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LT1107 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
FEATURES
s s s s s s s s s
DESCRIPTIO
Operates at Supply Voltages from 2V to 30V Consumes Only 320A Supply Current Works in Step-Up or Step-Down Mode Only Three External Components Required Low-Battery Detector Comparator On-Chip User Adjustable Current Limit Internal 1A Power Switch Fixed or Adjustable Output Voltage Versions Space Saving 8-Pin MiniDIP or SO-8 Package
The LT(R)1107 is a versatile micropower DC/DC converter. The device requires only three external components to deliver a fixed output of 5V or 12V. Supply voltage ranges from 2V to 12V in step-up mode and to 30V in step-down mode. The LT1107 functions equally well in step-up, stepdown, or inverting applications. The LT1107 is pin-for-pin compatible with the LT1111, but has a duty cycle of 70%, resulting in increased output current in many applications. The LT1107 can deliver 150mA at 5V from a 2AA cell input and 5V at 300mA from 24V in step-down mode. Quiescent current is just 320A, making the LT1107 ideal for power-conscious batteryoperated systems. The 63kHz oscillator is optimized to work with surface mount inductors and capacitors. Switch current limit can be programmed with a single resistor. An auxiliary gain block can be configured as a low-battery detector, linear post regulator, undervoltage lock-out circuit, or error amplifier.
, LTC and LT are registered trademarks of Linear Technology Corporation
APPLICATIO S
s s s s s s s s
Palmtop Computers 3V to 5V, 5V to 12V Converters 24V to 5V, 12V to 5V Converters LCD Bias Generators Peripherals and Add-On Cards Battery Backup Supplies Cellular Telephones Portable Instruments
TYPICAL APPLICATIO
Palmtop Computer Logic Supply
L1* 33H MBRS120T3 5V 150mA
82 80 78 VIN = 3V
47
EFFICIENCY (%)
76 74 72 70 68 66
ILIM 2 x AA ALKALINE CELLS
VIN SW1
+
47F LT1107-5
+
100F
SENSE GND SW2
* SUMIDA CD54-330K COILCRAFT DT3316-473
64
1107 TA01
U
Efficiency
VIN = 2V VIN = 2.5V 1 10 100 LOAD CURRENT (mA) 400
1107 TA02
U
U
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LT1107 ABSOLUTE AXI U RATI GS
Supply Voltage (VIN) ............................................... 36V SW1 Pin Voltage (VSW1) ......................................... 50V SW2 Pin Voltage (VSW2) ............................ - 0.5V to VIN Feedback Pin Voltage (LT1107) ................................ 5V Sense Pin Voltage (LT1107-5, LT1107-12) ............ 36V Maximum Power Dissipation ............................ 500mW Set Pin Voltage ...................................................... 5.5V
PACKAGE/ORDER I FOR ATIO
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 8 7 6 5 FB (SENSE)* SET AO GND
ORDER PART NUMBER LT1107CN8 LT1107CN8-5 LT1107CN8-12
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 8 FB(SENSE)* 7 SET 6 AO 5 GND
N8 PACKAGE 8-LEAD PLASTIC DIP * FIXED VERSIONS TJMAX = 90C, JA = 130C/W (N)
J8 PACKAGE 8-LEAD CERAMIC DIP TJMAX = 150C, JA = 120C/W (J)
LT1107MJ8 LT1107MJ8-5 LT1107MJ8-12
OBSOLETE PACKAGE
Consider the N8 Package for Alternate Source
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL IQ PARAMETER Quiescent Current Quiescent Current, Step-Up Mode Configuration VIN Input Voltage Comparator Trip Point Voltage VOUT Output Sense Voltage Comparator Hysteresis Output Hysteresis f OSC t ON Oscillator Frequency Duty Cycle, Step-Up Mode Switch ON Time, Step-Up Mode
The q denotes the specifications which apply over the full operating temperature range, VIN = 3V, military or commercial version, TA = 25C, unless otherwise noted.
CONDITIONS Switch OFF No Load Step-Up Mode Step-Down Mode LT1107 (Note 2) LT1107-5 (Note 3) LT1107-12 (Note 3) LT1107 LT1107-5 LT1107-12 Full Load ILIM Tied to VIN LT1107-5 LT1107-12
q q q q q q q q
2
U
U
W
WW
U
W
(Note 1)
Maximum Switch Current ...................................... 1.5A Operating Temperature Range LT1107C ................................................ 0C to 70C LT1107I ............................................ -45C to 85C LT1107M(OBSOLETE) ............... - 55C to 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LT1107CS8 LT1107CS8-5 LT1107CS8-12 LT1107IS8 S8 PART MARKING 1107 11075 110712 1107I
S8 PACKAGE 8-LEAD PLASTIC SO *FIXED VERSIONS TJMAX = 90C, JA = 150C/W
MIN
TYP 320 360 550
MAX 450
UNITS A A A
2 1.2 4.75 11.40 1.25 5 12 8 32 75 50 64 8.8 63 70 11
12.6 30.0 1.3 5.25 12.60 12.5 50 120 77 76 12.7
V V V V V mV mV mV kHz % s
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LT1107
The q denotes the specifications which apply over the full operating temperature range, VIN = 3V, military or commercial version, TA = 25C, unless otherwise noted.
SYMBOL PARAMETER Feedback Pin Bias Current Set Pin Bias Current VOL AV Gain Block Output Low Reference Line Regulation Gain Block Gain Current Limit Current Limit Temperature Coefficient Switch OFF Leakage Current VSW2 Maximum Excursion Below GND Measured at SW1 Pin, VSW1 = 12V ISW1 10A, Switch OFF CONDITIONS LT1107, VFB = 0V VSET = VREF ISINK = 300A, VSET = 1V 5V VIN 30V RL = 100k (Note 4) 220 to ILIM to VIN
q q q q q q
ELECTRICAL CHARACTERISTICS
MIN
TYP 70 70 0.15 0.02
MAX 120 300 0.4 0.075
UNITS nA nA V %/V V/V mA %/C
1000
6000 400 - 0.3 1 - 400 10 - 350
A mV
The q denotes the specifications which apply over the full operating temperature range, VIN = 3V, - 55C TA 125C, unless otherwise noted.
SYMBOL IQ fOSC DC tON PARAMETER Quiescent Current Oscillator Frequency Duty Cycle Switch ON Time Reference Line Regulation VSAT Switch Saturation Voltage, Step-Up Mode Switch Saturation Voltage, Step-Down Mode Step-Up Mode Step-Down Mode, VIN = 12V Step-Up Mode Step-Down Mode, VIN = 12V 2V VIN 5V, 0C TA 125C 2.4V VIN 5V, TA = - 55C 0C TA 125C, ISW = 500mA TA = - 55C, ISW = 400mA VIN = 12V, ISW = 500mA 0C TA 125C TA = - 55C CONDITIONS Switch OFF
q q q q q q
MIN 40 56 45 7 5
LT1107M TYP MAX 500 63 69 60 11 9 0.2 0.5 0.5 95 81 73 15 13 0.4 0.8 0.65 0.65 1.5 2.0
UNITS A kHz % % s s %/V %/V V V V V
The q denotes the specifications which apply over the full operating temperature range, VIN = 3V, 0C TA 70C, unless otherwise noted.
SYMBOL IQ fOSC DC tON PARAMETER Quiescent Current Oscillator Frequency Duty Cycle Switch ON Time Reference Line Regulation VSAT Switch Saturation Voltage, Step-Up Mode Switch Saturation Voltage, Step-Down Mode Step-Up Mode Step-Down Mode, VIN = 12V Step-Up Mode Step-Down Mode, VIN = 12V 2V VIN 5V VIN = 3V, ISW = 650mA VIN = 12V, ISW = 650mA CONDITIONS Switch OFF
q q q q q q q q q
MIN 50 62 50 8 6
LT1107C TYP 63 69 60 11 9 0.2 0.5 1.1
MAX 450 88 78 70 13.5 12.0 0.7 0.65 1.5
UNITS A kHz % % s s %/V V V
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: This specification guarantees that both the high and low trip points of the comparator fall within the 1.2V to 1.3V range.
Note 3: The output voltage waveform will exhibit a sawtooth shape due to the comparator hysteresis. The output voltage on the fixed-output versions will always be within the specified range. Note 4: 100k resistor connected between a 5V source and the AO pin.
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LT1107
TYPICAL PERFOR A CE CHARACTERISTICS
Saturation Voltage, Step-Up Mode (SW2 Pin Grounded)
1.2 1.0
1.4 1.3
SATURATION VOLTAGE (V)
SWITCH ON VOLTAGE (V)
VIN = 3V
0.8 VIN = 2V 0.6 0.4 0.2 0 VIN = 5V
1.2 1.1 1.0 0.9 0.8 0.7 0
SWITCH CURRENT (A)
0
0.2
0.4 0.6 0.8 SWITCH CURRENT (A)
Quiescent Current
400 350 400 380
QUIESCENT CURRENT (A)
QUIESCENT CURRENT (A)
300 250 200 150 100 -55 -35 -15
FREQUENCY (kHz)
5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G05
Switch ON Time Step-Up Mode
16 15 14
SWITCH ON TIME (s) DUTY CYCLE (%)
13 12 11 10 9 8 7 6 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G08
SWITCH ON TIME (s)
4
UW
1.0
1107 G01
Switch ON Voltage, Step-Down Mode (SW1 Pin Connected to VIN)
1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Maximum Switch Current vs RLIM
STEP-UP 2V VIN 5V
STEP-DOWN VIN = 12V
1.2
0.1
0.2
0.3 0.4 0.5 0.6 SWITCH CURRENT (A)
0.7
0.8
10
100 RLIM ()
1000
1107 G03
1107 G02
Quiescent Current
100 TA = 25C 90 80 70 60 50 40 30 0 3 6 9 12 15 18 21 24 27 30 INPUT VOLTAGE (V)
1107 G06
Oscillator Frequency
360 340 320 300 280 260 240 220 200
20 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G07
Duty Cycle Step-Up Mode
85 80 75 70 65 60 55 50 45 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G09
Switch ON Time Step-Down Mode
13 12 11 10 9 8 7 6 5 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G10
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LT1107
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum/Maximum Frequency vs ON Time, Step-Down Mode
100 90
FREQUENCY (kHz) FREQUENCY (kHz)
DUTY CYCLE (%)
80 70 60 50 40 30 4 5 6 7 -55C TA 125C
0C TA 70C
8 9 10 11 12 13 14 ON TIME (s)
1107 G11
LT1107-5 Output Voltage
5.3 5.2
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
12.10 12.05 12.00 11.95 11.90 11.85
TRIP POINT VOLTAGE (V)
5.1 5.0 4.9 4.8 4.7 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G16
PI FU CTI
S
GND (Pin 5): Ground. AO (Pin 6): Auxiliary Gain Block (GB) Output. Open collector, can sink 300A. SET (Pin 7): GB Input. GB is an op amp with positive input connected to SET pin and negative input connected to 1.25V reference. FB/SENSE (Pin 8): On the LT1107 (adjustable), this pin goes to the comparator input. On the LT1107-5 and LT1107-12, this pin goes to the internal application resistor that sets output voltage.
ILIM (Pin 1): Connect this pin to VIN for normal use. Where lower current limit is desired, connect a resistor between ILIM and VIN. A 220 resistor will limit the switch current to approximately 400mA. VIN (Pin 2): Input Supply Voltage. SW1 (Pin 3): Collector of Power Transistor. For step-up mode connect to inductor/diode. For step-down mode connect to VIN. SW2 (Pin 4): Emitter of Power Transistor. For step-up mode connect to ground. For step-down mode connect to inductor/diode. This pin must never be allowed to go more than a Schottky diode drop below ground.
UW
Minimum/Maximum Frequency vs ON Time, Step-Up Mode
100 90 80 70 60 50 40 30 6 7 8 9 10 11 12 13 14 15 16 ON TIME (s)
1107 G12
Duty Cycle Step-Down Mode
70 65 60 55 50 45 40 35 -55 -35 -15
0C TA 70C
TA = 25C -55C TA 125C
5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G13
LT1107-12 Output Voltage
12.20 12.15
1.30 1.29 1.28 1.27 1.26 1.25 1.24 1.23 1.22 1.21
LT1107 Feedback Voltage
11.80 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G17
1.20 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
1107 G18
UO
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LT1107
BLOCK DIAGRA S
LT1107 LT1107-5/LT1107-12
SET A2 VIN GAIN BLOCK/ ERROR AMP 1.25V REFERENCE A1 OSCILLATOR DRIVER COMPARATOR GND FB SW2
1107 BD01
OPERATIO
The LT1107 is a gated oscillator switcher. This type architecture has very low supply current because the switch is cycled when the feedback pin voltage drops below the reference voltage. Circuit operation can best be understood by referring to the LT1107 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.25V reference signal. When FB drops below 1.25V, A1 switches on the 63kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. The switch cycling action raises the output voltage and FB pin voltage. When the FB voltage is sufficient to trip A1, the oscillator is gated off. A small amount of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator output is low, the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 300A. The oscillator is set internally for 11s ON time and 5s OFF time in step-up mode, optimizing the device for converters where VOUT 3VIN. The combination of high duty cycle and the current limit feature enables continuous mode operation in many applications, increasing available output power.
6
W
SET AO A2 VIN GAIN BLOCK/ ERROR AMP 1.25V REFERENCE A1 OSCILLATOR DRIVER COMPARATOR R1 GND R2 220k SENSE SW2 LT1107-5: R1 = 73.5k LT1107-12: R1 = 25.5k
1107 BD02
AO
ILIM
SW1
ILIM
SW1
U
Gain block A2 can serve as a low-battery detector. The negative input of A2 is the 1.25V reference. A resistor divider from VIN to GND, with the mid-point connected to the SET pin provides the trip voltage in a low-battery detector application. AO can sink 300A (use a 22k resistor pull-up to 5V). A resistor connected between the ILIM pin and VIN sets maximum switch current. When the switch current exceeds the set value, the switch cycle is prematurely terminated. If current limit is not used, ILIM should be tied directly to VIN. Propagation delay through the current limit circuitry is approximately 1s. In step-up mode the switch emitter (SW2) is connected to ground and the switch collector (SW1) drives the inductor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The LT1107-5 and LT1107-12 are functionally identical to the LT1107. The -5 and -12 versions have on-chip voltage setting resistors for fixed 5V or 12V outputs. Pin 8 on the fixed versions should be connected to the output. No external resistors are needed.
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LT1107
APPLICATI S I FOR ATIO
Inductor Selection -- Step-Up Converter In a step-up, or boost converter (Figure 1), power generated by the inductor makes up the difference between input and output. Power required from the inductor is determined by:
PL = VOUT + V D - VIN(MIN) IOUT
()
where VD is the diode drop (0.5V for a 1N5818 Schottky). Energy required by the inductor per cycle must be equal or greater than:
PL / f OSC
in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds according to:
- R t VIN 1- e L IL (t) = (3) R where R is the sum of the switch equivalent resistance (0.8 typical at 25C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation:
t (4) L can be used. These equations assume that at t = 0, inductor current is zero. This situation is called "discontinuous mode operation" in switching regulator parlance. Setting "t" to the switch ON time from the LT1107 specification table (typically 11s) will yield IPEAK for a specific "L" and VIN. Once IPEAK is known, energy in the inductor at the end of the switch ON time can be calculated as: 1 E L = LI 2 (5) 2 PEAK EL must be greater than PL/fOSC for the converter to deliver the required power. For best efficiency IPEAK should be kept to 1A or less. Higher switch currents will cause excessive drop across the switch resulting in reduced efficiency. In general, switch current should be held to as low a value as possible in order to keep switch, diode and inductor losses at a minimum.
IL t =
()
VIN
U
As an example, suppose 12V at 60mA is to be generated from a 3V to 6V input. Recalling equation (1),
P L = 12V + 0.5V - 3V 60mA = 570mW
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(
)(
)
(6)
Energy required from the inductor is:
PL f OSC = 570mW = 9.05J 63kHz (7)
(1)
Picking an inductor value of 33H with 0.2 DCR results in a peak switch current of: I PEAK
-1 * 11s 3V = 1 - e 33H = 850mA 1
(2)
(8)
Substituting IPEAK into Equation 4 results in: EL = 1 33H 0.85A 2
(
)(
) 2 = 11.91J
(9)
Since 11.9J > 9.05J, the 33H inductor will work. This trial-and-error approach can be used to select the optimum inductor. A resistor can be added in series with the ILIM pin to invoke switch current limit. The resistor should be picked so the calculated IPEAK at minimum VIN is equal to the Maximum Switch Current (from Typical Performance Characteristic curves). Then, as VIN increases, peak switch current is held constant, resulting in increasing efficiency. Inductor Selection -- Step-Down Converter The step-down case (Figure 2) differs from the step-up in that the inductor current flows through the load during both the charge and discharge periods of the inductor. Current through the switch should be limited to ~650mA in this mode. Higher current can be obtained by using an external switch (see LT1111 and LT1110 data sheets). The ILIM pin is the key to successful operation over varying inputs. After establishing output voltage, output current and input voltage range, peak switch current can be calculated by the formula: V OUT + V D 2I (10) I PEAK = OUT DC V IN - VSW + V D
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LT1107
APPLICATI S I FOR ATIO
where DC = duty cycle (0.50 in step-down mode) VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818) IOUT = output current VOUT = output voltage VIN = minimum input voltage VSW is actually a function of switch current which is in turn a function of VIN, L, time, and VOUT. To simplify, 1.5V can be used for VSW as a very conservative value. Once IPEAK is known, inductor value can be derived from:
L= VIN (MIN) - VSW - VOUT I PEAK x t ON (11)
where tON = switch ON time (7s). Next, the current limit resistor RLIM is selected to give IPEAK from the Maximum Switch Current vs RLIM curve. The addition of this resistor keeps maximum switch current constant as the input voltage is increased. As an example, suppose 5V at 300mA is to be generated from a 12V to 24V input. Recalling Equation (10): 2 300mA 5 + 0.5 = 600mA (12) 0.50 12 - 1.5 + 0.5 Next, inductor value is calculated using Equation (11): I PEAK = L= 12 - 1.5 - 5 7s = 64H 600mA (13)
(
)
Use the next lowest standard value (56H). Then pick RLIM from the curve. For IPEAK = 600mA, RLIM = 56. Inductor Selection -- Positive-to-Negative Converter Figure 4 shows hookup for positive-to-negative conversion. All of the output power must come from the inductor. In this case, PL = VOUT + VD IOUT
(
)( )
8
U
In this mode the switch is arranged in common collector or step-down mode. The switch drop can be modeled as a 0.75V source in series with a 0.65 resistor. When the switch closes, current in the inductor builds according to:
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IL
()
VL 1 - e t= R
- R t L

(15)
where R = 0.65 + DCRL VL = VIN - 0.75V As an example, suppose -5V at 50mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), PL = -5V + 0.5V 50mA = 275mW Energy required from the inductor is:
(
)(
)
(16 )
PL 275mW = = 4.4J fOSC 63kHz
(17)
Picking an inductor value of 100H with 0.2 DCR results in a peak switch current of: -0.85 * 9 s 4.5V - 0.75V 1 - e 100H IPEAK = 0.65 + 0.2 = 325mA (18) Substituting IPEAK into Equation (04) results in:
( (
) )
EL =
1 100H 0.325A 2
(
)(
) 2 = 5.28J
(19)
Since 5.28J > 3.82J, the 100H inductor will work. With this relatively small input range, RLIM is not usually necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited to ~650mA. Step-Up (Boost Mode) Operation A step-up DC/DC converter delivers an output voltage higher than the input voltage. Step-up converters are not short-circuit protected since there is a DC path from input to output.
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(14)
LT1107
APPLICATI
S I FOR ATIO
The usual step-up configuration for the LT1107 is shown in Figure 1. The LT1107 first pulls SW1 low causing VIN - VCESAT to appear across L1. A current then builds up in L1. At the end of the switch ON time the current in L1 is1:
V IPEAK = IN t ON L
L1 VIN R3 ILIM LT1107 FB GND SW2 R1 VIN SW1 R2 D1 VOUT
+
C1
1107 F01
Figure 1. Step-Up Mode Hookup
Immediately after switch turn-off, the SW1 voltage pin starts to rise because current cannot instantaneously stop flowing in L1. When the voltage reaches VOUT + VD, the inductor current flows through D1 into C1, increasing VOUT. This action is repeated as needed by the LT1107 to keep VFB at the internal reference voltage of 1.25V. R1 and R2 set the output voltage according to the formula:
R2 VOUT = 1 + 1.25V R1
(
)
Step-Down (Buck Mode) Operation A step-down DC/DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1107 based step-down converter is shown in Figure 2. When the switch turns on, SW2 pulls up to VIN - VSW. This puts a voltage across L1 equal to VIN - VSW - VOUT, causing a current to build up in L1. At the end of the switch ON time, the current in L1 is equal to: I PEAK = VIN - VSW - VOUT L t ON (22)
Note 1: This simple expression neglects the effects of switch and coil resistance. This is taken into account in the "Inductor Selection" section.
U
VIN
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+
C2
R3 100 ILIM VIN LT1107 SW1 FB L1 SW2 GND D1 1N5818 VOUT R2 C1 R1
(20)
+
1107 F02
Figure 2. Step-Down Mode Hookup
When the switch turns off, the SW2 pin falls rapidly and actually goes below ground. D1 turns on when SW2 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go below -0.5V. A silicon diode such as the 1N4933 will allow SW2 to go to -0.8V, causing potentially destructive power dissipation inside the LT1107. Output voltage is determined by:
R2 VOUT = 1 + 1.25V R1
(
)
(23)
(21)
R3 programs switch current limit. This is especially important in applications where the input varies over a wide range. Without R3, the switch stays on for a fixed time each cycle. Under certain conditions the current in L1 can build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 100 resistor programs the switch to turn off when the current reaches approximately 700mA. When using the LT1107 in stepdown mode, output voltage should be limited to 6.2V or less. Higher output voltages can be accommodated by inserting a 1N5818 diode in series with the SW2 pin (anode connected to SW2). Inverting Configurations The LT1107 can be configured as a positive-to-negative converter (Figure 3), or a negative-to-positive converter (Figure 4). In Figure 3, the arrangement is very similar to a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As in the step-down mode, D1 must be a Schottky diode, and
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LT1107
APPLICATI
S I FOR ATIO
VOUTshould be less than 6.2V. More negative output voltages can be accommodated as in the prior section. In Figure 4, the input is negative while the output is positive. In this configuration, the magnitude of the input voltage can be higher or lower than the output voltage. A level shift, provided by the PNP transistor, supplies proper polarity feedback information to the regulator.
+VIN
+
C2 R3 ILIM VIN LT1107 SW2 GND D1 1N5818 SW1 FB L1 R1 C1 R2 -VOUT
1107 F03
+
Figure 3. Positive-to-Negative Converter
L1
D1
R3 ILIM VIN SW1 LT1107 FB GND -VIN SW2 R2
+
C1
+
C2
VOUT = R1 1.25V + 0.6V R2
1107 F04
()
Figure 4. Negative-to-Positive Converter
10
U
Using the ILIM Pin The LT1107 switch can be programmed to turn off at a set switch current, a feature not found on competing devices. This enables the input to vary over a wide range without exceeding the maximum switch rating or saturating the inductor. Consider the case where analysis shows the LT1107 must operate at an 800mA peak switch current with a 2V input. If VIN rises to 4V, the peak switch current will rise to 1.6A, exceeding the maximum switch current rating. With the proper resistor selected (see the "Maximum Switch Current vs RLIM" characteristic), the switch current will be limited to 800mA, even if the input voltage increases. Another situation where the ILIM feature is useful occurs when the device goes into continuous mode operation. This occurs in step-up mode when: VOUT + VDIODE VIN - VSW
+VOUT R1 2N3906
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<
1 1 - DC
(24)
When the input and output voltages satisfy this relationship, inductor current does not go to zero during the switch OFF time. When the switch turns on again, the current ramp starts from the non-zero current level in the inductor just prior to switch turn-on. As shown in Figure 5, the inductor current increases to a high level before the comparator turns off the oscillator. This high current can cause excessive output ripple and requires oversizing the output capacitor and inductor. With the ILIM feature, the switch turns off at the programmed current as shown in Figure 6, keeping output ripple to a minimum.
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LT1107
APPLICATI S I FOR ATIO U
1107 F05
1107 F06
SWITCH
ON OFF
Figure 5. No Current Limit Causes Large Inductor Current Build-Up
SWITCH
OFF
Figure 6. Current Limit Keeps Inductor Current Under Control
W
IL
IL ON
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PROGRAMMED CURRENT LIMIT
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LT1107
APPLICATI S I FOR ATIO
Figure 7 details current limit circuitry. Sense transistor A1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2's collector current flows in Q1's collector. This current is passed through internal 80 resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN sets the current limit. When sufficient switch current flows to develop a VBE across R1 + RLIM, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 800ns. The current trip point becomes less accurate for switch ON times less than 3s. Resistor values programming switch ON time for 800ns or less will cause spurious response in the switch circuitry although the device will still maintain output regulation. Using the Gain Block The gain block (GB) on the LT1107 can be used as an error amplifier, low-battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 1.25V reference. The positive input comes out on the SET pin. Arrangement of the gain block as a low-battery detector is straightforward. Figure 8 shows hookup. R1 and R2 need only be low enough in value so that the bias current of the SET input does not cause large errors. 33k for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to "snap" when the trip point is reached. Values in the 1M to 10M range are optimal. The addition of R3 will change the trip point, however. Output ripple of the LT1107, normally 50mV at 5VOUT can be reduced significantly by placing the gain block in front of the FB input as shown in Figure 9. This effectively reduces the comparator hysteresis by the gain of the gain block. Output ripple can be reduced to just a few millivolts using this technique. Ripple reduction works with stepdown or inverting modes as well. For this technique to be effective, output capacitor C1 must be large, so that each switching cycle increases VOUT by only a few millivolts. 1000F is a good starting value. C1 should be a low ESR type as well.
VBAT
12
U
RLIM (EXTERNAL) VIN Q3 DRIVER OSCILLATOR Q1 ILIM R1 80 (INTERNAL) SW1 Q2 SW2
1107 F07
W
U
UO
Figure 7. LT1107 Current Limit Circuitry
5V VIN R1 1.25V REF SET LT1107
47k
-
AO TO PROCESSOR
VBAT
+
GND
R2 R3 VLB - 1.25V 35.1A VLB = BATTERY TRIP POINT R2 = 33k 1107 F08 R3 = 1.6M
R1 =
(
)
Figure 8. Setting Low-Battery Detector Trip Point
L1
D1 VOUT
R3 270k
ILIM AO LT1107 FB GND
VIN SW1
R2
+
C1
SET SW2
R1
VOUT = R2 + 1 1.25V R1
( )( )
1107 F09
Figure 9. Output Ripple Reduction Using Gain Block
1107fa
LT1107
PACKAGE DESCRIPTIO U
J8 Package 8-Lead CERDIP (Narrow .300 Inch, Hermetic)
(Reference LTC DWG # 05-08-1110)
0.005 (0.127) MIN 0.405 (10.287) MAX 8 7 6 5 0.023 - 0.045 (0.584 - 1.143) HALF LEAD OPTION 0.045 - 0.068 (1.143 - 1.727) FULL LEAD OPTION 0.300 BSC (0.762 BSC) 0.025 (0.635) RAD TYP 1 2 3 0.220 - 0.310 (5.588 - 7.874) 4 0.200 (5.080) MAX 0.015 - 0.060 (0.381 - 1.524) 0 - 15 0.045 - 0.065 (1.143 - 1.651) 0.014 - 0.026 (0.360 - 0.660) 0.100 (2.54) BSC 0.125 3.175 MIN
J8 1298
CORNER LEADS OPTION (4 PLCS)
0.008 - 0.018 (0.203 - 0.457)
NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE OR TIN PLATE LEADS
OBSOLETE PACKAGE
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13
LT1107
PACKAGE DESCRIPTIO U
N8 Package 8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
.400* (10.160) MAX 8 7 6 5 .255 .015* (6.477 0.381) 1 .300 - .325 (7.620 - 8.255) 2 3 4 .130 .005 (3.302 0.127) .045 - .065 (1.143 - 1.651) .065 (1.651) TYP .125 (3.175) .020 MIN (0.508) MIN .018 .003 (0.457 0.076)
N8 0502
.009 - .015 (0.229 - 0.381)
(
+.035 .325 -.015 8.255 +0.889 -0.381
)
.100 (2.54) BSC
INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
NOTE: 1. DIMENSIONS ARE
1107fa
14
LT1107
PACKAGE DESCRIPTIO U
S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.045 .005 .050 BSC 8 N N .245 MIN .160 .005 .228 - .244 (5.791 - 6.197) 1 .030 .005 TYP 2 3 N/2 N/2 .150 - .157 (3.810 - 3.988) NOTE 3 .189 - .197 (4.801 - 5.004) NOTE 3 7 6 5 1 2 3 4 .053 - .069 (1.346 - 1.752) 0- 8 TYP .004 - .010 (0.101 - 0.254) .014 - .019 (0.355 - 0.483) TYP .050 (1.270) BSC
SO8 0502
RECOMMENDED SOLDER PAD LAYOUT
.010 - .020 x 45 (0.254 - 0.508) .008 - .010 (0.203 - 0.254)
.016 - .050 (0.406 - 1.270) NOTE: 1. DIMENSIONS IN
INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
1107fa
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.
15
LT1107
TYPICAL APPLICATI
RELATED PARTS
PART NUMBER LT1070/LT1070HV LT1071/LT1071HV LT1072/LT1072HV LT1082 LT1111 LT1170/LT1170HV LT1171/LT1171HV LT1172/LT1172HV LT1307/LT1307B LT1317/LT1317B LT1370/LT1370HV LT1371/LT1371HV DESCRIPTION 5A ISW, 40kHz, High Efficiency Switching Regulator 2.5A ISW, 40kHz, High Efficiency Switching Regulator 1.25A ISW, 40kHz, High Efficiency Switching Regulator 1A ISW, 60kHz, High Efficiency Switching Regulator 1A ISW, 72kHz, High Efficiency Switching Regulator 5A ISW, 100kHz, High Efficiency Switching Regulator 2.5A ISW, 100kHz, High Efficiency Switching Regulator 1.25A ISW, 100kHz, High Efficiency Switching Regulator 600mA ISW, 600kHz, High Efficiency Step-Up Switching Regulator 660mA ISW, 600kHz, High Efficiency Step-Up Switching Regulator 6A ISW, 500kHz, High Efficiency Switching Regulator 3A ISW, 500kHz, High Efficiency Switching Regulator COMMENTS VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50A, Can be Used for Buck, Boost, Inverting Applications, TO220-5 Packages VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50A, Can be Used for Buck, Boost, Inverting Applications, TO220-5 Package VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50A, Can be Used for Buck, Boost, Inverting Applications, N8, S8, S16, TO220-5 Packages VIN = 3V to 75V, VOUT = 100V, IQ = 4.5mA, ISD = <120A, Can be Used for Buck, Boost, Inverting Applications, DD, N8, TO220-5 Packages VIN = 2V to 30V, VOUT = 34V, IQ = 300A, Can be Used for Buck, Boost, Inverting Applications, N8, S8 Packages VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50A, Can be Used for Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50A, Can be Used for Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <100A, Can be Used for Buck, Boost, Inverting Applications, N8, S16, DD, TO220-5 Packages VIN = 1V to 12V, VOUT = 28V, IQ = 50A/1mA, ISD = <1A Ideal for Single Cell Applications, Low Battery Detect, MS8, N8, S8 Packages VIN = 1.5V to 12V, VOUT = 28V, IQ = 100A/4.8mA, ISD = <30A/28A Low Battery Detect, MS8, S8 Packages VIN = 2.7V to 30V, VOUT = 35V/42V, IQ = 4.5mA, ISD = <12A, Can be Used for Buck, Boost, Inverting Applications, DD, TO220-7 Packages VIN = 2.7V to 30V, VOUT = 35V/42V, IQ = 4mA, ISD = <12A, Can be Used for Buck, Boost, Inverting Applications, S20, DD, TO220-7 Packages
16 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
UO
24V-to-5V Step-Down Converter
24VIN 220 ILIM VIN SW1 22F LT1107-5 SENSE GND SW2 150H* 5V 300mA 330F
1107 TA03
+
+
1N5818
*COILTRONICS CTX150-4
1107fa LT/TP 1002 1K REV A * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 1993

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