram)
INTVCC/EXTVCC Power PowerforthetopandbottomMOSFETdriversandmost otherinternalcircuitryisderivedfromtheINTVCCpin.When theEXTVCCpinisleftopenortiedtoavoltagelessthan 4.7V,theVINLDO(lowdropoutlinearregulator)supplies 5.1VfromVINtoINTVCC.IfEXTVCCistakenabove4.7V, theVINLDOisturnedoffandtheEXTVCCLDOisturnedon. Onceenabled,theEXTVCCLDOsupplies5.1VfromEXTVCC toINTVCC.UsingtheEXTVCCpinallowstheINTVCCpower tobederivedfromahighefficiencyexternalsourcesuch asoneoftheLTC3858-1switchingregulatoroutputs. EachtopMOSFETdriverisbiasedfromthefloatingbootstrapcapacitor,CB,whichnormallyrechargesduringeach switchingcyclethroughanexternaldiodewhenthetop MOSFETturnsoff.IftheinputvoltageVINdecreasesto avoltageclosetoVOUT,theloopmayenterdropoutand attempt to turn on the top MOSFET continuously. The dropoutdetectordetectsthisandforcesthetopMOSFET offforaboutone-twelfthoftheclockperiodeverytenth cycletoallowCBtorecharge. Shutdown and Start-Up (RUN1, RUN2 and SS1, SS2 Pins) ThetwochannelsoftheLTC3858-1canbeindependently shutdownusingtheRUN1andRUN2pins.Pullingeitherof thesepinsbelow1.26Vshutsdownthemaincontrolloop forthatcontroller.Pullingbothpinsbelow0.7Vdisables bothcontrollersandmostinternalcircuits,includingthe INTVCCLDOs.Inthisstate,theLTC3858-1drawsonly8A ofquiescentcurrent. TheRUNpinmaybeexternallypulledupordrivendirectly bylogic.WhendrivingtheRUNpinwithalowimpedance source,donotexceedtheabsolutemaximumratingof 8Vonthispin.TheRUNpinhasaninternal11Vvoltage clampthatallowstheRUNpintobeconnectedthrougha resistortoahighervoltage(forexample,VIN),solongas themaximumcurrentintotheRUNpindoesnotexceed 100A. Thestart-upofeachcontroller'soutputvoltageVOUTis controlledbythevoltageontheSSpinforthatchannel. When the voltage on the SS pin is less than the 0.8V internalreference,theLTC3858-1regulatestheVFBvoltagetotheSSpinvoltageinsteadofthe0.8Vreference. ThisallowstheSSpintobeusedtoprogramasoft-start byconnectinganexternalcapacitorfromtheSSpinto SGND.Aninternal1Apull-upcurrentchargesthiscapacitorcreatingavoltagerampontheSSpin.AstheSS voltageriseslinearlyfrom0Vto0.8V(andbeyondupto theabsolutemaximumratingof6V),theoutputvoltage VOUTrisessmoothlyfromzerotoitsfinalvalue. Short-Circuit Latch-Off After the controller has been started and been given adequate time to ramp up the output voltage, the SS capacitorisusedinashort-circuittime-outcircuit.Specifically,oncethevoltageontheSSpinrisesabove2V (thearmingthreshold),theshort-circuittimeoutcircuitis enabled(seeFigure1).Iftheoutputvoltagefallsbelow 70%ofitsnominalregulatedvoltage,theSScapacitor beginsdischargingwithanet9Apull-downcurrenton theassumptionthattheoutputisinanovercurrentand/or short-circuitcondition.Iftheconditionlastslongenough toallowtheSSpinvoltagetofallbelow1.5V(thelatchoff threshold),thecontrollerwillshutdown(latchoff)until theRUNpinvoltageortheVINvoltageisrecycled. Thedelaytimefromwhenanshort-circuitoccursuntil thecontrollerlatchesoffcanbecalculatedusingthefollowingequation: tLATCH CSS VSS - 1.5V 9A
whereVSSistheinitialvoltage(mustbegreaterthan2V) ontheSSpinatthetimetheshort-circuitoccurs.Normally theSSpinvoltagewillhavebeenpulleduptotheINTVCC voltage(5.1V)bytheinternal1Apull-upcurrent. NotethatthetwocontrollersontheLTC3858-1haveseparate,independentshort-circuitlatchoffcircuits.Latchoff canbeoverridden/defeatedbyconnectingaresistor150k orlessfromtheSSpintoINTVCC.Thisresistorprovides enoughpull-upcurrenttoovercomethe9Apull-down currentpresentduringashort-circuit.Notethatthisresistoralsoshortensthesoft-startperiod.
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LTC3858-1 operaTion (Refer to the Functional Diagram)
INTVCC SS VOLTAGE 2V 0.8V LATCH-OFF COMMAND 0V 1A -9A OUTPUT VOLTAGE LATCH-OFF ENABLE
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1.5V
SS PIN CURRENT
1A
WhenacontrollerisenabledforBurstModeoperation,the minimumpeakcurrentintheinductorissettoapproximately30%ofthemaximumsensevoltageeventhough thevoltageontheITHpinindicatesalowervalue.Ifthe averageinductorcurrentishigherthantheloadcurrent, theerroramplifier,EA,willdecreasethevoltageontheITH pin.WhentheITHvoltagedropsbelow0.425V,theinternal sleepsignalgoeshigh(enabling"sleep"mode)andboth externalMOSFETsareturnedoff. Insleepmode,muchoftheinternalcircuitryisturnedoff, reducingthequiescentcurrent.Ifonechannelisshutdown andtheotherchannelisinsleepmode,theLTC3858-1 drawsonly170Aofquiescentcurrent.Ifbothchannels areinsleepmode,theLTC3858-1drawsonly300Aofquiescentcurrent.Insleepmode,theloadcurrentissupplied bytheoutputcapacitor.Astheoutputvoltagedecreases, theEA'soutputbeginstorise.Whentheoutputvoltage dropsenough,theITHpinisreconnectedtotheoutput oftheEA,thesleepsignalgoeslow,andthecontroller resumesnormaloperationbyturningonthetopexternal MOSFETonthenextcycleoftheinternaloscillator. WhenacontrollerisenabledforBurstModeoperation, theinductorcurrentisnotallowedtoreverse.Thereverse current comparator, IR, turns off the bottom external MOSFETjustbeforetheinductorcurrentreacheszero, preventing it from reversing and going negative. Thus, thecontrollerisindiscontinuousoperation. In forced continuous operation or when clocked by an externalclocksourcetousethephase-lockedloop(see Frequency Selection and Phase-Locked Loop section), theinductorcurrentisallowedtoreverseatlightloads or under large transient conditions. The peak inductor currentisdeterminedbythevoltageontheITHpin,just asinnormaloperation.Inthismode,theefficiencyatlight loads is lower than in Burst Mode operation. However, continuousoperationhastheadvantagesofloweroutput voltagerippleandlessinterferencetoaudiocircuitry.In forcedcontinuousmode,theoutputrippleisindependent ofloadcurrent. WhenthePLLIN/MODEpinisconnectedforpulse-skipping mode, the LTC3858-1 operates in PWM pulse-skipping mode at light loads. In this mode, constant frequency
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ARMING SOFT-START INTERVAL
tLATCH
Figure 1. Latch-Off Timing Diagram
Foldback Current Ontheotherhand,whentheoutputvoltagefallstoless than70%ofitsnominallevel,foldbackcurrentlimiting isalsoactivated,progressivelyloweringthepeakcurrent limitinproportiontotheseverityoftheovercurrentor short-circuitcondition.Evenifashort-circuitispresent andtheshort-circuitlatch-offisnotyetenabled(when SSvoltagehasnotyetreached2V),asafe,lowoutput currentisprovidedduetointernalcurrentfoldbackand actualpowerwastedislowduetotheefficientnatureof thecurrentmodeswitchingregulator.Foldbackcurrent limitingisdisabledduringthesoft-startinterval(aslong astheVFBvoltageiskeepingupwiththeSSvoltage). Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Forced Continuous Conduction) (PLLIN/MODE Pin) TheLTC3858-1canbeenabledtoenterhighefficiency BurstModeoperation,constantfrequencypulse-skipping mode,orforcedcontinuousconductionmodeatlowload currents.ToselectBurstModeoperation,tiethePLLIN/ MODEpintoground.Toselectforcedcontinuousoperation,tiethePLLIN/MODEpintoINTVCC.Toselectpulseskippingmode,tiethePLLIN/MODEpintoaDCvoltage greaterthan1.2VandlessthanINTVCC-1.3V.
LTC3858-1 operaTion (Refer to the Functional Diagram)
operation is maintained down to approximately 1% of designedmaximumoutputcurrent.Atverylightloads,the currentcomparator,ICMP ,mayremaintrippedforseveral cyclesandforcetheexternaltopMOSFETtostayofffor thesamenumberofcycles(i.e.,skippingpulses).The inductorcurrentisnotallowedtoreverse(discontinuous operation).Thismode,likeforcedcontinuousoperation, exhibitslowoutputrippleaswellaslowaudionoiseand reducedRFinterferencewhencomparedtoBurstMode operation. It provides higher light load efficiency than forcedcontinuousmode,butnotnearlyashighasBurst Modeoperation. Frequency Selection and Phase-Locked Loop (FREQ and PLLIN/MODE Pins) Theselectionofswitchingfrequencyisatradeoffbetween efficiency and component size. Low frequency operationincreasesefficiencybyreducingMOSFETswitching losses,butrequireslargerinductanceand/orcapacitance tomaintainlowoutputripplevoltage. TheswitchingfrequencyoftheLTC3858-1'scontrollers canbeselectedusingtheFREQpin. IfthePLLIN/MODEpinisnotbeingdrivenbyanexternal clocksource,theFREQpincanbetiedtoSGND,tiedto INTVCCorprogrammedthroughanexternalresistor.Tying FREQtoSGNDselects350kHzwhiletyingFREQtoINTVCC selects 535kHz. Placing a resistor between FREQ and SGNDallowsthefrequencytobeprogrammedbetween 50kHzand900kHz. Aphase-lockedloop(PLL)isavailableontheLTC3858-1 tosynchronizetheinternaloscillatortoanexternalclock source that is connected to the PLLIN/MODE pin. The phasedetectoradjuststhevoltage(throughaninternal lowpassfilter)oftheVCOinputtoaligntheturn-onof controller1'sexternaltopMOSFETtotherisingedgeof thesynchronizingsignal.Thus,theturn-onofcontroller 2'sexternaltopMOSFETis180degreesoutofphaseto therisingedgeoftheexternalclocksource. The VCO input voltage is pre-biased to the operating frequencysetbytheFREQpinbeforetheexternalclock isapplied.Ifprebiasedneartheexternalclockfrequency, thePLLlooponlyneedstomakeslightchangestothe VCOinputinordertosynchronizetherisingedgeofthe externalclock'stotherisingedgeofTG1.Theabilityto pre-biastheloopfilterallowsthePLLtolock-inrapidly withoutdeviatingfarfromthedesiredfrequency. The typical capture range of the phase-locked loop is from approximately 55kHz to 1MHz, with a guarantee over all manufacturing variations to be between 75kHz and850kHz. ThetypicalinputclockthresholdsonthePLLIN/MODE pinare1.6V(rising)and1.1V(falling). Output Overvoltage Protection Anovervoltagecomparatorguardsagainsttransientovershootsaswellasothermoreseriousconditionsthatmay overvoltagetheoutput.WhentheVFBpinrisesbymore than10%aboveitsregulationpointof0.800V,thetop MOSFETisturnedoffandthebottomMOSFETisturned onuntiltheovervoltageconditioniscleared. Power Good (PGOOD) Pin The PGOOD1 pin is connected to an open drain of an internalN-channelMOSFET.TheMOSFETturnsonand pullsthePGOOD1pinlowwhenthecorrespondingVFB1pin voltageisnotwithin10%ofthe0.8Vreferencevoltage. ThePGOOD1pinisalsopulledlowwhentheRUN1pin islow(shutdown).WhentheVFB1pinvoltageiswithin the10%requirement,theMOSFETisturnedoffandthe pinisallowedtobepulledupbyanexternalresistortoa sourcenogreaterthan6V. Theory and Benefits of 2-Phase Operation Why the need for 2-phase operation? Up until the 2-phasefamily,constantfrequencydualswitchingregulatorsoperatedbothchannelsinphase(i.e.,singlephase operation).Thismeansthatbothswitchesturnedonat thesametime,causingcurrentpulsesofuptotwicethe amplitudeofthoseforoneregulatortobedrawnfromthe inputcapacitorandbattery.Theselargeamplitudecurrent
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LTC3858-1 operaTion (Refer to the Functional Diagram)
pulsesincreasedthetotalRMScurrentflowingfromthe inputcapacitor,requiringtheuseofmoreexpensiveinput capacitorsandincreasingbothEMIandlossesintheinput capacitorandbattery. With 2-phase operation, the two channels of the dual switchingregulatorareoperated180degreesoutofphase. Thiseffectivelyinterleavesthecurrentpulsesdrawnbythe switches,greatlyreducingtheoverlaptimewheretheyadd together.TheresultisasignificantreductionintotalRMS inputcurrent,whichinturnallowslessexpensiveinput capacitorstobeused,reducesshieldingrequirementsfor EMIandimprovesrealworldoperatingefficiency. Figure2comparestheinputwaveformsforarepresentative singlephasedualswitchingregulatortotheLTC3858-1 2-phase dual switching regulator. An actual measurementoftheRMSinputcurrentundertheseconditions showsthat2-phaseoperationdroppedtheinputcurrent from2.53ARMSto1.55ARMS.Whilethisisanimpressive reductioninitself,rememberthatthepowerlossesare proportionaltoIRMS2,meaningthattheactualpowerwasted isreducedbyafactorof2.66.Thereducedinputripple voltagealsomeanslesspowerislostintheinputpower path,whichcouldincludebatteries,switches,trace/connectorresistancesandprotectioncircuitry.Improvements inbothconductedandradiatedEMIalsodirectlyaccrueas aresultofthereducedRMSinputcurrentandvoltage. Ofcourse,theimprovementaffordedby2-phaseoperationisafunctionofthedualswitchingregulator'srelative dutycycleswhich,inturn,aredependentupontheinput voltageVIN(DutyCycle=VOUT/VIN).Figure3showshow theRMSinputcurrentvariesforsingle-phaseand2-phase operationfor3.3Vand5Vregulatorsoverawideinput voltagerange. Itcanreadilybeseenthattheadvantagesof2-phaseoperationarenotjustlimitedtoanarrowoperatingrange, formostapplicationsisthat2-phaseoperationwillreduce theinputcapacitorrequirementtothatforjustonechannel operatingatmaximumcurrentand50%dutycycle.
3.0 2.5 INPUT RMS CURRENT (A) 2.0 1.5 1.0 0.5 0 SINGLE PHASE DUAL CONTROLLER
2-PHASE DUAL CONTROLLER
VO1 = 5V/3A VO2 = 3.3V/3A 0 10 20 30 INPUT VOLTAGE (V) 40
3858 F03
Figure 3. RMS Input Current Comparison
5V SWITCH 20V/DIV 3.3V SWITCH 20V/DIV INPUT CURRENT 5A/DIV INPUT VOLTAGE 500mV/DIV
IIN(MEAS) = 2.53ARMS
IIN(MEAS) = 1.55ARMS
38581 F01
Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
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LTC3858-1 applicaTions inForMaTion
The Typical Application on the first page is a basic LTC3858-1applicationcircuit.LTC3858-1canbeconfigured touseeitherDCR(inductorresistance)sensingorlow valueresistorsensing.Thechoicebetweenthetwocurrentsensingschemesislargelyadesigntradeoffbetween cost,powerconsumptionandaccuracy.DCRsensingis becoming popular because it saves expensive current sensingresistorsandismorepowerefficient,especially in high current applications. However, current sensing resistorsprovidethemostaccuratecurrentlimitsforthe controller.Otherexternalcomponentselectionisdriven bytheloadrequirement,andbeginswiththeselectionof RSENSE(ifRSENSEisused)andinductorvalue.Next,the powerMOSFETsandSchottkydiodesareselected.Finally, inputandoutputcapacitorsareselected. SENSE+ and SENSE- Pins TheSENSE+andSENSE-pinsaretheinputstothecurrent comparators.Thecommonmodevoltagerangeonthese pinsis0Vto28V(AbsMax),enablingtheLTC3858-1to regulateoutputvoltagesuptoanominal24V(allowing marginfortolerancesandtransients). TheSENSE+pinishighimpedanceoverthefullcommon moderange,drawingatmost1A.Thishighimpedance allows the current comparators to be used in inductor DCRsensing. TheimpedanceoftheSENSE-pinchangesdependingon thecommonmodevoltage.WhenSENSE-islessthan INTVCC-0.5V,asmallcurrentoflessthan1Aflowsout ofthepin.WhenSENSE-isaboveINTVCC+0.5V,ahigher current(~550A)flowsintothepin.BetweenINTVCC-0.5V andINTVCC+0.5V,thecurrenttransitionsfromthesmaller currenttothehighercurrent. Filtercomponentsmutualtothesenselinesshouldbe placedclosetotheLTC3858-1,andthesenselinesshould runclosetogethertoaKelvinconnectionunderneaththe currentsenseelement(showninFigure4).Sensingcurrent elsewhere can effectively add parasitic inductance andcapacitancetothecurrentsenseelement,degrading the information at the sense terminals and making the programmedcurrentlimitunpredictable.IfinductorDCR sensingisused(Figure5b),resistorR1shouldbeplaced closetotheswitchingnode,topreventnoisefromcoupling intosensitivesmall-signalnodes.
TO SENSE FILTER, NEXT TO THE CONTROLLER
38581 F04
COUT
INDUCTOR OR RSENSE
Figure 4. Sense Lines Placement with Inductor or Sense Resistor
VIN INTVCC BOOST TG SW LTC3858-1 BG
VIN
VOUT
SENSE+ SENSE- SGND
PLACE CAPACITOR NEAR SENSE PINS
38581 F05a
(5a) Using a Resistor to Sense Current
VIN INTVCC BOOST TG SW LTC3858-1 BG R1 C1* SENSE- SGND *PLACE C1 NEAR SENSE PINS (R1||R2) * C1 = L DCR RSENSE(EQ) = DCR R2 R1 + R2
38581 F05b
VIN
INDUCTOR L DCR VOUT
SENSE+
R2
(5b) Using the Inductor DCR to Sense Current Figure 5. Current Sensing Methods
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LTC3858-1 applicaTions inForMaTion
Low Value Resistors Current Sensing Atypicalsensingcircuitusingadiscreteresistorisshown in Figure 5a. RSENSE is chosen based on the required outputcurrent. The current comparator has a maximum threshold VSENSE(MAX)of50mV(typ).Thecurrentcomparatorthresholdvoltagesetsthepeakoftheinductorcurrent,yielding a maximum average output current, IMAX, equal to the peakvaluelesshalfthepeak-to-peakripplecurrent,IL. Tocalculatethesenseresistorvalue,usetheequation: RSENSE = VSENSE(MAX) IMAX + IL 2 using a good RLC meter, but the DCR tolerance is not alwaysthesameandvarieswithtemperature;consultthe manufacturers'datasheetsfordetailedinformation. UsingtheinductorripplecurrentvaluefromtheInductor ValueCalculationsection,thetargetsenseresistorvalue is: RSENSE(EQUIV) = VSENSE(MAX) IMAX + IL 2
Whenusingthecontrollerinverylowdropoutconditions, themaximumoutputcurrentlevelwillbereducedduetothe internalcompensationrequiredtomeetstabilitycriterion forbuckregulatorsoperatingatgreaterthan50%duty factor.AcurveisprovidedintheTypicalPerformanceCharacteristicssectiontoestimatethisreductioninpeakoutput currentdependingupontheoperatingdutyfactor. Inductor DCR Sensing Forapplicationsrequiringthehighestpossibleefficiency athighloadcurrents,theLTC3850iscapableofsensing thevoltagedropacrosstheinductorDCR,asshownin Figure5b.TheDCRoftheinductorrepresentsthesmall amountofDCresistanceofthecopperwire,whichcanbe lessthan1mfortoday'slowvalue,highcurrentinductors. Inahighcurrentapplicationrequiringsuchaninductor, powerlossthroughasenseresistorwouldcostseveral pointsofefficiencycomparedtoinductorDCRsensing. IftheexternalR1||R2*C1timeconstantischosentobe exactlyequaltotheL/DCRtimeconstant,thevoltagedrop acrosstheexternalcapacitorisequaltothedropacross theinductorDCRmultipliedbyR2/(R1+R2).R2scalesthe voltageacrossthesenseterminalsforapplicationswhere theDCRisgreaterthanthetargetsenseresistorvalue. Toproperlydimensiontheexternalfiltercomponents,the DCRoftheinductormustbeknown.Itcanbemeasured
Toensurethattheapplicationwilldeliverfullloadcurrent over the full operating temperature range, choose the minimumvaluefortheMaximumCurrentSenseThresholdVoltage(VSENSE(MAX))intheElectricalCharacteristics table. Next,determinetheDCRoftheinductor.Whenprovided, usethemanufacturer'smaximumvalue,usuallygivenat 20C.Increasethisvaluetoaccountforthetemperature coefficientofcopper,whichisapproximately0.4%/C.A conservativevalueforTL(MAX)is100C. ToscalethemaximuminductorDCRtothedesiredsense resistorvalue,usethedividerratio: RD = RSENSE(EQUIV ) DCRMAX at TL(MAX )
C1isusuallyselectedtobeintherangeof0.1Fto0.47F . ThisforcesR1||R2toaround2k,reducingerrorthatmight havebeencausedbytheSENSE+pin's1Acurrent. TheequivalentresistanceR1||R2isscaledtotheroom temperatureinductanceandmaximumDCR: R1|| R2 =
(
L DCR at 20C * C1
)
Thesenseresistorvaluesare: R1 = R1 * RD R1|| R2 ;R2 = RD 1 - RD
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LTC3858-1 applicaTions inForMaTion
ThemaximumpowerlossinR1isrelatedtodutycycle, andwilloccurincontinuousmodeatthemaximuminput voltage: PLOSS R1 = 30% of the current limit determined by RSENSE. Lower inductorvalues(higherIL)willcausethistooccurat lowerloadcurrents,whichcancauseadipinefficiencyin theupperrangeoflowcurrentoperation.InBurstMode operation,lowerinductancevalueswillcausetheburst frequencytodecrease. Inductor Core Selection OncethevalueforLisknown,thetypeofinductormust beselected.Highefficiencyconvertersgenerallycannot affordthecorelossfoundinlowcostpowderedironcores, forcingtheuseofmoreexpensiveferriteormolypermalloy cores.Actualcorelossisindependentofcoresizefora fixedinductorvalue,butitisverydependentoninductance valueselected.Asinductanceincreases,corelossesgo down.Unfortunately,increasedinductancerequiresmore turnsofwireandthereforecopperlosseswillincrease. Ferritedesignshaveverylowcorelossandarepreferred forhighswitchingfrequencies,sodesigngoalscanconcentrateoncopperlossandpreventingsaturation.Ferrite corematerialsaturates"hard,"whichmeansthatinductancecollapsesabruptlywhenthepeakdesigncurrentis exceeded.Thisresultsinanabruptincreaseininductor ripplecurrentandconsequentoutputvoltageripple.Do notallowthecoretosaturate! Power MOSFET and Schottky Diode (Optional) Selection TwoexternalpowerMOSFETsmustbeselectedforeach controllerintheLTC3858-1:oneN-channelMOSFETfor thetop(main)switch,andoneN-channelMOSFETforthe bottom(synchronous)switch. Thepeak-to-peakdrivelevelsaresetbytheINTVCCvoltage. Thisvoltageistypically5.1Vduringstart-up(seeEXTVCC Pin Connection). Consequently, logic-level threshold MOSFETsmustbeusedinmostapplications.Theonly exceptionisiflowinputvoltageisexpected(VIN<4V); then,sub-logiclevelthresholdMOSFETs(VGS(TH)<3V) shouldbeused.PaycloseattentiontotheBVDSSspecificationfortheMOSFETsaswell;manyofthelogic-level MOSFETsarelimitedto30Vorless.
( VIN(MAX) - VOUT ) * VOUT
R1
EnsurethatR1hasapowerratinghigherthanthisvalue. Ifhighefficiencyisnecessaryatlightloads,considerthis powerlosswhendecidingtouseinductorDCRsensing orsenseresistors.Lightloadpowerlosscanbemodestly higherwithaDCRnetworkthanwithasenseresistor,due totheextraswitchinglossesincurredthroughR1.However, DCRsensingeliminatesasenseresistor,reducesconductionlossesandprovideshigherefficiencyatheavyloads. Peakefficiencyisaboutthesamewitheithermethod. Inductor Value Calculation Theoperatingfrequencyandinductorselectionareinterrelatedinthathigheroperatingfrequenciesallowtheuse ofsmallerinductorandcapacitorvalues.Sowhywould anyoneeverchoosetooperateatlowerfrequencieswith larger components? The answer is efficiency. A higher frequency generally results in lower efficiency because ofMOSFETgatechargelosses.Inadditiontothisbasic trade-off,theeffectofinductorvalueonripplecurrentand lowcurrentoperationmustalsobeconsidered. Theinductorvaluehasadirecteffectonripplecurrent. The inductor ripple current IL decreases with higher inductanceorhigherfrequencyandincreaseswithhigher VIN: IL = V 1 VOUT 1- OUT ( f) (L) VIN
Accepting larger values of IL allows the use of low inductances,butresultsinhigheroutputvoltageripple andgreatercorelosses.Areasonablestartingpointfor settingripplecurrentisIL=0.3(IMAX).Themaximum ILoccursatthemaximuminputvoltage. Theinductorvaluealsohassecondaryeffects.ThetransitiontoBurstModeoperationbeginswhentheaverage inductorcurrentrequiredresultsinapeakcurrentbelow
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LTC3858-1 applicaTions inForMaTion
SelectioncriteriaforthepowerMOSFETsincludetheonresistance, RDS(ON), Miller capacitance, CMILLER, input voltageandmaximumoutputcurrent.Millercapacitance, CMILLER,canbeapproximatedfromthegatechargecurve usually provided on the MOSFET manufacturers' data sheet. CMILLER is equal to the increase in gate charge alongthehorizontalaxiswhilethecurveisapproximately flatdividedbythespecifiedchangeinVDS.Thisresultis thenmultipliedbytheratiooftheapplicationappliedVDS tothegatechargecurvespecifiedVDS.WhentheICis operatingincontinuousmodethedutycyclesforthetop andbottomMOSFETsaregivenby: MainSwitchDuty Cycle = VOUT VIN VIN - VOUT VIN synchronousMOSFETlossesaregreatestathighinput voltagewhenthetopswitchdutyfactorisloworduring ashort-circuitwhenthesynchronousswitchisonclose to100%oftheperiod. Theterm(1+)isgenerallygivenforaMOSFETinthe formofanormalizedRDS(ON)vsTemperaturecurve,but =0.005/Ccanbeusedasanapproximationforlow voltageMOSFETs. TheoptionalSchottkydiodesD1andD2showninFigure10 conductduringthedead-timebetweentheconductionof thetwopowerMOSFETs.Thispreventsthebodydiodeof thebottomMOSFETfromturningon,storingchargeduring thedead-timeandrequiringareverserecoveryperiodthat couldcostasmuchas3%inefficiencyathighVIN.A1A to3ASchottkyisgenerallyagoodcompromiseforboth regionsofoperationduetotherelativelysmallaverage current.Largerdiodesresultinadditionaltransitionlosses duetotheirlargerjunctioncapacitance. CIN and COUT Selection TheselectionofCINissimplifiedbythe2-phasearchitectureanditsimpactontheworst-caseRMScurrentdrawn throughtheinputnetwork(battery/fuse/capacitor).Itcanbe shownthattheworst-casecapacitorRMScurrentoccurs whenonlyonecontrollerisoperating.Thecontrollerwith thehighest(VOUT)(IOUT)productneedstobeusedinthe formulashowninEquation1todeterminethemaximum RMScapacitorcurrentrequirement.Increasingtheoutputcurrentdrawnfromtheothercontrollerwillactually decreasetheinputRMSripplecurrentfromitsmaximum value.Theout-of-phasetechniquetypicallyreducesthe inputcapacitor'sRMSripplecurrentbyafactorof30% to70%whencomparedtoasinglephasepowersupply solution. Incontinuousmode,thesourcecurrentofthetopMOSFET isasquarewaveofdutycycle(VOUT)/(VIN).Toprevent largevoltagetransients,alowESRcapacitorsizedforthe maximumRMScurrentofonechannelmustbeused.The maximumRMScapacitorcurrentisgivenby: CIN RequiredIRMS
1/2 IMAX ( VOUT ) ( VIN - VOUT ) (1) VIN
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Synchronous SwitchDuty Cycle =
The MOSFET power dissipations at maximum output currentaregivenby: V 2 PMAIN = OUT (IMAX ) (1+ ) RDS(ON) + VIN 2 I ( VIN) MAX (RDR ) (CMILLER ) * 2 1 1 + ( f) VINTVCC - VTHMIN VTHMIN PSYNC = VIN - VOUT 2 (IMAX ) (1+ )RDS(ON) VIN
whereisthetemperaturedependencyofRDS(ON)and RDR(approximately2)istheeffectivedriverresistance attheMOSFET'sMillerthresholdvoltage.VTHMINisthe typicalMOSFETminimumthresholdvoltage. BothMOSFETshaveI2RlosseswhilethetopsideN-channel equationincludesanadditionaltermfortransitionlosses, whicharehighestathighinputvoltages.ForVIN<20V thehighcurrentefficiencygenerallyimproveswithlarger MOSFETs,whileforVIN>20Vthetransitionlossesrapidly increasetothepointthattheuseofahigherRDS(ON)device withlowerCMILLERactuallyprovideshigherefficiency.The
LTC3858-1 applicaTions inForMaTion
Equation1hasamaximumatVIN=2VOUT,whereIRMS =IOUT/2.Thissimpleworst-caseconditioniscommonly usedfordesignbecauseevensignificantdeviationsdonot offermuchrelief.Notethatcapacitormanufacturers'ripple currentratingsareoftenbasedononly2000hoursoflife. Thismakesitadvisabletofurtherderatethecapacitor,or tochooseacapacitorratedatahighertemperaturethan required. Several capacitors may be paralleled to meet sizeorheightrequirementsinthedesign.Duetothehigh operatingfrequencyoftheLTC3858-1,ceramiccapacitors canalsobeusedforCIN.Alwaysconsultthemanufacturer ifthereisanyquestion. Thebenefitofthe2-phaseoperationcanbecalculated byusingEquation1forthehigherpowercontrollerand thencalculatingthelossthatwouldhaveresultedifboth controller channels switched on at the same time. The totalRMSpowerlostislowerwhenbothcontrollersare operatingduetothereducedoverlapofcurrentpulses requiredthroughtheinputcapacitor'sESR.Thisiswhy theinputcapacitor'srequirementcalculatedaboveforthe worst-casecontrollerisadequateforthedualcontroller design.Also,theinputprotectionfuseresistance,battery resistance,andPCboardtraceresistancelossesarealso reducedduetothereducedpeakcurrentsina2-phase system.Theoverallbenefitofamultiphasedesignwill onlybefullyrealizedwhenthesourceimpedanceofthe powersupply/batteryisincludedintheefficiencytesting. ThesourcesofthetopMOSFETsshouldbeplacedwithin 1cmofeachotherandshareacommonCIN(s).Separating thesourcesandCINmayproduceundesirablevoltageand currentresonancesatVIN. Asmall(0.1Fto1F)bypasscapacitorbetweenthechip VIN pin and ground, placed close to the LTC3858-1, is alsosuggested.A10resistorplacedbetweenCIN(C1) and the VIN pin provides further isolation between the twochannels. The selection of COUT is driven by the effective series resistance (ESR). Typically, once the ESR requirement issatisfied,thecapacitanceisadequateforfiltering.The outputripple(VOUT)isapproximatedby: VOUT 1 IL ESR + 8 * f * COUT where f is the operating frequency, COUT is the output capacitanceandIListheripplecurrentintheinductor. Theoutputrippleishighestatmaximuminputvoltage sinceILincreaseswithinputvoltage. Setting Output Voltage TheLTC3858-1outputvoltagesareeachsetbyanexternalfeedbackresistordividercarefullyplacedacrossthe output,asshowninFigure6.Theregulatedoutputvoltage isdeterminedby: R VOUT = 0.8V 1+ B RA To improve the frequency response, a feedforward capacitor,CFF,maybeused.Greatcareshouldbetakento routetheVFBlineawayfromnoisesources,suchasthe inductorortheSWline.
VOUT 1/2 LTC3858-1 VFB RA
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RB
CFF
Figure 6. Setting Output Voltage
Soft-Start (SS Pins) Thestart-upofeachVOUTiscontrolledbythevoltageon therespectiveSSpin.WhenthevoltageontheSSpin islessthantheinternal0.8Vreference,theLTC3858-1 regulatestheVFBpinvoltagetothevoltageontheSSpin insteadof0.8V.TheSSpincanbeusedtoprograman externalsoft-startfunction. Soft-startisenabledbysimplyconnectingacapacitorfrom theSSpintoground,asshowninFigure7.Aninternal 1A current source charges the capacitor, providing a
1/2 LTC3858-1 SS CSS SGND
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Figure 7. Using the SS Pin to Program Soft-Start
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LTC3858-1 applicaTions inForMaTion
linearrampingvoltageattheSSpin.TheLTC3858-1will regulatetheVFBpin(andhenceVOUT)accordingtothe voltageontheSSpin,allowingVOUTtorisesmoothlyfrom 0Vtoitsfinalregulatedvalue.Thetotalsoft-starttimewill beapproximately: 0.8 V tSS = CSS * 1A INTVCC Regulators TheLTC3858-1featurestwoseparateinternalP-channel lowdropoutlinearregulators(LDO)thatsupplypowerat theINTVCCpinfromeithertheVINsupplypinortheEXTVCCpindependingontheconnectionoftheEXTVCCpin. INTVCCpowersthegatedriversandmuchoftheinternal circuitry.TheVINLDOandtheEXTVCCLDOregulateINTVCCto5.1V.Eachofthesecansupplyapeakcurrentof 50mAandmustbebypassedtogroundwithaminimum of4.7FlowESRcapacitor.Regardlessofwhattypeof bulkcapacitorisused,anadditional1Fceramiccapacitor placeddirectlyadjacenttotheINTVCCandPGNDICpinsis highlyrecommended.Goodbypassingisneededtosupply thehightransientcurrentsrequiredbytheMOSFETgate driversandtopreventinteractionbetweenthechannels. HighinputvoltageapplicationsinwhichlargeMOSFETs arebeingdrivenathighfrequenciesmaycausethemaximumjunctiontemperatureratingfortheLTC3858-1tobe exceeded.TheINTVCCcurrent,whichisdominatedbythe gatechargecurrent,maybesuppliedbyeithertheVINLDO ortheEXTVCCLDO.WhenthevoltageontheEXTVCCpin islessthan4.7V,theVINLDOisenabled.PowerdissipationfortheICinthiscaseishighestandisequaltoVIN* IINTVCC.Thegatechargecurrentisdependentonoperating frequencyasdiscussedintheEfficiencyConsiderations section. The junction temperature can be estimated by usingtheequationsgiveninNote2oftheElectricalCharacteristics.Forexample,theLTC3858-1INTVCCcurrent islimitedtolessthan15mAfroma40Vsupplywhennot usingtheEXTVCCsupplyat70Cambienttemperaturein theSSOPpackage: TJ=70C+(15mA)(40V)(90C/W)=125C Topreventthemaximumjunctiontemperaturefrombeingexceeded,theinputsupplycurrentmustbechecked whileoperatinginforcedcontinuousmode(PLLIN/MODE =INTVCC)atmaximumVIN. WhenthevoltageappliedtoEXTVCCrisesabove4.7V,the VINLDOisturnedoffandtheEXTVCCLDOisenabled.The EXTVCCLDOremainsonaslongasthevoltageappliedto EXTVCCremainsabove4.5V.TheEXTVCCLDOattempts toregulatetheINTVCCvoltageto5.1V,sowhileEXTVCC islessthan5.1V,theLDOisindropoutandtheINTVCC voltageisapproximatelyequaltoEXTVCC.WhenEXTVCC isgreaterthan5.1V,uptoanabsolutemaximumof14V, INTVCCisregulatedto5.1V. Using the EXTVCC LDO allows the MOSFET driver and controlpowertobederivedfromoneoftheswitching regulator outputs (4.7V VOUT 14V) during normal operationandfromtheVINLDOwhentheoutputisout ofregulation(e.g.,start-up,short-circuit).Ifmorecurrent isrequiredthroughtheEXTVCCLDOthanisspecified,an externalSchottkydiodecanbeaddedbetweentheEXTVCC andINTVCCpins.Inthiscase,donotapplymorethan6V totheEXTVCCpinandmakesurethatEXTVCCVIN. Significantefficiencyandthermalgainscanberealized bypoweringINTVCCfromtheoutput,sincetheVINcurrentresultingfromthedriverandcontrolcurrentswillbe scaledbyafactorof(DutyCycle)/(SwitcherEfficiency). For5Vto14Vregulatoroutputs,thismeansconnecting theEXTVCCpindirectlytoVOUT.TyingtheEXTVCCpinto a 8.5V supply reduces the junction temperature in the previousexamplefrom125Cto: TJ=70C+(15mA)(8.5V)(90C/W)=82C However,for3.3Vandotherlowvoltageoutputs,additionalcircuitryisrequiredtoderiveINTVCCpowerfrom theoutput. ThefollowinglistsummarizesthefourpossibleconnectionsforEXTVCC:
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LTC3858-1 applicaTions inForMaTion
1.EXTVCCLeftOpen(orGrounded).ThiswillcauseINTVCC tobepoweredfromtheinternal5.1Vregulatorresultinginanefficiencypenaltyofupto10%athighinput voltages. 2.EXTVCCConnectedDirectlytoVOUT.Thisisthenormal connectionfora5Vto14Vregulatorandprovidesthe highestefficiency. 3.EXTVCCConnectedtoanExternalSupply.Ifanexternal supplyisavailableinthe5Vto14Vrange,itmaybe usedtopowerEXTVCC.EnsurethatEXTVCCCIN VIN MTOP TG1 1/2 LTC3858-1 EXTVCC SW MBOT BG1 PGND
VIN BAT85 BAT85 BAT85
VN2222LL L RSENSE
VOUT COUT
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D
Figure 8. Capacitive Charge Pump for EXTVCC
Topside MOSFET Driver Supply (CB, DB) Externalbootstrapcapacitors,CB,connectedtotheBOOST pinssupplythegatedrivevoltagesforthetopsideMOSFETs. CapacitorCBintheFunctionalDiagramischargedthough externaldiodeDBfromINTVCCwhentheSWpinislow. WhenoneofthetopsideMOSFETsistobeturnedon,the driverplacestheCBvoltageacrossthegate-sourceofthe desiredMOSFET.ThisenhancestheMOSFETandturnson thetopsideswitch.Theswitchnodevoltage,SW,risesto VINandtheBOOSTpinfollows.WiththetopsideMOSFET
Fault Conditions: Overvoltage Protection (Crowbar) Theovervoltagecrowbarisdesignedtoblowasystem inputfusewhentheoutputvoltageoftheregulatorrises muchhigherthannominallevels.Thecrowbarcauseshuge currentstoflow,thatblowthefusetoprotectagainsta shortedtopMOSFETiftheshortoccurswhilethecontrollerisoperating. Acomparatormonitorstheoutputforovervoltageconditions. The comparator detects faults greater than 10% abovethenominaloutputvoltage.Whenthiscondition
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LTC3858-1 applicaTions inForMaTion
issensed,thetopMOSFETisturnedoffandthebottom MOSFETisturnedonuntiltheovervoltageconditionis cleared. The bottom MOSFET remains on continuously foraslongastheovervoltageconditionpersists;ifVOUT returns to a safe level, normal operation automatically resumes. AshortedtopMOSFETwillresultinahighcurrentcondition whichwillopenthesystemfuse.Theswitchingregulator willregulateproperlywithaleakytopMOSFETbyaltering thedutycycletoaccommodatetheleakage. Phase-Locked Loop and Frequency Synchronization TheLTC3858-1hasaninternalphase-lockedloop(PLL) comprisedofaphasefrequencydetector,alowpassfilter, andavoltage-controlledoscillator(VCO).Thisallowsthe turn-onofthetopMOSFETofcontroller1tobelockedto therisingedgeofanexternalclocksignalappliedtothe PLLIN/MODEpin.Theturn-onofcontroller2'stopMOSFET isthus180degreesoutofphasewiththeexternalclock. Thephasedetectorisanedgesensitivedigitaltypethat provideszerodegreesphaseshiftbetweentheexternal andinternaloscillators.Thistypeofphasedetectordoes notexhibitfalselocktoharmonicsoftheexternalclock. Iftheexternalclockfrequencyisgreaterthantheinternal oscillator'sfrequency,fOSC,thencurrentissourcedcontinuouslyfromthephasedetectoroutput,pullinguptheVCO input.WhentheexternalclockfrequencyislessthanfOSC, currentissunkcontinuously,pullingdowntheVCOinput. Iftheexternalandinternalfrequenciesarethesamebut exhibitaphasedifference,thecurrentsourcesturnonfor anamountoftimecorrespondingtothephasedifference. ThevoltageattheVCOinputisadjusteduntilthephase andfrequencyoftheinternalandexternaloscillatorsare identical.Atthestableoperatingpoint,thephasedetector outputishighimpedanceandtheinternalfiltercapacitor, CLP,holdsthevoltageattheVCOinput. NotethattheLTC3858-1canonlybesynchronizedtoan external clock whose frequency is within range of the LTC3858-1's internal VCO, which is nominally 55kHz to1MHz.Thisisguaranteedtobebetween75kHzand 850kHz.
1000 900 800 FREQUENCY (kHz) 700 600 500 400 300 200 100 0 15 25 35 45 55 65 75 85 95 105 115 125 FREQ PIN RESISTOR (k )
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Figure 9. Relationship Between Oscillator Frequency and Resistor Value at the FREQ Pin
Typically, the external clock (on the PLLIN/MODE pin) inputhighthresholdis1.6V,whiletheinputlowthreshold is1.1V. RapidphaselockingcanbeachievedbyusingtheFREQ pin to set a free-running frequency near the desired synchronizationfrequency.TheVCO'sinputvoltageis prebiasedatafrequencycorrespondingtothefrequency setbytheFREQpin.Onceprebiased,thePLLonlyneeds to adjust the frequency slightly to achieve phase lock andsynchronization.Althoughitisnotrequiredthatthe free-runningfrequencybenearexternalclockfrequency, doingsowillpreventtheoperatingfrequencyfrompassing throughalargerangeoffrequenciesasthePLLlocks. Table2summarizesthedifferentstatesinwhichtheFREQ pincanbeused.
Table 2
FREQ PIN 0V INTVCC Resistor AnyoftheAbove PLLIN/MODE PIN DCVoltage DCVoltage DCVoltage ExternalClock FREQUENCY 350kHz 535kHz 50kHz-900kHz Phase-Lockedto ExternalClock
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Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time durationthattheLTC3858-1iscapableofturningonthetop MOSFET.Itisdeterminedbyinternaltimingdelaysandthe gatechargerequiredtoturnonthetopMOSFET.Lowduty cycleapplicationsmayapproachthisminimumon-time limitandcareshouldbetakentoensurethat: tON(MIN) < VOUT VIN f 1.TheVINcurrentistheDCinputsupplycurrentgiven intheElectricalCharacteristicstable,whichexcludes MOSFETdriverandcontrolcurrents.VINcurrenttypicallyresultsinasmall(<0.1%)loss. 2.INTVCCcurrentisthesumoftheMOSFETdriverand control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs.EachtimeaMOSFETgateisswitchedfrom lowtohightolowagain,apacketofcharge,dQ,moves fromINTVCCtoground.TheresultingdQ/dtisacurrent out of INTVCC that is typically much larger than the controlcircuitcurrent.Incontinuousmode,IGATECHG =f(QT+QB),whereQTandQBarethegatechargesof thetopsideandbottomsideMOSFETs. SupplyingINTVCCfromanoutput-derivedpowersource through EXTVCC will scale the VIN current required forthedriverandcontrolcircuitsbyafactorof(Duty Cycle)/(Efficiency).Forexample,ina20Vto5Vapplication,10mAofINTVCCcurrentresultsinapproximately 2.5mAofVINcurrent.Thisreducesthemidcurrentloss from10%ormore(ifthedriverwaspowereddirectly fromVIN)toonlyafewpercent. 3.I2RlossesarepredictedfromtheDCresistancesofthe fuse(ifused),MOSFET,inductor,currentsenseresistor,andinputandoutputcapacitorESR.Incontinuous modetheaverageoutputcurrentflowsthroughLand RSENSE,butis"chopped"betweenthetopsideMOSFET andthesynchronousMOSFET.IfthetwoMOSFETshave approximatelythesameRDS(ON),thentheresistance ofoneMOSFETcansimplybesummedwiththeresistancesofL,RSENSEandESRtoobtainI2Rlosses.For example,ifeachRDS(ON)=30m,RL=50m,RSENSE =10mandRESR=40m(sumofbothinputand output capacitance losses), then the total resistance is130m.Thisresultsinlossesrangingfrom3%to 13%astheoutputcurrentincreasesfrom1Ato5Afor a5Voutput,ora4%to20%lossfora3.3Voutput. EfficiencyvariesastheinversesquareofVOUTforthe sameexternalcomponentsandoutputpowerlevel.The combinedeffectsofincreasinglyloweroutputvoltages andhighercurrentsrequiredbyhighperformancedigital systemsisnotdoublingbutquadruplingtheimportance oflosstermsintheswitchingregulatorsystem!
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Ifthedutycyclefallsbelowwhatcanbeaccommodated bytheminimumon-time,thecontrollerwillbegintoskip cycles.Theoutputvoltagewillcontinuetoberegulated, buttheripplevoltageandcurrentwillincrease. Theminimumon-timefortheLTC3858-1isapproximately 95ns.However,asthepeaksensevoltagedecreasesthe minimumon-timegraduallyincreasesuptoabout130ns. Thisisofparticularconcerninforcedcontinuousapplicationswithlowripplecurrentatlightloads.Ifthedutycycle dropsbelowtheminimumon-timelimitinthissituation, asignificantamountofcycleskippingcanoccurwithcorrespondinglylargercurrentandvoltageripple. Efficiency Considerations Thepercentefficiencyofaswitchingregulatorisequalto theoutputpowerdividedbytheinputpowertimes100%. Itisoftenusefultoanalyzeindividuallossestodetermine whatislimitingtheefficiencyandwhichchangewould producethemostimprovement.Percentefficiencycan beexpressedas: %Efficiency=100%-(L1+L2+L3+...) whereL1,L2,etc.aretheindividuallossesasapercentageofinputpower. Althoughalldissipativeelementsinthecircuitproduce losses, four main sources usually account for most of thelossesinLTC3858-1circuits:1)ICVINcurrent,2)INTVCCregulatorcurrent,3)I2Rlosses,4)topsideMOSFET transitionlosses.
LTC3858-1 applicaTions inForMaTion
4.TransitionlossesapplyonlytothetopsideMOSFET(s), and become significant only when operating at high input voltages (typically 15V or greater). Transition lossescanbeestimatedfrom: TransitionLoss=(1.7)*VIN*2*IO(MAX)*CRSS*f Other"hidden"lossessuchascoppertraceandinternal batteryresistancescanaccountforanadditional5%to 10%efficiencydegradationinportablesystems.Itis veryimportanttoincludethese"system"levellosses duringthedesignphase.Theinternalbatteryandfuse resistancelossescanbeminimizedbymakingsurethat CINhasadequatechargestorageandverylowESRat theswitchingfrequency.A25Wsupplywilltypically require a minimum of 20F to 40F of capacitance having a maximum of 20m to 50m of ESR. The LTC3858-1 2-phase architecture typically halves this input capacitance requirement over competing solutions.OtherlossesincludingSchottkyconductionlosses duringdead-timeandinductorcorelossesgenerally accountforlessthan2%totaladditionalloss. Checking Transient Response Theregulatorloopresponsecanbecheckedbylookingat theloadcurrenttransientresponse.Switchingregulators takeseveralcyclestorespondtoastepinDC(resistive) loadcurrent.Whenaloadstepoccurs,VOUTshiftsby anamountequaltoILOAD(ESR),whereESRistheeffectiveseriesresistanceofCOUT.ILOADalsobeginsto chargeordischargeCOUTgeneratingthefeedbackerror signalthatforcestheregulatortoadapttothecurrent changeandreturnVOUTtoitssteady-statevalue.During thisrecoverytimeVOUTcanbemonitoredforexcessive overshoot or ringing, which would indicate a stability problem.OPTI-LOOPcompensationallowsthetransient responsetobeoptimizedoverawiderangeofoutput capacitanceandESRvalues.The availability of the ITH pin not only allows optimization of control loop behavior, but it also provides a DC coupled and AC filtered closed-loop response test point. The DC step, rise time and settling at this test point truly reflects the closed-loop response. Assumingapredominantlysecondordersystem,phase marginand/ordampingfactorcanbeestimatedusingthe percentageofovershootseenatthispin.Thebandwidth canalsobeestimatedbyexaminingtherisetimeatthe pin. The ITH external components shown in Figure 12 circuitwillprovideanadequatestartingpointformost applications. TheITHseriesRC-CCfiltersetsthedominantpole-zero loopcompensation.Thevaluescanbemodifiedslightly (from0.5to2timestheirsuggestedvalues)tooptimize transientresponseoncethefinalPClayoutisdoneand theparticularoutputcapacitortypeandvaluehavebeen determined. The output capacitors need to be selected becausethevarioustypesandvaluesdeterminetheloop gainandphase.Anoutputcurrentpulseof20%to80% offull-loadcurrenthavingarisetimeof1sto10swill produceoutputvoltageandITHpinwaveformsthatwill giveasenseoftheoverallloopstabilitywithoutbreaking thefeedbackloop. Placing a resistive load and a power MOSFET directly acrosstheoutputcapacitoranddrivingthegatewithan appropriatesignalgeneratorisapracticalwaytoproduce arealisticloadstepcondition.Theinitialoutputvoltage stepresultingfromthestepchangeinoutputcurrentmay notbewithinthebandwidthofthefeedbackloop,sothis signalcannotbeusedtodeterminephasemargin.This iswhyitisbettertolookattheITHpinsignalwhichisin the feedback loop and is the filtered and compensated controlloopresponse. ThegainoftheloopwillbeincreasedbyincreasingRC andthebandwidthoftheloopwillbeincreasedbydecreasingCC.IfRCisincreasedbythesamefactorthatCC isdecreased,thezerofrequencywillbekeptthesame, thereby keeping the phase shift the same in the most criticalfrequencyrangeofthefeedbackloop.Theoutput voltagesettlingbehaviorisrelatedtothestabilityofthe closed-loopsystemandwilldemonstratetheactualoverall supplyperformance. Asecond,moreseveretransientiscausedbyswitching inloadswithlarge(>1F)supplybypasscapacitors.The dischargedbypasscapacitorsareeffectivelyputinparallel withCOUT,causingarapiddropinVOUT.Noregulatorcan alteritsdeliveryofcurrentquicklyenoughtopreventthis suddenstepchangeinoutputvoltageiftheloadswitch resistanceislowanditisdrivenquickly.Iftheratioof
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LTC3858-1 applicaTions inForMaTion
CLOADtoCOUTisgreaterthan1:50,theswitchrisetime shouldbecontrolledsothattheloadrisetimeislimited toapproximately25*CLOAD.Thusa10Fcapacitorwould requirea250srisetime,limitingthechargingcurrent toabout200mA. Design Example As a design example for one channel, assume VIN = 12V(nominal),VIN=22V(max),VOUT=3.3V,IMAX=6A, VSENSE(MAX)=50mVandf=350kHz. Theinductancevalueischosenfirstbasedona30%ripple currentassumption.Thehighestvalueofripplecurrent occursatthemaximuminputvoltage.TietheFREQpin to GND, generating 350kHz operation. The minimum inductancefor30%ripplecurrentis: IL = VOUT VOUT 1- ( f) (L) VIN ThepowerdissipationonthetopsideMOSFETcanbeeasily estimated.ChoosingaFairchildFDS6982SdualMOSFET .At resultsin:RDS(ON)=0.035/0.022,CMILLER=215pF maximuminputvoltagewithT(estimated)=50C: PMAIN =
2 3.3V 6A 1+ 0.005 50C - 25C 22V 2 6A 2.5 215pF * 0.035 + 22V 2 1 1 5V - 2.3V + 2.3V 350kHz = 433mW
()
(
)(
)
(
)(
)
(
)(
)
(
)
Ashort-circuittogroundwillresultinafoldedbackcurrentof: 25mV 1 95ns 22V ISC = - = 3.9A 0.006 2 3.9H withatypicalvalueofRDS(ON)and=(0.005/C)(25C) = 0.125. The resulting power dissipated in the bottom MOSFETis: P = 3.9A SYNC
(
)
A3.9Hinductorwillproduce29%ripplecurrent.The peakinductorcurrentwillbethemaximumDCvalueplus onehalftheripplecurrent,or6.88A.Increasingtheripple currentwillalsohelpensurethattheminimumon-time of95nsisnotviolated.Theminimumon-timeoccursat maximumVIN: V 3.3V tON(MIN) = OUT = = 429ns 22V 350kHz VIN f
(
)2 (1.125)(0.022) = 376mW
whichislessthanfull-loadconditions. CINischosenforanRMScurrentratingofatleast3Aat temperatureassumingonlythischannelison.COUTis chosenwithanESRof0.02forlowoutputripplevoltage.Theoutputrippleincontinuousmodewillbehighest atthemaximuminputvoltage.Theoutputvoltageripple duetoESRisapproximately: VORIPPLE=RESR(IL)=0.02(1.75A)=35mVP-P
()
(
)
TheequivalentRSENSEresistorvaluecanbecalculatedby usingtheminimumvalueforthemaximumcurrentsense threshold(43mV): RSENSE 43mV = 0.006 6.88A
Choosing0.5%resistors:RA=24.9kandRB=77.7kyields anoutputvoltageof3.296V.
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LTC3858-1 applicaTions inForMaTion
PC Board Layout Checklist Whenlayingouttheprintedcircuitboard,thefollowing checklistshouldbeusedtoensureproperoperationof theIC.Theseitemsarealsoillustratedgraphicallyinthe layoutdiagramofFigure10.Figure11illustratesthecurrent waveformspresentinthevariousbranchesofthe2-phase synchronousregulatorsoperatinginthecontinuousmode. Checkthefollowinginyourlayout: 1.ArethetopN-channelMOSFETsMTOP1andMTOP2 locatedwithin1cmofeachotherwithacommondrain connectionatCIN?Donotattempttosplittheinput decouplingforthetwochannelsasitcancausealarge resonantloop. 2.Arethesignalandpowergroundskeptseparate?The combinedICsignalgroundpinandthegroundreturn ofCINTVCCmustreturntothecombinedCOUT(-)terminals.ThepathformedbythetopN-channelMOSFET, SchottkydiodeandtheCINcapacitorshouldhaveshort leadsandPCtracelengths.Theoutputcapacitor(-) terminals should be connected as close as possible tothe(-)terminalsoftheinputcapacitorbyplacing thecapacitorsnexttoeachotherandawayfromthe Schottkyloopdescribedabove. 3.DotheLTC3858-1VFBpins'resistivedividersconnect to the (+) terminals of COUT? The resistive divider mustbeconnectedbetweenthe(+)terminalofCOUT andsignalground.Thefeedbackresistorconnections shouldnotbealongthehighcurrentinputfeedsfrom theinputcapacitor(s). 4.AretheSENSE-andSENSE+leadsroutedtogetherwith minimumPCtracespacing?Thefiltercapacitorbetween SENSE+andSENSE-shouldbeascloseaspossible totheIC.EnsureaccuratecurrentsensingwithKelvin connectionsattheSENSEresistor. 5. Is the INTVCC decoupling capacitor connected close totheIC,betweentheINTVCCandthepowerground pins?ThiscapacitorcarriestheMOSFETdrivers'currentpeaks.Anadditional1Fceramiccapacitorplaced immediatelynexttotheINTVCCandPGNDpinscanhelp improvenoiseperformancesubstantially. 6.Keeptheswitchingnodes(SW1,SW2),topgatenodes (TG1,TG2),andboostnodes(BOOST1,BOOST2)away fromsensitivesmall-signalnodes,especiallyfromthe oppositeschannel'svoltageandcurrentsensingfeedbackpins.Allofthesenodeshaveverylargeandfast movingsignalsandthereforeshouldbekeptonthe "outputside"oftheLTC3858-1andoccupyminimum PCtracearea. 7.Useamodified"starground"technique:alowimpedance, large copper area central grounding point on thesamesideofthePCboardastheinputandoutput capacitors with tie-ins for the bottom of the INTVCC decouplingcapacitor,thebottomofthevoltagefeedback resistivedividerandtheSGNDpinoftheIC. PC Board Layout Debugging Startwithonecontrolleronatatime.Itishelpfultouse aDC-50MHzcurrentprobetomonitorthecurrentinthe inductor while testing the circuit. Monitor the output switchingnode(SWpin)tosynchronizetheoscilloscope totheinternaloscillatorandprobetheactualoutputvoltage aswell.Checkforproperperformanceovertheoperating voltageandcurrentrangeexpectedintheapplication.The frequencyofoperationshouldbemaintainedovertheinput voltagerangedowntodropoutanduntiltheoutputload dropsbelowthelowcurrentoperationthreshold--typically10%ofthemaximumdesignedcurrentlevelinBurst Modeoperation. Thedutycyclepercentageshouldbemaintainedfromcycle tocycleinawell-designed,lownoisePCBimplementation. Variationinthedutycycleatasubharmonicratecansuggestnoisepickupatthecurrentorvoltagesensinginputs orinadequateloopcompensation.Overcompensationof theloopcanbeusedtotameapoorPClayoutifregulator bandwidth optimization is not required. Only after eachcontrollerischeckedforitsindividualperformance shouldbothcontrollersbeturnedonatthesametime. A particularly difficult region of operation is when one controllerchannelisnearingitscurrentcomparatortrip pointwhentheotherchannelisturningonitstopMOSFET. Thisoccursaround50%dutycycleoneitherchanneldue
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LTC3858-1 applicaTions inForMaTion
SS1 LTC3858-1 ITH1 VFB1 SENSE1+ SENSE1- FREQ PGOOD1 TG1 SW1 CB1 BOOST1 BG1 fIN PLLIN/MODE RUN1 RUN2 SGND SENSE2- SENSE2+ VFB2 ITH2 SS2 VIN PGND EXTVCC INTVCC BG2 BOOST2 SW2 TG2 L2 CB2 RSENSE VOUT2 VOUT1 1F CERAMIC COUT1 M1 M2 D1 RPU1 PGOOD1 VPULL-UP (<6V) L1 RSENSE VOUT1
CVIN
RIN
+
GND
+
CIN
+
CINTVCC
VIN 1F CERAMIC M3
COUT2
M4
D2
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Figure 10. Recommended Printed Circuit Layout Diagram
tothephasingoftheinternalclocksandmaycauseminor dutycyclejitter. Reduce VIN from its nominal level to verify operation oftheregulatorindropout.Checktheoperationofthe undervoltagelockoutcircuitbyfurtherloweringVINwhile monitoringtheoutputstoverifyoperation. Investigatewhetheranyproblemsexistonlyathigheroutputcurrentsoronlyathigherinputvoltages.Ifproblems
coincidewithhighinputvoltagesandlowoutputcurrents, lookforcapacitivecouplingbetweentheBOOST,SW,TG, and possibly BG connections and the sensitive voltage andcurrentpins.Thecapacitorplacedacrossthecurrent sensingpinsneedstobeplacedimmediatelyadjacentto thepinsoftheIC.Thiscapacitorhelpstominimizethe effectsofdifferentialnoiseinjectionduetohighfrequency capacitive coupling. If problems are encountered with highcurrentoutputloadingatlowerinputvoltages,look
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LTC3858-1 applicaTions inForMaTion
SW1
L1
RSENSE1
VOUT1
D1
COUT1
RL1
VIN RIN
CIN
SW2
L2
RSENSE2
VOUT2
BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH.
D2
COUT2
RL2
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Figure 11. Branch Current Waveforms
forinductivecouplingbetweenCIN,Schottkyandthetop MOSFETcomponentstothesensitivecurrentandvoltage sensingtraces.Inaddition,investigatecommonground pathvoltagepickupbetweenthesecomponentsandthe SGNDpinoftheIC. An embarrassing problem, which can be missed in an otherwiseproperlyworkingswitchingregulator,results whenthecurrentsensingleadsarehookedupbackwards.
Theoutputvoltageunderthisimproperhookupwillstill bemaintainedbuttheadvantagesofcurrentmodecontrol willnotberealized.Compensationofthevoltageloopwill be much more sensitive to component selection. This behaviorcanbeinvestigatedbytemporarilyshortingout thecurrentsensingresistor--don'tworry,theregulator willstillmaintaincontroloftheoutputvoltage.
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LTC3858-1 Typical applicaTions
RB1 215k CF1 15pF RA1 68.1k CITH1A 150pF RITH1 15k CITH1 820pF C1 1nF LTC3858-1 SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 TG1 D1 VIN SS1 INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 RITH2 27k ITH2 SW2 BG2 MBOT2 D2 MTOP2 CB2 0.47F L2 7.2H RSENSE2 10m VOUT2 8.5V 3A COUT2 150F CINT 4.7F CIN 22F VIN 9V TO 38V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 3.3H RSENSE1 7m COUT1 150F VOUT1 3.3V 5A
CSS1 0.1F
CSS2 0.1F CITH2 680pF
CITH2A 100pF RA2 44.2k CF2 39pF RB2 442k
VFB2 SENSE2- SENSE2+
C2 1nF
COUT1, COUT2: SANYO 10TPD150M L1: SUMIDA CDEP105-3R2M L2: SUMIDA CDEP105-7R2M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
38581 F12
Efficiency vs Output Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 VIN = 12V Burst Mode OPERATION 1 10
38581 F12b
Start-Up
SW Node Waveforms
VOUT = 8.5V
VOUT = 3.3V
VOUT2 2V/DIV SW1 5V/DIV VOUT1 2V/DIV SW2 5V/DIV
0 0.00001 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A)
20ms/DIV
3858 F12c
1s/DIV
3858 F12d
Figure 12. High Efficiency Dual 8.5V/3.3V Step-Down Converter
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LTC3858-1 Typical applicaTions
High Efficiency Dual 2.5V/3.3V Step-Down Converter
RB1 143k CF1 22pF RA1 68.1k CITH1A 100pF RITH1 22k CITH1 820pF C1 1nF
LTC3858-1 SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 TG1
INTVCC 100k MBOT1 CB1 0.47F MTOP1 D1 VIN CIN 22F VIN 4V TO 38V L1 2.4H RSENSE1 7m COUT1 150F VOUT1 2.5V 5A
CSS1 0.01F SS1
INTVCC
CSS2 0.01F CITH2 820pF
PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 SW2 BG2
CINT 4.7F D2 MTOP2 CB2 0.47F
L2 3.2H
RSENSE2 7m
RITH2 15k
ITH2
MBOT2
VOUT2 3.3V COUT2 5A 150F
CITH2A 150pF RA2 68.1k CF2 15pF RB2 215k
VFB2 SENSE2- SENSE2+
C2 1nF
COUT1, COUT2: SANYO 10TPD150M L1: SUMIDA CDEP105-2R5 L2: SUMIDA CDEP105-3R2M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
38581 F13
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0
LTC3858-1 Typical applicaTions
High Efficiency Dual 12V/5V Step-Down Converter
RB1 422k CF1 33pF RA1 30.1k CITH1A 100pF RITH1 33k CITH1 680pF CSS1 0.01F C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3858-1 SS1 VIN INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 RITH2 17k ITH2 SW2 BG2 MBOT2 CINT 4.7F D2 MTOP2 CB2 0.47F L2 4.3H RSENSE2 7m VOUT2 5V COUT2 5.5A 150F TG1 D1 CIN 22F VIN 12.5V TO 38V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 8.8H RSENSE1 10m COUT1 47F VOUT1 12V 3A
RFREQ 60k CSS2 0.01F CITH2 680pF
CITH2A 100pF RA2 75k CF2 15pF RB2 393k
VFB2 SENSE2- SENSE2+
C2 1nF
COUT1: KEMET T525D476M016E035 COUT2: SANYO 10TPD150M L1: SUMIDA CDEP105-8R8M L2: SUMIDA CDEP105-4R3M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
38581 TA02a
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LTC3858-1 Typical applicaTions
High Efficiency Dual 24V/5V Step-Down Converter
RB1 487k CF1 18pF RA1 16.9k CITH1A 100pF RITH1 46k CITH1 680pF CSS1 0.01F C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3858-1 SS1 VIN INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 RITH2 17k ITH2 SW2 BG2 MBOT2 CINT 4.7F D2 MTOP2 CB2 0.47F L2 4.3H RSENSE2 7m VOUT2 5V COUT2 5A 150F TG1 D1 CIN 22F VIN 24.5V TO 38V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 22H RSENSE1 25m VOUT1 24V 1A
COUT1 22F 25V x2 CERAMIC
RFREQ 60k CSS2 0.01F CITH2 680pF
CITH2A 100pF RA2 75k CF2 15pF RB2 392k
VFB2 SENSE2- SENSE2+ COUT2: SANYO 10TPD150M L1: SUMIDA CDRH105R-220M L2: SUMIDA CDEP105-4R3M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP
C2 1nF
38581 TA04
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LTC3858-1 Typical applicaTions
High Efficiency Dual 1V/1.2V Step-Down Converter
RB1 28.7k CF1 56pF RA1 115k CITH1A 220pF RITH1 3.93k CITH1 1000pF C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3858-1 VIN SS1 INTVCC CINT 4.7F D2 MTOP2 CB2 0.47F L2 0.47H RSENSE2 4m VOUT2 1.2V COUT2 8A 220F 2 TG1 D1 CIN 22F VIN 12V CB1 0.47F MTOP1 INTVCC 100k L1 MBOT1 0.47H RSENSE1 4m COUT1 220F 2 VOUT1 1V 8A
CSS1 0.01F
RFREQ 60k CSS2 0.01F CITH2 1000pF
PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 SW2 BG2
RITH2 3.43k
ITH2
MBOT2
CITH2A 220pF RA2 115k CF2 56pF RB2 57.6k
VFB2 SENSE2- SENSE2+
C2 1nF
COUT1, COUT2: SANYO 2RSTPE220M L1: SUMIDA CDEP105-3R2M L2: SUMIDA CDEP105-7R2M MTOP1, MTOP2: RENESAS RJK0305 MBOT1, MBOT2: RENESAS RJK0328
38581 TA03a
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LTC3858-1 Typical applicaTions
High Efficiency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing
RB1 28.7k CF1 56pF RA1 115k CITH1A 200pF RITH1 3.93k CITH1 1000pF C1 0.1F RS1 1.18k SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3858-1 VIN SS1 INTVCC CINT 4.7F D2 MTOP2 CB2 0.47F L2 0.47H VOUT2 1.2V COUT2 8A 220F 2 TG1 D1 CIN 22F VIN 12V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 0.47H COUT1 220F 2 VOUT1 1V 8A
CSS1 0.01F
RFREQ 65k CSS2 0.01F CITH2 1000pF
PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 SW2 BG2
RITH2 3.93k
ITH2
MBOT2
CITH2A 220pF RA2 115k CF2 56pF RB2 57.6k
VFB2 SENSE2- SENSE2+ RS2 1.18k
38581 TA05
C2 0.1F
COUT1, COUT2: SANYO 2R5TPE220M L1, L2: SUMIDA IHL P2525CZERR47M06 MTOP1, MTOP2: RENESAS RJK0305 MBOT1, MBOT2: RENESAS RJK0328
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LTC3858-1 package DescripTion
(ReferenceLTCDWG#05-08-1712RevB)
UFD Package 28-Lead Plastic QFN (4mm x 5mm)
0.70 0.05
4.50
0.05 3.10
0.05 2.50 REF 2.65 0.05 3.65 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC 3.50 REF 4.10 0.05 5.50 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 45 CHAMFER 27 28 0.40 1 2 5.00 0.10 (2 SIDES) 0.10
2.50 REF R = 0.115 TYP
3.50 REF 3.65 0.10 2.65 0.10
(UFD28) QFN 0506 REV B
0.200 REF 0.00 - 0.05
0.25 BOTTOM VIEW--EXPOSED PAD
0.05
0.50 BSC
NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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LTC3858-1 package DescripTion
GN Package 28-Lead Plastic SSOP (Narrow .150 Inch)
(ReferenceLTCDWG#05-08-1641)
.045 .005
.386 - .393* (9.804 - 9.982) 28 27 26 25 24 23 22 21 20 19 18 17 1615
.033 (0.838) REF
.254 MIN
.150 - .165
.229 - .244 (5.817 - 6.198)
.150 - .157** (3.810 - 3.988)
.0165 .0015 RECOMMENDED SOLDER PAD LAYOUT
.0250 BSC
1
45
23
4
56
7
8
9 10 11 12 13 14 .004 - .0098 (0.102 - 0.249)
.015 .004 (0.38 0.10) .0075 - .0098 (0.19 - 0.25) .016 - .050 (0.406 - 1.270)
NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 0 - 8 TYP
.0532 - .0688 (1.35 - 1.75)
.008 - .012 (0.203 - 0.305) TYP
.0250 (0.635) BSC
GN28 (SSOP) 0204
3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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LTC3858-1 revision hisTory
REV B DATE 11/09 DESCRIPTION ChangetoAbsoluteMaximumRatings ChangetoElectricalCharacteristics ChangetoTypicalPerformanceCharacteristics ChangetoPinFunctions TextChangestoOperationSection TextChangestoApplicationsInformationSection ChangetoTable2 ChangetoFigure10 ChangestoRelatedParts
(Revision history begins at Rev B)
PAGE NUMBER 2 3,4 6 8,9 11,12,13 20,21,22,23,25 22 27 38
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However,noresponsibilityisassumedforitsuse.LinearTechnologyCorporationmakesnorepresentationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
LTC3858-1 relaTeD parTs
PART NUMBER DESCRIPTION COMMENTS Phase-LockableFixedOperatingFrequency50kHzto900kHz, 4VVIN38V,0.8VVOUT24V,IQ=50A, Phase-LockableFixedOperatingFrequency50kHzto900kHz, 4VVIN24V,0.8VVOUT14V,IQ=170A, Phase-LockableFixedOperatingFrequency140kHzto650kHz, 4VVIN36V,0.8VVOUT10V,IQ=30A, Phase-LockableFixedOperatingFrequency140kHzto650kHz, 4VVIN36V,0.8VVOUT10V,IQ=80A, AdjustableFixedOperatingFrequency100kHzto500kHz, 4VVIN60V,1.23VVOUT36V,IQ=120A,TSSOP-16 Fixed200kHzOperatingFrequency,4VVIN60V,1.23VVOUT36V, IQ=100A,TSSOP-16 LTC3857/LTC3857-1 LowIQ,DualOutput2-PhaseSynchronousStep-Down DC/DCControllerwith99%DutyCycle LTC3868/LTC3868-1 LowIQ,DualOutput2-PhaseSynchronousStep-Down DC/DCControllerwith99%DutyCycle LTC3834/LTC3834-1 LowIQ,SynchronousStep-DownDC/DCController LTC3835/LTC3835-1 LowIQ,SynchronousStep-DownDC/DCController LT3845 LT3800 LTC3824 LowIQ,HighVoltageSynchronousStep-Down DC/DCController LowIQ,HighVoltageSynchronousStep-Down DC/DCController
LowIQ,HighVoltageDC/DCController,100%DutyCycle SelectableFixed200kHzto600kHzOperatingFrequency, 4VVIN60V,0.8VVOUTVIN,IQ=40A,MSOP-10E Phase-LockableFixedOperatingFrequency250kHzto780kHz, 4VVIN30V,0.8VVOUT5.25V Phase-LockableFixedFrequency250kHzto770kHz, 4.5VVIN38V,0.8VVOUT12.5V Phase-LockableFixedOperatingFrequency250kHzto750kHz, 4VVIN24V,VOUTUpto13.5V
LTC3850/LTC3850-1 Dual2-Phase,HighEfficiencySynchronousStep-Down LTC3850-2 DC/DCControllers,RSENSEorDCRCurrentSensingand Tracking LTC3855 Dual,Multiphase,SynchronousDC/DCStep-Down ControllerwithDiffampandDCRTemperature Compensation TripleOutput,MultiphaseSynchronousStep-Down DC/DCController,RSENSEorDCRCurrentSensingand Tracking
LTC3853
LTC3854 LTC3775 LTC3851A/ LTC3851A-1 LTC3878/LTC3879 LTM4600HV LTM4601AHV
SmallFootprintWideVINRangeSynchronousStep-Down Fixed400kHzOperatingFrequency,4.5VVIN38V, DC/DCController 0.8VVOUT5.25V,2mmx3mmQFN-12,MSOP-12 HighFrequencySynchronousVoltageModeStep-Down DC/DCController NoRSENSETMWideVINRangeSynchronousStep-Down DC/DCController NoRSENSEConstantOn-TimeSynchronousStep-Down DC/DCController 10ADC/DCModule(R)CompletePowerSupply 12ADC/DCModuleCompletePowerSupply FastTransientResponse,tON(MIN)=30ns,4VVIN38V, 0.6VVOUT0.8VIN,MSOP-16E,3mmx3mmQFN-16 Phase-LockableFixedOperatingFrequency250kHzto750kHz, 4VVIN38V,0.8VVOUT5.25V,MSOP-16E,3mmx3mmQFN-16, SSOP-16 VeryFastTransientResponse,tON(MIN)=43ns,4VVIN38V, VOUTUp90%ofVIN,MSOP-16E,3mmx3mmQFN-16,SSOP-16 HighEfficiency,CompactSize,UltraFastTMTransientResponse, 4.5VVIN28V,0.8VVOUT5V,15mmx15mmx2.8mm HighEfficiency,CompactSize,UltrafastTransientResponse, 4.5VVIN28V,0.8VVOUT5V,15mmx15mmx2.8mm
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Linear Technology Corporation
(408)432-1900 FAX: (408) 434-0507 www.linear.com
LT 0110 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
LINEAR TECHNOLOGY CORPORATION 2009

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