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FEATURES
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LTC3735 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs DESCRIPTION
The LTC(R)3735 is a 2-phase synchronous step-down switching regulator controller that drives all N-channel power MOSFETs in a constant frequency architecture. The output voltage is programmable by six VID bits during normal operation and by external resistors during initial boot-up and deeper sleep state. The LTC3735 drives its two output stages out-of-phase at frequencies up to 550kHz to minimize the RMS ripple currents in both input and output capacitors. This antiphase technique also doubles the apparent switching frequency, improving the transient response while operating each phase at an optimum frequency for efficiency. Thermal design is further simplified by cycle-by-cycle current sharing between the two phases. An Intel compatible PSIB input is provided to select between two modes of operation. Fully enhanced synchronous mode achieves a very small output ripple and very fast transient response while power saving mode realizes very high efficiency. OPTI-LOOP(R) compensation allows the transient response to be optimized for a wide range of output capacitance and ESR values.
L, LT, LTC, LTM, OPTI-LOOP PolyPhase, Linear Technology and the Linear logo are registered , trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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Output Stages Operate Antiphase 1% Output Voltage Accuracy 6-Bit IMVP-IV VID Code: VOUT = 0.7V to 1.708V Intel Compatible Power Saving Mode (PSIB) Stage Shedding Improves Low Current Efficiency Power Good Output with Adaptive Masking Lossless Voltage Positioning Dual Input Supply Capability for Load Sharing Resistor Programmable VOUT at Boot-Up and Deeper Sleep State Resistor Programmable Deep Sleep Offset Programmable Fixed Frequency: 210kHz to 550kHz Adjustable Soft-Start Current Ramping Foldback Output Current Limit Short-Circuit Shutdown Timer with Defeat Option Overvoltage Protection Available in 36-Lead SSOP (0.209 Wide) and 38-Lead (5mm x 7mm) Packages
APPLICATIONS
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Mobile and Desktop Computers Internet Servers
TYPICAL APPLICATION
MCH_PG DPRSLPVR STP_CPUB PSIB FREQSET VID5-VID0 PGOOD ITH RUN/SS 0.1F SGND LTC3735 PVCC 4.7F BAT54A 0.47F SW2 SW1 0.47F RBOOT BOOST1 RDPRSLP BOOST2 RDPSLP VOA+ OAOUT VOA- 56.2k 1.27M 13.3k 549k VOA+ SENSE2+ SENSE2- 12.7k 13.3k TG1 SW1 BG1 PGND M2 D1 VOUT 0.7V TO 1.708V 40A M1 1H 0.002 VIN 5V TO 24V
SENSE1+ SENSE1- TG2 SW2 BG2 M4 D2 M3 1H 0.002
+
100pF 232k VOA+ 4.5V TO 7V
RC 4.74k CC 470pF
COUT 330F 2V x5
CIN 10F 35V x4
3735 F01
Figure 1. High Current 2-Phase Step-Down Converter
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LTC3735 ABSOLUTE MAXIMUM RATINGS (Note 1)
Input Supply Voltage (PVCC) ....................... 7V to - 0.3V Topside Driver Voltages (BOOST1,2) ......... 38V to -0.3V Switch Voltage (SW1, 2) ............................... 32V to -5V Boosted Driver Voltages (BOOST1-SW1, BOOST2-SW2) ............... 7V to -0.3V DPRSLPVR, STP_CPUB, MCH_PG, PGOOD, RDPRSLP, RDPSLP, RBOOT Voltages ......... 5V to -0.3V RUN/SS, PSIB, FREQSET Voltages .............7V to - 0.3V VID0-VID5 Voltages ....................................5V to - 0.3V VFB, Voltage ................................................ 2V to -0.3V VOA+, VOA- ............................................... 3.6V to -0.3V Peak Gate Drive Current <1s (TG1, TG2, BG1, BG2).................................................5A Operating Ambient Temperature Range (Note 2) ................................................... -40C to 85C Junction Temperature (Note 3) ............................. 125C Storage Temperature Range SSOP ................................................. -65C to 150C QFN.................................................... -65C to 125C QFN Reflow Peak Body Temperature .................... 260C Lead Temperature (Soldering, 10 sec) .................. 300C
PIN CONFIGURATION
TOP VIEW DPRSLPVR VFB DPRSLPVR FREQSET PSIB VOA+ VOA- OAOUT STP_CPUB SGND SENSE1+ SENSE1- 1 2 3 4 5 6 7 8 9 10 11 36 MCH_PG 35 PGOOD 34 BOOST1 33 TG1 32 SW1 31 BOOST2 30 TG2 29 SW2 28 PVCC 27 BG1 26 PGND 25 BG2 24 VID5 23 VID4 22 VID3 21 VID2 20 VID1 19 VID0 FREQSET 1 PSIB 2 VOA+ 3 VOA- 4 OAOUT 5 STP_CPUB 6 SGND 7 SENSE1+ 8 SENSE1- 9 SENSE2+ 10 SENSE2- 11 RDPRSLP 12 13 14 15 16 17 18 19 RBOOT VID0 RDPSLP RUN/SS VID1 ITH NC 39 TOP VIEW MCH_PG BOOST1 PGOOD SW1 31 NC 30 BOOST2 29 TG2 28 SW2 27 PVCC 26 BG1 25 PGND 24 BG2 23 VID5 22 VID4 21 VID3 20 VID2 TG1 VFB
38 37 36 35 34 33 32
SENSE2+ 12 SENSE2- 13 RDPRSLP 14 RDPSLP 15 RUN/SS 16 ITH 17 RBOOT 18
G PACKAGE 36-LEAD PLASTIC SSOP
UHF PACKAGE 38-LEAD (7mm x 5mm) PLASTIC QFN
TJMAX = 125C, JA = 85C/W
TJMAX = 125C, JA = 34C/W EXPOSED PAD (PIN 39) IS SIGNAL GROUND, MUST BE CONNECTED TO PCB AND SGND
ORDER INFORMATION
LEAD FREE FINISH LTC3735EG#PBF LTC3735EUHF#PBF TAPE AND REEL LTC3735EG#TRPBF LTC3735EUHF#TRPBF PART MARKING* LTC3735 LTC3735 PACKAGE DESCRIPTION 36-Lead Plastic SSOP 38-Lead (7mm x 5mm) Plastic QFN TEMPERATURE RANGE -40C to 85C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LTC3735 ELECTRICAL CHARACTERISTICS
SYMBOL Reference VSENSEMAX VLOADREG VREFLNREG VPSIB IPSIB VOVL gm gmOL VACTIVE IQ UVR IRUN/SS VRUN/SS VRUN/SSLO ISCL ISDLHO ISENSE DFMAX TG1, 2 tr TG1, 2 tf BG1, 2 tr BG1, 2 tf TG/BG t1D BG/TG t2D tON(MIN) RATTEN ATTENERR VIDTHLOW VIDTHHIGH VIDLEAK PARAMETER Regulated Feedback Voltage Maximum Current Sense Threshold Output Voltage Load Regulation Main Control Loop ITH Voltage = 0.5V; Measured at VFB (Note 4) ITH Voltage = Max; VCM = 1.7V
l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VPVCC = 5V, VRUN/SS = 5V unless otherwise noted.
CONDITIONS MIN TYP 0.600 59 72 0.1 -0.1 0.02 0.57 VPSIB = 0V Measured with Respect to VFB = 0.6V ITH = 1.2V, Sink/Source 25A (Note 4) ITH = 1.2V, (gm * ZL; No Ext Load) (Note 4) VID = 010110, ITH = 0.5V (0C to 85C) VID = 010110, ITH = 0.5V (Note 2) (Note 5) VRUN/SS = 0V PVCC Lowered Until the RUN/SS Pin is Pulled Low VRUN/SS = 1.9V VRUN/SS Rising VRUN/SS Rising from 3V VRUN/SS, Ramping Negative Soft-Short Condition VFB = 0.375V, VRUN/SS = 4.5V VFB = 0.375V, VRUN/SS = 4.5V Each Channel: VSENSE1-, 2- = VSENSE1+, 2+ = 0V In Dropout, VSENSEMAX 45mV (Note 6) CLOAD = 3300pF CLOAD = 3300pF (Note 6) CLOAD = 3300pF CLOAD = 3300pF -85 95 -5 3.2 -2.3 1.0
l l
MAX
UNITS V
85 0.5 -0.5 0.1 0.63 -1 0.68 7.5 1.370 1.376 3 100 4.2 -0.8 1.9
mV % % %/V V A V mmho V/mV V V mA A V A V V V A
(Note 4) Measured in Servo Loop, ITH Voltage: 1.2V to 0.7V l l Measured in Servo Loop, ITH Voltage: 1.2V to 2V VPVCC = 4.5V to 7V
Reference Voltage Line Regulation Forced Continuous Threshold Forced Continuous Current Output Overvoltage Threshold Transconductance Amplifier gm Transconductance Amplifier Gain Output Voltage in Active Mode Input DC Supply Current Normal Mode Shutdown Undervoltage RUN/SS Reset Soft-Start Charge Current RUN/SS Pin ON Threshold RUN/SS Pin Latchoff Threshold RUN/SS Discharge Current Shutdown Latch Disable Current Total Sense Pins Source Current Maximum Duty Factor Top Gate Transition Time: Rise Time Fall Time Bottom Gate Transition Time: Rise Time Fall Time
0.6 -0.5 0.66 6 3 1.356 1.356 2 20 3.7 -1.5 1.5 3.9 3.2 -1.5 1.5 -60 98.5 30 40 60 50 50 60 100 5.33
0.64 4.5 1.342 1.336
VRUN/SSARM RUN/SS Pin Latchoff Arming
5
A A %
90 90 90 90
ns ns ns ns ns ns ns k
Top Gate Off to Bottom Gate On Delay CLOAD = 3300pF Each Driver (Note 6) Synchronous Switch-On Delay Time Bottom Gate Off to Top Gate On Delay CLOAD = 3300pF Each Driver (Note 6) Top Switch-On Delay Time Minimum On-Time VID Top Resistance Resistive Divider Error VID0 to VID5 Logic Threshold Low VID0 to VID5 Logic Threshold High VID0 to VID5 Leakage 0.7 (Note 8)
l
Tested with a Square Wave (Note 7)
VID Parameters -0.25 0.25 0.3 1 % V V A
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LTC3735 ELECTRICAL CHARACTERISTICS
SYMBOL Oscillator IFREQSET fNOM fLOW fHIGH VPGL IPGOOD VPG tMASK tDELAY IB VOS CM CMRR ICL AVOL GBP SR VO(MAX) FREQSET Input Current Nominal Frequency Lowest Frequency Highest Frequency PGOOD Voltage Low PGOOD Leakage Current PGOOD Trip Thresholds VFREQSET = 0V VFREQSET = 1.2V VFREQSET = 0V VFREQSET 2.4V IPGOOD = 2mA VPGOOD = 5V VFB with Respect to Set Output Voltage VFB Ramping Negative VFB Ramping Positive -7 7 100 -10 11 110 15 15 VOA+ = VOA- 1.2V, IOUT = 1mA 0 IOUT = 1mA IOUT = 1mA IOUT = 1mA RL = 2k IOUT = 1mA 46 10 70 35 30 2 5 PVCC - 1.2 PVCC - 0.9 0.8 200 5 PVCC - 1.4 320 190 490 -2 355 210 550 0.1 -1 390 240 610 0.3 1 -13 13 120 A kHz kHz kHz V A % % s cycles nA mV V dB mA V/mV MHz V/s V PARAMETER
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VPVCC = 5V, VRUN/SS = 5V unless otherwise noted.
CONDITIONS MIN TYP MAX UNITS
PGOOD Output
PGOOD Mask Timer MCH_PG Delay Time Input Bias Current Input Offset Voltage Magnitude Common Mode Input Voltage Range Common Mode Rejection Ratio Output Source Current Open-Loop DC Gain Gain-Bandwidth Product Slew Rate Maximum High Output Voltage
Operational Amplifier
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3735E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC3735EG: TJ = TA + (PD * 85C/W) LTC3735EUHF: TJ = TA + (PD * 34C/W)
Note 4: The LTC3735 is tested in a feedback loop that servos VITH to a specified voltage and measures the resultant VFB. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 6: Rise and fall times are measured using 10% and 90% levels. Delay times are measured using 50% levels. Note 7: The minimum on-time condition corresponds to the on inductor peak-to-peak ripple current 40% IMAX (see Minimum On-Time Considerations in the Applications Information section). Note 8: The ATTENERR specification is in addition to the output voltage accuracy specified at VID code = 010110.
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LTC3735 TYPICAL PERFORMANCE CHARACTERISTICS
Active Mode Efficiency (Figure 14)
100 VIN = 7.5V EFFICIENCY (%) VIN = 20V 80 EFFICIENCY (%) 80 VIN = 7.5V VID = 1.468V PSI = 0 90
Deeper Sleep Mode Efficiency (Figure 14)
PSI = 0 100
Efficiency vs Input Voltage (Figure 14)
IOUT = 20A VOUT = 1.6V
90 EFFICIENCY (%)
90
80 70
70
VIN = 20V
70
60 60 0.01
60
0
5
10 15 20 LOAD CURRENT (A)
25
30
3735 G01
0.1 1 LOAD CURRENT (A)
10
3735 G02
50
5
15 10 INPUT VOLTAGE (V)
20
3735 G03
Supply Current vs PVCC Voltage and Mode
2500 ON 50 75
Maximum Current Sense Threshold vs Duty Factor
80 70 60 VSENSE (mV) VSENSE (mV) 0 20 40 60 DUTY FACTOR (%) 80 100
3735 G05
Maximum Current Sense Threshold vs Percent of Nominal Output Voltage (Foldback)
2000 SUPPLY CURRENT (A)
1500
50 40 30 20
1000
25 500 SHUTDOWN 4 6 5 PVCC VOLTAGE (V) 7
3735 G04
10 0 0 50 100 0 25 75 PERCENT OF NOMINAL OUTPUT VOLTAGE (%)
3735 G06
0
Maximum Current Sense Threshold vs VRUN/SS (Soft-Start)
80 70 60 VSENSE (mV) VSENSE (mV) 50 40 30 20 10 0 0 1 2 3 VRUN/SS (V) 4 5
3735 G07
Maximum Current Sense Threshold vs Sense Common Mode Voltage
76 90 80 70 72 VSENSE (mV) 0 1 3 4 2 COMMON MODE VOLTAGE (V) 5
3735 G08
Current Sense Threshold vs ITH Voltage
VSENSE(CM) = 1.25V
60 50 40 30 20 10 0 -10 -20
68
64
60
-30
0
0.5
1
1.5 VITH (V)
2
2.5
3735 G09
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LTC3735 TYPICAL PERFORMANCE CHARACTERISTICS
Load Regulation (Without AVP)
0.0 VPSIB = 5V VIN = 15V FIGURE 1 100
SENSE Pins Total Source Current
NORMALIZED VOUT (%)
-0.1 ISENSE (A)
50
-0.2
0
-0.3
-50
-0.4
0
5
15 10 LOAD CURRENT (A)
20
25
3735 G10
-100
0
2
4
6
3735 G12
VSENSE COMMON MODE VOLTAGE (V)
Maximum Current Sense Threshold vs Temperature
78 1.8 1.6 76 RUN/SS CURRENT (A) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 68 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125
RUN/SS Current vs Temperature
-12 CURRENT SENSE INPUT CURRENT (A)
Current Sense Pin Input Current vs Temperature
VOUT = 1.6V
-11
VSENSE (mV)
74
-10
72
-9
70
-8
0 -50
-25
0 25 50 75 TEMPERATURE (C)
100
125
-7 -50 -25
50 0 75 25 TEMPERATURE (C)
100
125
3735 G13
3735 G14
3735 G18
Oscillator Frequency vs Temperature
700 600 FREQUENCY (kHz) 500 400 300 200 100 - 50 - 25 VFREQSET = 0V VOUT(AC) 20mV/DIV IL1 1A/DIV VFREQSET = 1.2V
Constant Frequency Low Current Mode (Figure 14)
VIN = 15V, VOUT = 1.6V, ILOAD = 400mA VRON 2V/DIV VCC - CORE 500mV/DIV PGOOD 2V/DIV VPSIB = 0V 2s/DIV
3735 G17
Start-Up Sequence (Figure 14)
C1 MAX 3.28V V BOOT = 1.37V C2 MAX 1.37V C3 MAX 3.24V C4 MAX 3.20V 500s/DIV
3735 G16
VFREQSET = 2.4V
VID = 1.228V
IL2 1A/DIV
MCH -PGOOD 2V/DIV
50 25 75 0 TEMPERATURE (C)
100
125
3735 G19
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LTC3735 TYPICAL PERFORMANCE CHARACTERISTICS
VRUN/SS Shutdown Latch Thresholds vs Temperature
4.5 SHUTDOWN LATCH THRESHOLDS (V) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -50 -25 0 25 50 75 TEMPERATURE (C) 100 125 20s/DIV
3735 G22
Load Step (Figure 14)
VIDs 32A IOUT 10A/DIV 7.2A 1.364V VOUT 100mV/DIV 1.230V 1 0 1.356V
VID Transition (Figure 14)
LATCH ARMING
LATCHOFF THRESHOLD
VOUT 200mV /DIV 0.844V PGOOD 2V/DIV
50s/DIV
3735 G23
3735 G21
PIN FUNCTIONS
(G/UHF)
VFB (Pin 1/Pin 37): Input to the error amplifier that compares the feedback voltage to the internal 0.6V reference voltage. DPRSLPVR (Pin 2/Pin 38): Deeper Sleep State Input. When the signal to this pin is high, the voltage regulator enters deeper sleep state and its output is determined by the parallel resistor value of RDPRSLP and RDPSLP . When the signal is low, the voltage regulator exits deeper sleep state. FREQSET (Pin 3/Pin 1): Frequency Set Pin. Apply a DC voltage between 0V and 5V to set the operating frequency of the internal oscillator. This frequency is the switching frequency of each phase. PSIB (Pin 4/Pin 2): Power Status Indicator Input. When the signal to this pin is high, both channels operate in fully synchronous switching mode for fastest transient and lowest ripple. When the signal is low, controller enters power saving mode, providing high efficiency at light load. VOA+, VOA- (Pins 5, 6/Pins 3, 4): Inputs to the Internal Operational Amplifier. OAOUT (Pin 7/Pin 5): Output of the Internal Operational Amplifier.
STP_CPUB (Pin 8/Pin 6): Deep Sleep State Input. When the signal to this pin is low, the voltage regulator enters deep sleep state and its output voltage is a certain percentage lower than the VID commands. This offset percentage is set by the resistor connected to the RDPSLP pin. When the signal to this pin is high, the voltage regulator exits deep sleep state. SGND (Pin 9/Pin 7): Signal Ground. This pin is common to both controllers. Route separately to the PGND pin. SENSE1+, SENSE2+ (Pins 10,12/Pins 8, 9): The (+) Input to Each Differential Current Comparator. The ITH pin voltage and built-in offsets between SENSE- and SENSE+ pins in conjunction with RSENSE set the current trip threshold. SENSE1-, SENSE2- (Pins 11,13/Pins 10, 11): The (-) Input to Each Differential Current Comparator. RDPRSLP (Pin 14/Pin 12): Deeper Sleep State Resistor Pin. Connect a resistor from this pin to VOA+. This resistor in conjunction with RDPSLP resistor sets the output voltage of the regulator in deeper sleep state. RDPSLP (Pin 15/Pin 14): Deep Sleep Resistor Pin. Connect a resistor from this pin to VOA+. This resistor sets the percentage offset of output voltage in deep sleep state.
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LTC3735 PIN FUNCTIONS
(G/UHF)
RUN/SS (Pin 16/Pin 15): Combination of Soft-Start, Run Control Input and Short-Circuit Detection Timer. A capacitor to ground at this pin sets the ramp time to full current output. Forcing this pin below 1V causes the IC to shut down all internal circuitry. All functions are disabled in shutdown. ITH (Pin 17/Pin 16): Error Amplifier Output and Switching Regulator Compensation Point. Both current comparator's thresholds increase with this control voltage. The normal voltage range of this pin is from 0V to 2.4V RBOOT (Pin 18/Pin 17): Boot-Up Resistor Pin. Connect a resistor from this pin to VOA+. This resistor sets the output voltage during the initial boot-up. VID0-VID5 (Pins 19, 20, 21, 22, 23, 24/Pins 18, 19, 20, 21, 22, 23): VID Control Logic Input Pins. BG2, BG1 (Pins 25, 27/Pins 24, 26): High Current Gate Drives for Bottom N-Channel MOSFETs. Voltage swing at these pins is from ground to PVCC. PGND (Pin 26/Pin 25): Driver Power Ground. Connect to sources of bottom N-channel MOSFETs and the (-) terminals of CIN. PVCC (Pin 28/Pin 27): Power Supply Pin. The internal control circuits and on-chip gate drivers are powered from this voltage source. Decouple to PGND with a minimum of 4.7F X5R/X7R ceramic capacitor placed directly adjacent to the IC.
SW2, SW1 (Pins 29, 32/Pins 28, 32): Switch Node Connections to Inductors. Voltage swing at these pins is from a Schottky diode (external) voltage drop below ground to VIN. TG2, TG1 (Pins 30, 33/Pins 29, 33): High Current Gate Drives for Top N-Channel MOSFETs. These are the outputs of floating drivers with a voltage swing equal to PVCC superimposed on the switch node voltage SW. BOOST2, BOOST1 (Pins 31, 34/Pins 30, 34): Bootstrapped Supplies to the Topside Floating Drivers. External capacitors are connected between the BOOST and SW pins, and Schottky diodes are connected between the BOOST and PVCC pins. PGOOD (Pin 35/Pin 35): Power Good Indicator Output. This pin is open drain when output is within 10% of its set point. When output is not within the 10% window, this pin is pulled to ground. An internal timer watches over VID, state transitions overvoltage or undervoltage conditions, then masks PGOOD from going low for 110s. MCH_PG (Pin 36/Pin 36): MCH Power Good Input. Output voltage remains VBOOT for 15 clock cycles after the assertion of MCH_PG. This delay is only sensitive to the rising edge of the MCH_PG logic signal. SGND (Exposed Pad Pin 39, UHF Only): Signal Ground. Connect to Pins 7 and 25. The Exposed Pad must be soldered to the PCB. NC (Pins 13, 31, UHF Only): No Connect.
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LTC3735 FUNCTIONAL DIAGRAM
R3 MCH_PG R4 RDPSLP STP_CPUB R6 RDPRSLP DPRSLPVR R5 RBOOT
DELAY FREQSET OSCILLATOR CLK1 CLK2 TO SECOND CHANNEL DUPLICATE FOR SECOND CONTROLLER CHANNEL DROP OUT DET - + - VOA+ + R2 VOA- - I1 R1 OAOUT 5.33(VFB) SLOPE COMP + - 0.60V PSI VREF OV 5V PVCC 1.5A 6V DPRSLPVR MD VID CHANGE RATTEN 5.33k SHDN RST 5.33(VFB) RUN SOFTSTART - EA + + - 0.66V VFB 0.60V - + - + I2 PVCC A1 + VFB 0.54V DPRSLPVR SHDN 0.66V S R Q Q COMPOSITE PG
MD PVCC BOOST DB VIN
DPRSLPVR STP_CPUB VID CHANGE PGOOD 110s BLANKING RUN
TOP BOT PSI TOP ON SWITCH LOGIC BOT PVCC
TG
CB D1
+
CIN
SW
BG PGND
COUT
+
RSENSE
VOUT
L
-
++
-
+ 36k SENSE - 36k SENSE
3V 0.5A PSIB
54k
54k 2.4V VFB
+
ITH
CC
SGND
CC2 RUN/SS + - 1.5V
RC
CSS
RUN 6-BIT VID DECODER
RVID VID0 VID1 VID2 VID3 VID4 VID5
3735 FD
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LTC3735 OPERATION
Main Control Loop The LTC3735 uses a constant frequency, current mode stepdown architecture with the two output stages operating 180 degrees out of phase. During normal operation, each top MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the main current comparator, I1, resets the RS latch. The peak inductor current at which I1 resets the RS latch is controlled by the voltage on the ITH pin, which is the output of error amplifier EA. The VOA+ pin receives the voltage feedback signal, which is compared to the internal reference voltage by the EA. When the load current increases, it causes a slight decrease in EA inverting input node relative to the 0.6V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by current comparator I2, or the beginning of the next cycle. The top MOSFET drivers are biased from floating bootstrap capacitor CB, which normally is recharged during each off cycle through an external diode when the top MOSFET turns off. As VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector detects this and forces the top MOSFET off for about 500ns every sixth cycle to allow CB to recharge. The main control loop is shut down by pulling the RUN/ SS pin low. Releasing RUN/SS allows an internal 1.5A current source to charge soft-start capacitor CSS. When CSS reaches 1.5V, the main control loop is enabled with the internal ITH voltage clamped at approximately 30% of its maximum value. As CSS continues to charge, the internal ITH voltage is gradually released allowing normal, full-current operation. Frequency Programming and Antiphase Operation The switching frequency of the LTC3735 is determined by the DC voltage at the FREQSET pin. A DC voltage ranging from 0V to 2.4V moves the internal oscillator frequency from 210kHz to 550kHz.
(Refer to Functional Diagram)
This frequency is the actual switching frequency of either channel. Because the two channels operate 180C out of phase, the apparent frequency at both VIN and VOUT is twice the actual switching frequency, minimizing ripple voltages and speeding up transient responses. Low Current Operation (PSIB) The PSIB pin selects between two modes of operation. When PSIB is above 0.6V, both channels operate in full synchronous switching mode. Both bottom drivers (BG1, BG2) are kept on once they are turned on until their respective oscillator sets the RS latch. The inductor current can therefore go from output back to input power supply and could potentially boost the input supply to dangerous voltage levels--BEWARE! This mode of operation is also of lower efficiency, given both channels are fully enabled and much current can circulate between input and output. However, this mode provides faster transient response, lower input noise and minimum output ripple. When PSIB is below 0.6V, the bottom drivers (BG1, BG2) are turned off if the inductor current starts to reverse. This mode of operation prevents current going from output back to input and eliminates the conduction power loss related to circulating current. If the DPRSLPVR signal goes high in this mode, Channel 2 will be shut off and only Channel 1 will be active in supplying load current. This further eliminates power MOSFET gate driving and transition losses of Channel 2. Since DPRSLPVR indicates the entry to deeper sleep state, this "channel shedding" technique optimizes the voltage regulator efficiency at light loads. Table 1 summarizes the operation modes for different pin configurations.
Table 1. Low Current Operation Modes
PSIB High DPRSLPVR OPERATION MODE High or Low Both Channels ON, Fully Synchronous Switching, Inductor Current is Allowed to Reverse Low High Both Channels ON; Reverse Current is Prevented Channel 2 is Shut Off, Reverse Current is Prevented
Low Low
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LTC3735 OPERATION
(Refer to Functional Diagram)
Output Voltage at Start-Up and at Deeper Sleep State Under normal conditions, the output voltage of the regulator is commanded by six VID bits, except at start-up and at deeper sleep state. At start-up, the RUN/SS capacitor starts to charge up and its voltage limits the inrush current from the input power source. This linearly rising current limit provides a controlled output voltage rise. During start-up, the VID command is ignored and the output set point is determined by the value of the resistor connected to the RBOOT pin. The VID bits continue to be ignored for 15 switching cycles after the completion of the following two conditions: 1) output voltage has risen up and has regulated 2) MCH_PG signal has asserted. After 15 switching cycles, output voltage is fully commanded by VID bits. In deeper sleep state, the VID command and STP_CPUB signal are ignored and the output set point is determined by the parallel value of the resistors at the RDPRSLP pin and RDPSLP pin. Operational Amplifier and Deep Sleep Offset The internal operational amplifier provides a programmable output offset at deep sleep state (when the STP_CPUB signal is low). The offset percentage is programmed by the resistor from RDPSLP to VOA+ and the resistor from output to VOA+. The amplifier has an output slew rate of 5V/s and is capable of driving capacitive loads with an output RMS current typically up to 40mA. The open-loop gain of the amplifier is >120dB and the unity-gain bandwidth is 2MHz. Output Overvoltage Protection An overvoltage comparator, OV, guards against transient overshoots (>10%) as well as other more serious conditions that may overvoltage the output. In this case, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared.
Power Good The PGOOD pin is connected to the drain of an internal N-channel MOSFET. The MOSFET turns on when the output voltage is not within 10% of its nominal set point. When the output voltage is within 10% of its nominal set point, the MOSFET turns off and PGOOD is high impedance. PGOOD monitors the VBOOT voltage when MCH_PG is not asserted. During VID, deep sleep or deeper sleep transitions, PGOOD is masked from going low for 110s, preventing the system from resetting during CPU mode changes. When VID bits, STP_CPUB or DPRSLPVR signals change again after a previous transition, but before the timer expires, the internal timer resets. Short-Circuit Detection The RUN/SS capacitor is used initially to limit the inrush current from the input power source. Once the controllers have been given time, as determined by the capacitor on the RUN/SS pin, to charge up the output capacitors and provide full-load current, the RUN/SS capacitor is then used as a short-circuit timeout circuit. If the output voltage falls to less than 70% of its nominal output voltage the RUN/SS capacitor begins discharging assuming that the output is in a severe overcurrent and/or short-circuit condition. If the condition lasts for a long enough period as determined by the size of the RUN/SS capacitor, the controller will be shut down until the RUN/SS pin voltage is recycled. This built-in latchoff can be overidden by providing a current >5A to the RUN/SS pin. This current shortens the soft-start period but also prevents net discharge of the RUN/SS capacitor during a severe overcurrent and/or short-circuit condition. Foldback current limiting is activated when the output voltage falls below 70% of its nominal level whether or not the shortcircuit latchoff circuit is enabled.
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LTC3735 APPLICATIONS INFORMATION
The basic LTC3735 application circuit is shown in Figure1 on the first page of this data sheet. External component selection begins with the selection of the inductors based on ripple current requirements and continues with the current sensing resistors using the calculated peak inductor current and/or maximum current limit. Next, the power MOSFETs, D1 and D2 are selected. The operating frequency and the inductor are chosen based mainly on the amount of ripple current. Finally, CIN is selected for its ability to handle the input ripple current (that PolyPhase(R) operation minimizes) and COUT is chosen with low enough ESR to meet the output ripple voltage and load step specifications (also minimized with PolyPhase). Current mode architecture provides inherent current sharing between output stages. The circuit shown in Figure1 can be configured for operation up to an input voltage of 28V (limited by the external MOSFETs). Current mode control allows the ability to connect the two output stages to two different input power supply rails. A heavy output load can take some power from each input supply according to the selection of the RSENSE resistors. RSENSE Selection For Output Current RSENSE1,2 are chosen based on the required peak output current. The LTC3735 current comparator has a maximum threshold of 72mV/RSENSE and an input common mode range of SGND to PVCC. The current comparator threshold sets the peak inductor current, yielding a maximum average output current IMAX equal to the peak value less half the peak-to-peak ripple current, IL. Assuming a common input power source for each output stage and allowing a margin for variations in the LTC3735 and external component values yields: RSENSE = 2(40mV/IMAX) Operating Frequency The LTC3735 uses a constant frequency architecture with the frequency determined by an internal capacitor. This capacitor is charged by a fixed current plus an additional current which is proportional to the DC voltage applied to the FREQSET pin. The FREQSET voltage is internally set to 1.2V. It is recommended that this pin is actively biased with a resistor divider to prevent noise getting into the system. A graph for the voltage applied to the FREQSET pin vs frequency is given in Figure2. As the operating frequency is increased the gate drive and switching losses will be higher, reducing efficiency (see Efficiency Considerations). The maximum switching frequency is approximately 550kHz.
600 550 OPERATING FREQUENCY (kHz) 500 450 400 350 300 250 200 150 100 0 0.5 1.0 1.5 2.0 2.5 FREQSET PIN VOLTAGE (V) 3.0
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Figure 2. Operating Frequency vs VFREQSET
Inductor Value Calculation and Output Ripple Current The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values. So why would anyone ever choose to operate at lower frequencies with larger components? The answer is efficiency. A higher frequency generally results in lower efficiency because MOSFET gate charge and transition losses increase directly with frequency. In addition to this basic tradeoff, the effect of inductor value on ripple current and low current operation must also be considered. The PolyPhase approach reduces both input and output ripple currents while optimizing individual output stages to run at a lower fundamental frequency, enhancing efficiency. The inductor value has a direct effect on ripple current. The inductor ripple current IL, decreases with higher inductance or frequency and increases with higher VIN: IL = VOUT VOUT 1- fL VIN
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where f is the individual output stage operating frequency.
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LTC3735 APPLICATIONS INFORMATION
In a 2-phase converter, the net ripple current seen by the output capacitor is much smaller than the individual inductor ripple currents due to ripple cancellation. The details on how to calculate the net output ripple current can be found in Linear Technology Application Note 77. Figure 3 shows the net ripple current seen by the output capacitors for 1- and 2-phase configurations. The output ripple current is plotted for a fixed output voltage as the duty factor is varied between 10% and 90% on the x-axis. The graph can be used in place of tedious calculations, simplifying the design process. Accepting larger values of IL allows the use of low inductances, but can result in higher output voltage ripple. A reasonable starting point for setting ripple current is IL = 0.4(IOUT)/2, where IOUT is the total load current. Remember, the maximum IL occurs at the maximum input voltage. The individual inductor ripple currents are determined by the frequency, inductance, input and output voltages.
1.0 0.9 0.8 0.7 IO(P-P) 0.6 VO/fL 0.5 0.4 0.3 0.2 0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 DUTY FACTOR (VOUT/VIN) 0.8 0.9 1-PHASE 2-PHASE
is very dependent on inductor type selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! A variety of inductors designed for high current, low voltage applications are available from manufacturers such as Sumida, Coilcraft, Coiltronics, Toko and Panasonic. Power MOSFET, D1 and D2 Selection Two external power MOSFETs must be selected for each output stage with the LTC3735: one N-channel MOSFET for the top (main) switch, and one N-channel MOSFET for the bottom (synchronous) switch. The peak-to-peak drive levels are set by the PVCC voltage. This voltage typically ranges from 4.5V to 7V. Consequently, logic-level threshold MOSFETs must be used in most applications. Pay close attention to the BVDSS specification for the MOSFETs as well; most of the logiclevel MOSFETs are limited to 30V or less. Selection criteria for the power MOSFETs include the "ON" resistance RDS(ON), gate charge QG, reverse transfer capacitance CRSS, breakdown voltage BVDSS and maximum continuous drain current ID(MAX). When the LTC3735 is operating at continuous mode in a step-down configuration, the duty cycles for the top and bottom MOSFETs of each power stage are approximately: Top MOSFET Duty Cycle = VOUT VIN VIN - VOUT VIN (1) (2)
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Figure 3. Normalized Output Ripple Current vs Duty Factor [IRMS 0.3 (IO(P-P)]
Inductor Core Selection Once the values for L1 and L2 are known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy, or Kool M cores. Actual core loss is independent of core size for a fixed inductor value, but it
Bottom MOSFET Duty Cycle =
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The conduction losses of the top and bottom MOSFETs are therefore: PCONTOP = VOUT VIN I * OUT * (1+ * T ) *RDS(ON)(3) 2 I * OUT * (1+ * T ) 2
2 2
V -V PCONBOT = IN OUT VIN * RDS(ON)
(4)
where IOUT is the total output current at full load, T is the difference between MOSFET operating temperature and room temperature, and is the temperature dependency of RDS(ON). is roughly 0.004/C ~ 0.006/C for low voltage MOSFETs. The power losses of driving the top and bottom MOSFETs are simply: PDRTOP = QG * PVCC * f PDRBOT = QG * PVCC * f (5) (6)
where RDR is the effective driver resistance (of approximately 2), VDR is the driving voltage (= PVCC) and VTH(MIN) is the minimum gate threshold voltage of the MOSFET. Please notice that the switching loss of the bottom MOSFET is effectively negligible because the current conduction of the antiparalleling diode. This effect is often referred as zero-voltage-transition (ZVT). Similarly when the LTC3735 converter works under fully synchronous mode at light load, the reverse inductor current can also go through the body diode of the top MOSFET and make the turn-on loss to be negligible. However, equations 7 and 8 have to be used in calculating the worst-case power loss, which happens at highest load level. The selection criteria of power MOSFETs start with the stress check: VIN < BVDSS IMAX < ID(MAX) and PCONTOP + PSWTOP < top MOSFET maximum power dissipation specification PCONBOT + PSWBOT < bottom MOSFET maximum power dissipation specification The maximum power dissipation allowed for each MOSFET depends heavily on MOSFET manufacturing and packaging, PCB layout and power supply cooling method. Maximum power dissipation data are usually specified in MOSFET data sheets under different PCB mounting conditions. The next step of selecting power MOSFETs is to minimize the overall power loss: POVL = PTOP + PBOT = (PCONTOP + PDRTOP + PSWTOP) + (PCONBOT + PDRBOT + PSWBOT) For typical mobile CPU applications where the ratio between input and output voltages is higher than 2:1, the bottom MOSFET conducts load current most of the time while the main losses of the top MOSFET are for switching and driving. Therefore a low RDS(ON) part (or multiple parts in parallel) would minimize the conduction loss of the bottom
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Use QG data at VGS = PVCC in MOSFET data sheets. f is the switching frequency as described previously. Please notice that the above gate driving losses are usually not dissipated by the MOSFETs. Instead they are mainly dissipated on the internal drivers of the LTC3735, if there are no resistors connected between the drive pins (TG, BG) and the gates of the MOSFETs. The calculation of MOSFET switching loss is complicated by several factors including the wide distribution of power MOSFET threshold voltage, the nonlinearity of current rising/falling characteristic and the Miller Effect. Given the data in a typical power MOSFET data sheet, the switching losses of the top and bottom MOSFETs can only be estimated as follows: V 2 *I PSWTOP = IN OUT * f * CRSS *RDR * 4 per Phase 1 1 + VDR - VTH(MIN) VTH(MIN) PSWBOT 0 (7)
(8)
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LTC3735 APPLICATIONS INFORMATION
MOSFET while a higher RDS(ON) but lower QG and CRSS part would be desirable for the top MOSFET. The Schottky diodes, D1 and D2 in Figure 1 conduct during the dead-time between the conduction of the top and bottom MOSFETs. This helps reduce the current flowing through the body diode of the bottom MOSFET. A body diode usually has a forward conduction voltage higher than that of a Schottky and is thus detrimental to efficiency. The charge storage and reverse recovery of a body diode also cause high frequency rings at the switching nodes (the conjunction nodes between the top and bottom MOSFETs), which are again not desired for efficiency or EMI. Some power MOSFET manufacturers integrate a Schottky diode with a power MOSFET, eliminating the need to parallel an external Schottky. These integrated Schottky-MOSFETs, however, have smaller MOSFET die sizes than conventional parts and are thus not suitable for high current applications. CIN and COUT Selection In continuous mode, the source current of each top N-channel MOSFET is a square wave of duty cycle VOUT/ VIN. A low ESR input capacitor sized for the maximum RMS current must be used. The details of a closed form equation can be found in Linear Technology Application Note 77. Figure 4 shows the input capacitor ripple current for a 2-phase configuration with the output voltage fixed and input voltage varied. The input ripple current is normalized against the DC output current. The graph can be used in place of tedious calculations. The minimum input ripple current can be achieved when the input voltage is twice the output voltage. In the graph of Figure 4, the 2-phase local maximum input RMS capacitor currents are reached when: VOUT 2k - 1 = VIN 4 where k = 1, 2 These worst-case conditions are commonly used for design, considering input/output variations and long term reliability. Note that capacitor manufacturer's ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. Always consult the capacitor manufacturer if there is any question.
0.6 0.5 DC LOAD CURRENT 0.4 0.3 0.2 0.1 0 1-PHASE 2-PHASE
RMS INPUT RIPPLE CURRNET
0.1
0.2
0.3 0.4 0.5 0.6 0.7 DUTY FACTOR (VOUT/VIN)
0.8
0.9
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Figure 4. Normalized RMS Input Ripple Current vs Duty Factor for 1 and 2 Output Stages
It is important to note that the efficiency loss is proportional to the input RMS current squared and therefore a 2-phase implementation results in 75% less power loss when compared to a single phase design. Battery/input protection fuse resistance (if used), PC board trace and connector resistance losses are also reduced by the reduction of the input ripple current in a 2-phase system. The required amount of input capacitance is further reduced by the factor, 2, due to the reduction in input RMS current. The selection of COUT is driven by the required effective series resistance (ESR). Typically once the ESR requirement has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirements. The steady state output ripple (VOUT) is determined by: 1 VOUT IRIPPLE ESR + 16 * f * COUT where f = operating frequency of each stage, COUT = output capacitance and IRIPPLE = interleaved inductor ripple currents. IRIPPLE can be calculated from the duty factor and the IL of each stage. A closed form equation can be found in
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Linear Technology Application Note 77. Assuming inductors are selected to have same ripple percentage for both 1-phase and 2-phase configurations, Figure 5 shows the reduction of output ripple current by 2-phase operation. Not only the ripple amplitude is more than halved, but the ripple frequency is also doubled. Compared with the output voltage ripple for 1-phase: 1 VOUT IRIPPLE ESR + 8 * f * COUT VOUT of 2-phase is less than 50% of that of 1-phase, given the same output capacitor ESRs. Or, to have same VOUT 2-phase only need half the number of output capacitors that are needed in 1-phase. The output ripple varies with input voltage since IL is a function of input voltage. The output ripple will be less than 25mV at max VIN with IL = 0.4IOUT(MAX)/2 assuming: COUT required ESR < 4(RSENSE) and COUT > 1/(16f)(RSENSE) The LTC3735 employs OPTI-LOOP technique to compensate the switching regulator loop with external components (through ITH pin). OPTI-LOOP compensation speeds up regulator's transient response, minimizes output capacitance and effectively removes constraints on output capacitor ESR. It opens a much wider selection of output capacitor types and a variety of capacitor manufactures are available for high current, low voltage switching regulators.
50
Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance through-hole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest (ESR)(size) product of any aluminum electrolytic at a somewhat higher price. An additional ceramic capacitor in parallel with OS-CON type capacitors is recommended to reduce the inductance effects. In surface mount applications, multiple capacitors may have to be paralleled to meet the ESR or RMS current handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. New special polymer (SP) surface mount capacitors from Panasonic offer very low ESR also but have much lower capacitive density per unit volume. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. Several excellent choices are the AVX TPS, AVX TPSV or the KEMET T510 series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Other capacitor types include Sanyo OS-CON, POSCAPs, Kemet AO-CAPs, Nichicon PL series and Sprague 595D series. Consult the manufacturer for other specific recommendations. A combination of capacitors will often result in maximizing performance and minimizing overall cost and size. PVCC Decoupling The PVCC pin supplies power to the top and bottom gate drivers and therefore must be bypassed to power ground with a minimum of 4.7F ceramic or tantalum capacitor. Since the gate driving currents are of high amplitude and high slew rate, this bypassing capacitor should be placed very close to the PVCC and PGND pins to minimize the parasitic inductance. Do NOT apply greater than 7V to the PVCC pin. The PVCC pin also supplies current to the internal control circuitry of the LTC3735. This supply current is much lower than that of the current for the external MOSFET gate drive. Ceramic capacitors are very good for high frequency filtering and a 0.1F ~ 1F ceramic capacitor should be placed adjacent to the PVCC and SGND pins.
IRIPPLE OF 2-PHASE * 100% IRIPPLE OF 1-PHASE
40
30
20
10
0 0.1
0.2
0.3 0.4 0.5 0.6 0.7 DUTY FACTOR (VOUT/VIN)
0.8 0.9
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Figure 5. Output Ripple Current Reduction of 2-Phase Over Single Phase
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LTC3735 APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (CB,DB) (Refer to Functional Diagram) External bootstrap capacitors CB1 and CB2 connected to the BOOST1 and BOOST2 pins supply the gate drive voltages for the topside MOSFETs. Capacitor CB in the Functional Diagram is charged though diode DB from PVCC when the SW pin is low. When the topside MOSFET turns on, the driver places the CB voltage across the gate-source of the desired MOSFET. This enhances the MOSFET and turns on the topside switch. The switch node voltage, SW, rises to VIN and the BOOST pin rises to VIN + PVCC. The value of the boost capacitor CB needs to be 30 to 100 times that of the total input capacitance of the topside MOSFET(s). The reverse breakdown of DB must be greater than PVCC(MAX). VID Output Voltage Programming After 27s ~ 71s tBOOT delay, the output voltage of the regulator is digitally programmed as defined in Table 2 using the VID0 to VID5 logic input pins. The VID logic inputs program a precision, 0.25% internal feedback resistive divider. The LTC3735 has an output voltage range of 0.700V to 1.708V in 16mV steps. Refering to the Functional Diagram, there is a resistor, RVID, from VFB to ground. The value of RVID is controlled by the six VID input pins. Another internal resistor, 5.33k (RATTEN), completes the resistive divider. The output voltage is thus set by the ratio of (RVID + 5.33k) to RVID. Each VID digital pin is a high impedance input. Therefore they must be actively pulled high or pulled low. The logic low threshold of the VID pins is 0.3V; the logic high threshold is 0.7V. Soft-Start/Run Function The RUN/SS pin provides three functions: 1) run/shutdown, 2) soft-start and 3) an optional short-circuit latchoff timer. Soft-start reduces the input power sources' surge currents by gradually increasing the controller's current limit. The latchoff timer prevents very short, extreme load transients from tripping the overcurrent latch. A small pull-up current (>5A) supplied to the RUN/SS pin will prevent the overcurrent latch from operating. The following paragraph describes how the functions operate. An internal 1.5A current source charges up the soft-start capacitor, CSS. When the voltage on RUN/SS reaches 1.5V, the controller is permitted to start operating. As the voltage on RUN/SS increases from 1.5V to 3.0V, the internal current limit is increased from 25mV/RSENSE to 72mV/ RSENSE. The output current thus ramps up slowly, eliminating the starting surge current required from the input power supply. If RUN/SS has been pulled all the way to ground there is a delay before starting of approximately: 1.5V tDELAY = C = (1s/F ) CSS 1.5A SS The time for the output current to ramp up is then: 3V - 1.5V tIRAMP = C = (1s/F ) CSS 1.5A SS By pulling the RUN/SS pin below 1V the LTC3735 is put into low current shutdown (IQ < 100A). The RUN/SS pin can be driven directly from logic as shown in Figure 6. Diode D1 in Figure 6 reduces the start delay but allows CSS to ramp up slowly providing the soft-start function. The RUN/SS pin has an internal 6V zener clamp (see Functional Diagram).
PVCC 3.3V OR 5V D1 RUN/SS RSS* CSS *OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF RUN/SS
CSS
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Figure 6. RUN/SS Pin Interfacing
Start-Up Sequence (Refer to the Functional Diagram) After soft-start, the output voltage of the regulator settles at a voltage level equal to VBOOT. VBOOT = 0.6V * R2 * (R3 +R5) R5 * (R1+R2)
By using different R5 resistors, VBOOT can be programmed.
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Table 2. VID Output Voltage Programming VID5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 VID4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 VID3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 VID2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 VID1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 VID0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 LTC3735 1.708V 1.692V 1.676V 1.660V 1.644V 1.628V 1.612V 1.596V 1.580V 1.564V 1.548V 1.532V 1.516V 1.500V 1.484V 1.468V 1.452V 1.436V 1.420V 1.404V 1.388V 1.372V 1.356V 1.340V 1.324V 1.308V 1.292V 1.276V 1.260V 1.244V 1.228V 1.212V VID5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 VID4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 VID3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 VID2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 VID1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 VID0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 LTC3735 1.196V 1.180V 1.164V 1.148V 1.132V 1.116V 1.100V 1.084V 1.068V 1.052V 1.036V 1.020V 1.004V 0.988V 0.972V 0.956V 0.940V 0.924V 0.908V 0.892V 0.876V 0.860V 0.844V 0.828V 0.812V 0.796V 0.780V 0.764V 0.748V 0.732V 0.716V 0.700V
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LTC3735 APPLICATIONS INFORMATION
After the output voltage enters the 10% regulation window centered at VBOOT, the internal power good comparator issues a logic high signal. Refer to the timing diagram in Figure 7. This signal then enters a logic AND gate, with MCH_PG being the other input, and the output of the gate is PG shown in Figure 7. This composite PG signal is then delayed by tBOOT amount of time and then becomes MD. As soon as MD is asserted, the output voltage changes from VBOOT to VVID, a voltage level totally controlled by the six VID bits. In the LTC3735, the time tBOOT is set to be 15 switching cycles: 1 tBOOT = 15 fS If fS is set at 210kHz, tBOOT = 71s If fS is set at 550kHz, tBOOT = 27s Output Voltage Set in Deep Sleep and Deeper Sleep States (Refer to the Functional Diagram) The output voltage can be offset by the STP_CPUB signal. When STP_CPUB becomes low, the output voltage will be a certain percentage lower than that set by the VID bits in Table 2. This state is defined to be the deep sleep state. Referring to the Functional Diagram, we can caluculate the STP_CPUB offset to be: STP% = - R3 * 100% R3 +R4
By using different R4 resistors, STP_CPUB offset can be programmed. The output voltage could also be set by external resistors R6 and R4 when DPRSLPVR input is high. This state is defined to be the deeper sleep state. The output voltage is set to VDPRSLPVR, regardless of the VID setting: VDPRSLPVR = 0.6V * R2 * (R3 +R6||R4) (R6||R4) * (R1+R2)
RUN/SS
1.5V VVID
VBOOT VOUT 90% VBOOT INTERNAL PG (OUTPUT OF INTERNAL POWER GOOD COMPARATOR)
By using different value R6 resistors, VDPRSLPVR can be programmed. (The digital input threshold voltage is set to 1.8V for STP_CPUB, DPRSLPVR and MCH_PG inputs.) Power Good Masking The PGOOD output monitors VOUT. When VOUT is not within 10% of the set point, PGOOD is pulled low with an internal MOSFET. When VOUT is within the regulation window, PGOOD is high impedance. PGOOD should be pulled up by an external resistor. During VID changes, deep sleep and deeper sleep transitions, the output voltage can initially be out of the 10% window of the newly set regulation point. To avoid nuisance indications from PGOOD, a timer masks PGOOD for 110s. If VOUT is still out of regulation after this blanking time, PGOOD goes low. Any overvoltage or undervoltage condition is also masked for 110s before it is reported by PGOOD.
MCH_PG
COMPOSITE PG (=(INTERNAL PG) AND (MCH_PG))
MD
tBOOT
VID BITS
INVALID
VALID
TIME
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Figure 7. Start-Up Timing Diagram
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LTC3735 APPLICATIONS INFORMATION
The masking circuitry also adaptively tracks VID and state changes. If a new change in VID or state happens before the 110s masking timer expires, the timer resets and starts a fresh count of 110s. This prevents the system from rebooting under frequent output voltage transitions. Refer to Figure 8 for the PGOOD timing diagram. During start-up, PGOOD is actively pulled low until the RUN/ SS pin voltage reaches its arming voltage, which is 4.2V typically, only then is the PGOOD pull-low signal released. When RUN/SS goes low, PGOOD goes low simultaneously.
VID BITS
begins discharging on the assumption that the output is in an overcurrent condition. If the condition lasts for a long enough period as determined by the size of the CSS, the controller will be shut down until the RUN/SS pin voltage is recycled. If the overload occurs during start-up, the time can be approximated by: tLO1 (CSS * 0.7V)/(1.5A) = 4.6 * 105 (CSS) If the overload occurs after start-up, the voltage on CSS will continue charging and will provide additional time before latching off: tLO2 (CSS * 2V)/(1.5A) = 1.3 * 106 (CSS) This built-in overcurrent latchoff can be overridden by providing a pull-up resistor, RSS, to the RUN/SS pin as shown in Figure 6. This resistance shortens the softstart period and prevents the discharge of the RUN/SS capacitor during a severe overcurrent and/or short-circuit condition. When deriving the 5A current from PVCC as in the figure, current latchoff is always defeated. Why should you defeat current latchoff? During the prototyping stage of a design, there may be a problem with noise pickup or poor layout causing the protection circuit to latch off the controller. Defeating this feature allows troubleshooting of the circuit and PC layout. The internal short-circuit and foldback current limiting still remains active, thereby protecting the power supply system from failure. A decision can be made after the design is complete whether to rely solely on foldback current limiting or to enable the latchoff feature by removing the pull-up resistor. The value of the soft-start capacitor CSS may need to be scaled with output voltage, output capacitance and load current characteristics. The minimum soft-start capacitance is given by: CSS > (COUT )(VOUT)(10-4)(RSENSE) A recommended soft-start capacitor of CSS = 0.1F will be sufficient for most applications.
VOUT
INTERNAL PG (OUTPUT OF INTERNAL POWER GOOD COMPARATOR)
PGOOD MASKING 110s PGOOD TIME
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110s
Figure 8. PGOOD Timing Diagram
Fault Conditions: Overcurrent Latchoff The RUN/SS pin also provides the ability to latch off the controller when an overcurrent condition is detected. The RUN/SS capacitor, CSS, is used initially to limit the inrush current. After the controller has been started and been given adequate time to charge up the output capacitors and provide full load current, the RUN/SS capacitor is used for a short-circuit timer. If the output voltage falls to less than 70% of its nominal value after CSS reaches 4.2V, CSS
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LTC3735 APPLICATIONS INFORMATION
Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3735 is capable of turning on the top MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that: tON (MIN ) < VOUT VIN ( f ) AVP -35.5 * RSENSE R3 * , m R AVP (9)
V if gm *R3 > 10 * OUT 0.6V
If the duty cycle falls below what can be accommodated by the minimum on-time, the LTC3735 will begin to skip cycles resulting in variable frequency operation. The output voltage will continue to be regulated, but the ripple current and ripple voltage will increase. The minimum on-time for the LTC3735 is generally less than 150ns. However, as the peak sense voltage decreases, the minimum on-time gradually increases. This is of particular concern in forced continuous applications with low ripple current at light loads. If the duty cycle drops below the minimum on-time limit in this situation, a significant amount of cycle skipping can occur with correspondingly larger ripple current and ripple voltage. If an application can operate close to the minimum on-time limit, an inductor must be chosen that has a low enough inductance to provide sufficient ripple amplitude to meet the minimum on-time requirement. As a general rule, keep the inductor ripple current of each phase equal to or greater than 15% of IOUT(MAX) at VIN(MAX). Active Voltage Positioning Active voltage positioning can be used to minimize peak-topeak output voltage excursion under worst-case transient loading conditions. The open-loop DC gain of the control loop is reduced depending upon the maximum load step specifications. Active voltage positioning can easily be added to the LTC3735. Figure 9 shows the equivalent circuit for implementing AVP The load line slope is estimated to be: .
where RSENSE is the current sense resistor, m is the number of phases, (m = 2 for LTC3735) R3 and RAVP are defined in Figure 9. gm is the transconductance gain for the error amplifier, it is about 4.5mmho for LTC3735. Rewriting Equation 9 we can estimate the AVP resistor to be: R AVP 35.5 *R3 *RSENSE m*| AVP|
VOUT+ R3 RAVP
(10)
VOA+ R2 R1 VOA-
+ -
OAOUT
FB VID 0.6V
- +
ITH
3735 F09
Figure 9. Simplified Schematic Diagram for AVP Design in LTC3735
We also adopt the current sense resistors as part of voltage positioning slopes. So the total load line slope is estimated to be: AVP -35.5 * RSENSE R3 RSENSE * - , m R AVP m
V if gm *R3 >> OUT 0.6V
(11)
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LTC3735 APPLICATIONS INFORMATION
Rewriting this equation, we can estimate the RAVP value to be: R AVP 35.5 *R3 m *| AVP| -1 RSENSE (12) and higher currents required by high performance digital systems is not doubling but quadrupling the importance of loss terms in the switching regulator system! 2) Transition losses apply only to the topside MOSFET(s), and are significant only when operating at high input voltages (typically 12V or greater). Transition losses can be estimated from: Transition Loss = per Phase
2 VIN *IOUT
Typically the calculation results based on these equations have 10% tolerance. So the resistor values need to be fine tuned. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: %Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC3735 circuits: 1) I2R losses, 2) Topside MOSFET transition losses, 3) PVCC supply current and 4) CIN loss. 1) I2R losses are predicted from the DC resistances of the fuse (if used), MOSFET, inductor, and current sense resistor. In continuous mode the average output current flows through L and RSENSE, but is "chopped" between the topside MOSFET and the synchronous MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L, RSENSE and ESR to obtain I2R losses. For example, if each RDS(ON) = 10m, RL = 10m, and RSENSE = 5m, then the total resistance is 25m. This results in losses ranging from 2% to 8% as the output current increases from 3A to 15A per output stage for a 5V output, or a 3% to 12% loss per output stage for a 3.3V output. Efficiency varies as the inverse square of VOUT for the same external components and output power level. The combined effects of increasingly lower output voltages
4
* f * CRSS * R DR *
1 1 + VDR - VTH(MIN) VTH(MIN)
3) PVCC drives both top and bottom MOSFETs. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from PVCC to ground. The resulting dQ/dt is a current out of PVCC that is typically much larger than the control circuit current. In continuous mode, IGATECHG = (QT + QB)f, where QT and QB are the gate charges of the topside and bottom side MOSFETs and f is the switching frequency. 4) The input capacitor has the difficult job of filtering the large RMS input current to the regulator. It must have a very low ESR to minimize the AC I2R loss and sufficient capacitance to prevent the RMS current from causing additional upstream losses in fuses or batteries. The LTC3735 2-phase architecture typically halves the input and output capacitor requirements over 1-phase solutions. Other losses, including COUT ESR loss, Schottky diode conduction loss during dead time, inductor core loss and internal control circuitry supply current generally account for less than 2% additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in DC (resistive)
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LTC3735 APPLICATIONS INFORMATION
load current. When a load step occurs, VOUT shifts by an amount equal to ILOAD(ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT generating the feedback error signal that forces the regulator to adapt to the current change and return VOUT to its steady-state value. During this recovery time VOUT can be monitored for excessive overshoot or ringing, which would indicate a stability problem. The availability of the ITH pin not only allows optimization of control loop behavior but 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. Assuming a predominantly second order system, phase margin and/or damping factor can be estimated using the percentage of overshoot seen at this pin. The bandwidth can also be estimated by examining the rise time at the pin. The ITH external components shown in the Figure 1 circuit will provide an adequate starting point for most applications. The ITH series RC-CC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.2 to 5 times their suggested values) to optimize transient response once the final PC layout is done and the particular output capacitor type and value have been determined. The output capacitors need to be decided upon first because the various types and values determine the loop gain and phase. An output current pulse of 20% to 80% of full-load current having a rise time of <1s will produce output voltage and ITH pin waveforms that will give a sense of the overall loop stability without breaking the feedback loop. The initial output voltage step resulting from the step change in output current may not be within the bandwidth of the feedback loop, so this signal cannot be used to determine phase margin. This is why it is better to look at the ITH pin signal which is in the feedback loop and is the filtered and compensated control loop response. The gain of the loop will be increased by increasing RC and the bandwidth of the loop will be increased by decreasing CC. If RC is increased by the same factor that CC is decreased, the zero frequency will be kept the same, thereby keeping the phase the same in the most critical frequency range of the feedback loop. The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual overall supply performance. Automotive Considerations: Plugging into the Cigarette Lighter As battery-powered devices go mobile, there is a natural interest in plugging into the cigarette lighter in order to conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging into the supply from hell. The main battery line in an automobile is the source of a number of nasty potential transients, including load-dump, reverse-battery and double-battery. Load-dump is the result of a loose battery cable. When the cable breaks connection, the field collapse in the alternator can cause a positive spike as high as 60V which takes several hundred milliseconds to decay. Reverse-battery is just what it says, while double-battery is a consequence of tow truck operators finding that a 24V jump start cranks cold engines faster than 12V. The network shown in Figure 10 is the most straightforward approach to protect a DC/DC converter from the ravages of an automotive power line. The series diode prevents current from flowing during reverse-battery, while the transient suppressor clamps the input voltage during load-dump. Note that the transient suppressor should not conduct during double-battery operation, but must still clamp the input voltage below breakdown of the converter. Although the LT3735 has a maximum input voltage of 32V, most applications will be limited to 30V by the MOSFET BVDSS.
VBAT 12V PVCC PVCC LTC3735
3735 F10
+
Figure 10. Automotive Application Protection
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LTC3735 APPLICATIONS INFORMATION
Design Example As a design example, assume VIN = 12V (nominal), VIN = 21V (max), VOUT = 1.5V, IMAX = 35A, and f = 350kHz (each phase). The inductance value is chosen first based on a 40% ripple current assumption. The highest value of ripple current occurs at the maximum input voltage. The minimum inductance for 40% ripple current is: L VOUT VOUT 1.5V * 1- = 350kHz * ( 40% * 17.5A ) * f * I VIN 1.5V 1- 21V = 0.57H Using L = 0.6H, a common "off-the-shelf" value results in 38%ripple current. The peak inductor current will be the maximum DC current plus one half of the ripple current, or 21A. Tie the FREQSET pin to 1.2V, resistively divided down from PVCC to have 350kHz operation for each phase. The minimum on-time also occurs at maximum input voltage: tON(MIN) = VOUT 1.5V = = 204ns VIN * f 21V * 350kHz 1.5V 35A PTOP = * * 1+ 0.005 * ( 85C - 25C) * 21V 2
2
(
)
0.008 +
21V 2 * 17.5A * 350kHz * 307pF * 2
1 1 2 * + = 1.26W 5V - 1V 1V Equation 4 gives the worst-case power loss dissipated by the bottom MOSFET (assuming FDS7760 and TJ = 85C again): PBOT = 21V - 1.5V 35A * * 2 21V
2
(1+ 0.005 * (85C - 25C)) * 0.008
= 2.95W Therefore it is necessary to have two FDS7760s in parallel to split the power loss. A short-circuit to ground will result in a folded back current of about: 25mV 1 200ns * 21V ISC = +* = 16A 0.002 2 0.6H The worst-case power dissipation by the bottom MOSFET under short-circuit conditions is: 1 - 200ns 2 350kHz PBOT = * (16A ) * 1 350kHz
which is larger than 150ns, the typical minimum on time of the LTC3735. RSENSE1 and RSENSE2 can be calculated by using a conservative maximum sense voltage threshold of 40mV and taking into account of the peak current: RSENSE = 40mV = 0.002 21A
(1+ 0.005 * (85C - 25C)) * 0.008
= 2.48W which is less than normal, full load conditions. The nominal duty cycle of this application is equation 1: DC = 1.5V = 12.5% 12V
The power loss dissipated by the top MOSFET can be calculated with equations 3 and 7. Using a Fairchild FDS7760 as an example: RDS(ON) = 8m, QG = 55nC at 5V VGS, CRSS = 307pF VTH(MIN) = 1V. At maximum input voltage with , TJ(estimated) = 85C at an elevated ambient temperature:
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LTC3735 APPLICATIONS INFORMATION
Using Figure 4, the RMS input ripple current will be: IINRMS = 35A * 0.22 = 7.7A An input capacitor(s) with a 8A RMS current rating is required. The output capacitor ripple current is calculated by using the inductor ripple current and multiplying by the factor obtained from Figure 3. The output ripple will be highest at the maximum input voltage since the duty cycle is less than 50%. The maximum output current ripple is: IOUT(MAX) = 1.5V * 0.77 = 5.5AP-P 350kHz * 0.6H stable. The power ground returns to the sources of the bottom N-channel MOSFETs, anodes of the Schottky diodes, and (-) plates of CIN, which should have the shortest trace length possible. 3) Are the SENSE - and SENSE+ leads routed together with minimum PC trace spacing? The filter capacitors between SENSE+ and SENSE - pin pairs should be as close as possible to the LTC3735. Ensure accurate current sensing with Kelvin connections at the current sense resistor. See Figure 11. 4) Does the (+) plate of CIN connect to the drains of the topside MOSFETs as closely as possible? This capacitor provides the AC current to the MOSFETs. Keep the input current path formed by the input capacitor, top and bottom MOSFETs, and the Schottky diode on the same side of the PC board in a tight loop to minimize conducted and radiated EMI. 5) Keep the "noisy" nodes, SW, BOOST, TG and BG away from sensitive small-signal nodes. Ideally the switch nodes should be placed at the furthest point from the LTC3735. The diagram in Figure 12 illustrates all branch currents in a 2-phase switching regulator. It becomes very clear after studying the current waveforms why it is critical to keep the high-switching-current paths to a small physical size. High electric and magnetic fields will radiate from these "loops" just as radio stations transmit signals. The output capacitor ground should return to the negative terminal of the input capacitor and not share a common ground path with any switched current paths. The left half of the circuit gives rise to the "noise" generated by a switching regulator.
PADS OF SENSE RESISTOR
Assuming the ESR of output capacitor(s) is 5m, the output ripple voltage is: 1 VOUT 5.5AP-P 5m + 16 * 350kHz * ( 4 * 270F ) = 28.4mVP-P PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3735. Check the following in your layout: 1) Are the signal and power grounds segregated? Keep the SGND at one end of a PC board to prevent MOSFET currents from traveling under the IC. The IC signal ground pin should be used to hook up all control circuitry on one side of the IC, routing the copper through SGND, under the IC covering the "shadow" of the package, connecting to the PGND pin and then continuing on to the (-) plate of COUT. 2) Is the PVCC decoupling capacitor connected immediately adjacent to the PVCC and PGND pins? A 1F ceramic capacitor of the X7R or X5R material is small enough to fit very close to the IC to minimize the ill effects of the large current pulses drawn to drive the power MOSFETs. An additional 4.7F ~ 10F of ceramic, tantalum or other low ESR capacitor is recommended in order to keep PVCC
TRACE TO INDUCTOR
TRACE TO OUTPUT CAP (+)
3735 F11
SENSE +
SENSE -
Figure 11. Proper Current Sense Connections
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25
LTC3735 APPLICATIONS INFORMATION
SW1
L1
RSENSE1
D1
VIN RIN CIN
VOUT COUT RL
+
+
SW2
L2
RSENSE2
BOLD LINES INDICATE HIGH, SWITCHING CURRENT LINES. KEEP LINES TO A MINIMUM LENGTH.
D2
3735 F12
Figure 12. Instantaneous Current Path Flow in a Multiple Phase Switching Regulator
The ground terminations of the sychronous MOSFETs and Schottky diodes should return to the negative plate(s) of the input capacitor(s) with a short isolated PC trace since very high switched currents are present. A separate isolated path from the negative plate(s) of the input capacitor(s) should be used to tie in the IC power ground pin (PGND) and the signal ground pin (SGND). This technique keeps inherent signals generated by high current pulses from taking alternate current paths that have finite impedances during the total period of the switching regulator. External OPTI-LOOP compensation allows overcompensation for PC layouts which are not optimized but this is not the recommended design procedure.
Simplified Visual Explanation of How a 2-Phase Controller Reduces Both Input and Output RMS Ripple Current A multiphase power supply significantly reduces the amount of ripple current in both the input and output capacitors. The RMS input ripple current is divided by, and the effective ripple frequency is multiplied up by the number of phases used (assuming that the input voltage is greater than the number of phases used times the output voltage). The output ripple amplitude is also reduced by, and the effective ripple frequency is increased by the number of phases used. Figure 13 graphically illustrates the principle.
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LTC3735 APPLICATIONS INFORMATION
SINGLE PHASE SW V SW1 V SW2 V DUAL PHASE
ICIN
IL1 ICOUT IL2 ICIN ICOUT RIPPLE
3735 F13
Figure 4 illustrates the RMS input current drawn from the input capacitance vs the duty cycle as determined by the ratio of input and output voltage. The peak input RMS current level of the single phase system is reduced by 50% in a 2-phase solution due to the current splitting between the two stages. An interesting result of the 2-phase solution is that the VIN which produces worst-case ripple current for the input capacitor, VOUT = VIN/2, in the single phase design produces zero input current ripple in the 2-phase design. The output ripple current is reduced significantly when compared to the single phase solution using the same inductance value because the VOUT/L discharge current term from the stage that has its bottom MOSFET on subtracts current from the (VIN - VOUT)/L charging current resulting from the stage which has its top MOSFET on. The output ripple current is: IRIPPLE = 2VOUT fL 1- 2D (1- D) 1- 2D + 1
Figure 13. Single and 2-Phase Current Waveforms
The worst-case RMS ripple current for a single stage design peaks at an input voltage of twice the output voltage. The worst-case RMS ripple current for a two stage design results in peak outputs of 1/4 and 3/4 of input voltage. When the RMS current is calculated, higher effective duty factor results and the peak current levels are divided as long as the currents in each stage are balanced. Refer to Linear Technology Application Note 19 for a detailed description of how to calculate RMS current for the single stage switching regulator. Figures 3 and 4 illustrate how the input and output currents are reduced by using an additional phase. The input current peaks drop in half and the frequency is doubled for this 2-phase converter. The input capacity requirement is thus reduced theoretically by a factor of four! Ceramic input capacitors with their low ESR characteristics can be used.
where D is duty factor. The input and output ripple frequency is increased by the number of stages used, reducing the output capacity requirements. When VIN is approximately equal to 2(VOUT) as illustrated in Figures 3 and 4, very low input and output ripple currents result.
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27
LTC3735 TYPICAL APPLICATION
Figure 14 shows a typical application using the LTC3735 to power the mobile CPU core. The input can vary from 5V to 24V; the output voltage can be programmed from 0.7V to 1.708V with a maximum current of 32A. By only modifying the external MOSFET and inductor selection, higher load current capability (up to 40A) can be achieved. The power supply in Figure 14 receives a VRON signal for ON/OFF control. After soft-start, the output voltage is set at 1.2V until the assertion of the MCH_PG signal. After about a 50s delay, the VID5-VID0 bits gain the control over the output voltage and program it between 0.7V and
36 2 8 4 3 19 20 21 22 23 24 35 17 3.3k 1000pF 1M 1F X5R 5V 4.7F X5R SW2 SW1 VOUT 47pF 16 470pF 9 1 28 34 31 BAT54 0.1F 0.1F 33 32 27 26 10 11 30 29 25 12 13 18 14 15 5 7 6 13.3k 56.2k 1.27M 1nF 12.7k 10 13.3k 100 VOA+ 1M 1% S1+ PSIB 100 S2+ 1nF 1F L2 0.8H D2 10 2.2F 0.002 S2+ C5: PANSONIC SP CAPS EEFSX0D181R OR SANYO POSCAP 2R5TPE220M9 D1, D2: B340A L1, L2: CDEP 104-OR8MC-L Q1, Q3: IRF7811W OR Si7860DP Q2, Q4: IRF7811W 2 OR Si7856DP 3.3V 3.3V CLK_EN# 3.3V 2k 1M 249k MMBT3904 80.6k PGOOD 2N7002 4.12k 43.2k 1F BAT54C
3735 F14
1.708V. When the STP_CPUB signal is low, a deep sleep state is indicated and the output voltage is decreased by about 1.04%. When the DPRSLPVR signal is high, a deeper sleep state is indicated and the output voltage becomes 0.748V regardless of the states of the VID bits. Active voltage positioning is accomplished with a resistor from the ITH to the VOA+ pin. Lower resistance yields a steeper AVP slope while higher resistance provides a flatter slope. Finally, the PGOOD output is masked for 110s during VID change or state transition.
VCCP_PG/MCH_PG 3.3V 2k PGOOD 5V 100k DPRSLPVR STP_CPUB PSIB VID0 VID1 VID2 VRON 232k 100pF 470pF VID3 VID4 VID5
MCH_PG DPRSLPVR STP_CPUB PSIB VID0 VID1 VID2 VID3 VID4 VID5 PGOOD ITH RUN/SS SGND VFB PVCC BOOST1 BOOST2
TG1 SW1 BG1 PGND SENSE1-
Q1
1F L1 0.8H D1
0.002 S1+ 10 10 C1 10F x4 35V X5R VIN 5V ~ 24V VOUT 0.7V ~ 1.708V AT 32A
Q2
FREQSET SENSE1+
Si1034X
+
LTC3735
TG2 SW2 BG2
Q3 Q4
C5 x3
VOA+
SENSE2+ SENSE2- RBOOT RDPRSLP RDPSLP VOA+ OAOUT VOA-
1.9k IMVP4_PG MMBT3904
549k
VRON
Figure 14. 5V to 24V Input, 0.7V to 1.708V Output, 32A IMVP-IV Compatible Power Supply
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28
LTC3735 PACKAGE DESCRIPTION
(Reference LTC DWG # 05-08-1640)
12.50 - 13.10* (.492 - .516) 1.25 0.12 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19
G Package 36-Lead Plastic SSOP (5.3mm)
7.8 - 8.2
5.3 - 5.7
7.40 - 8.20 (.291 - .323)
0.42 0.03 RECOMMENDED SOLDER PAD LAYOUT 5.00 - 5.60** (.197 - .221)
0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 2.0 (.079) MAX
0 - 8
0.09 - 0.25 (.0035 - .010)
0.55 - 0.95 (.022 - .037)
0.65 (.0256) BSC
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .152mm (.006") PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .254mm (.010") PER SIDE
0.22 - 0.38 (.009 - .015) TYP
0.05 (.002) MIN
G36 SSOP 0204
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29
LTC3735 PACKAGE DESCRIPTION
(Reference LTC DWG # 05-08-1701 Rev C)
UHF Package 38-Lead Plastic QFN (5mm x 7mm)
0.70 0.05
5.50 0.05 4.10 0.05 3.00 REF
5.15 0.05
3.15 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC 5.5 REF 6.10 0.05 7.50 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED PIN 1 NOTCH R = 0.30 TYP OR 0.35 x 45 CHAMFER 37 38 0.40 0.10 1 2
5.00 0.10
0.75 0.05 0.00 - 0.05
3.00 REF
PIN 1 TOP MARK (SEE NOTE 6)
7.00 0.10
5.50 REF
5.15 0.10
3.15 0.10
(UH) QFN REF C 1107
0.200 REF 0.25 0.05 0.50 BSC
R = 0.125 TYP
R = 0.10 TYP
BOTTOM VIEW--EXPOSED PAD
NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE M0-220 VARIATION WHKD 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS
30
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm 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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LTC3735 REVISION HISTORY
REV A DATE 4/11 DESCRIPTION Updated Figure 14 Updated Related Parts PAGE NUMBER 28 32
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
31
LTC3735 RELATED PARTS
PART NUMBER LTC3816 LTC3732 LTC3734 DESCRIPTION Single Phase DC/DC Controller for Intel IMVP-6/6+/6.5 CPUs 3-Phase, 5-Bit VID, 600kHz, Synchronous Controller Single Phase DC/DC Controller for IMVP-4 COMMENTS 7-Bit IMVP-6 VID: 0.00V VOUT 1.50V, 4.5V VIN 36V, Very Low Duty Cycle Capable 5-Bit VRM 9/9.1: 1.10V VOUT 1.85V 6-Bit IMVP-4 VID: 0.70V VOUT 1.708V, 4.5V VIN 30V, Lossless Voltage Positioning Phase-Lockable Fixed 250kHz to 780kHz Frequency, 4V VIN 38V, 0.8V VOUT 12.5V Phase-Lockable Fixed 250kHz to 770kHz Frequency, 4.5V VIN 38V, 0.8V VOUT 5V Phase-Lockable Fixed 250kHz to 780kHz Frequency, 4V VIN 30V, 0.8V VOUT 5.25V Operates with Power Blocks, DRMOS Devices or External Drivers/MOSFETs, 3V VIN 24V, tON(MIN) = 20ns Phase-Lockable Fixed Frequency 250kHz to 770kHz, 4.5V VIN 38V, 0.8V VOUT 12V Phase-Lockable Fixed 250kHz to 770kHz Frequency, 4.5V VIN 38V, 0.8V VOUT 5V
LTC3869/LTC3869-2 Dual, 2-Phase Synchronous Step-Down DC/DC Controllers with Excellent Current Share when Paralleled LTC3856 2-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diff Amp and DCR Temperature Compensation
LTC3850/LTC3850-1 Dual 2-Phase, High Efficiency Synchronous Step-Down DC/DC LTC3850-2 Controller, RSENSE or DCR Current Sensing and Tracking LTC3860 LTC3855 LTC3829 LTC3853 Dual, Multiphase, Synchronous Step-Down DC/DC Controller with Diff Amp and Three-State Output Drive Dual, Multiphase, Synchronous Step-Down DC/DC Controller with Diff Amp and DCR Temperature Compensation 3-Phase, Single Output Synchronous Step-Down Controller with Diff Amp and DCR Temperature Compensation
Triple Output, Multiphase Synchronous Step-Down DC/DC Controller, Phase-Lockable Fixed 250kHz to 750kHz Frequency, 4V VIN 24V, VOUT3 Up to 13.5V RSENSE or DCR Current Sensing and Tracking
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32 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0411 REV A * PRINTED IN USA
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2002

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