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LT1307/LT1307B Single Cell Micropower 600kHz PWM DC/DC Converters
FEATURES
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DESCRIPTION
The LT (R)1307/LT1307B are micropower, fixed frequency DC/DC converters that operate from an input voltage as low as 1V. First in the industry to achieve true current mode PWM performance from a single cell supply, the LT1307 features automatic shifting to power saving Burst Mode operation at light loads. High efficiency is maintained over a broad 100A to 100mA load range. The LT1307B does not shift into Burst Mode operation at light loads, eliminating low frequency output ripple at the expense of light load efficiency. The devices contain a lowbattery detector with a 200mV reference and shut down to less than 5A. No load quiescent current of the LT1307 is 50A and the internal NPN power switch handles a 500mA current with a voltage drop of just 295mV. Unlike competitive devices, large electrolytic capacitors are not required with the LT1307/LT1307B in single cell applications. The high frequency (600kHz) switching allows the use of tiny surface mount multilayer ceramic (MLC) capacitors along with small surface mount inductors. The devices work with just 10F of output capacitance and require only 1F of input bypassing. The LT1307/LT1307B are available in 8-lead MSOP, PDIP and SO packages.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation.
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Uses Small Ceramic Capacitors 50A Quiescent Current (LT1307) 1mA Quiescent Current (LT1307B) Operates with VIN as Low as 1V 600kHz Fixed Frequency Operation Starts into Full Load Low Shutdown Current: 3A Low-Battery Detector 3.3V at 75mA from a Single Cell Automatic Burst ModeTM Operation at Light Load (LT1307) Continuous Switching at Light Load (LT1307B) Low VCESAT Switch: 295mV at 500mA
APPLICATIONS
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Pagers Cordless Telephones GPS Receivers Battery Backup Portable Electronic Equipment Glucose Meters Diagnostic Medical Instrumentation
TYPICAL APPLICATION
L1 10H D1 C1: MURATA-ERIE GRM235Y5V105Z01 MARCON THCS50E1E105Z TOKIN 1E105ZY5U-C103-F 3.3V C2: MURATA-ERIE GRM235Y5V106Z01 75mA MARCON THCS50E1E105Z TOKIN 1E106ZY5U-C304-F D1: MOTOROLA MBR0520L C2 L1: COILCRAFT D01608C-103 10F SUMIDA CD43-100 MURATA ERIE LQH3C100 FOR 5V OUTPUT: R1 = 1M, R2 = 329k
1307 F01
Single Cell to 3.3V Converter Efficiency
90
C1 1F 1.5V CELL SHUTDOWN
VIN LBI
SW FB R1 1.02M 1% R2 604k 1%
EFFICIENCY (%)
LT1307 SHDN LBO GND VC 100k 680pF
Figure 1. Single Cell to 3.3V Boost Converter
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80 70 60
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VIN = 1.5V VIN = 1V VIN = 1.25V
50 0.1
1 10 LOAD CURRENT (mA)
100
1307 TA01
1
LT1307/LT1307B ABSOLUTE AXI U RATI GS
Junction Temperature........................................... 125C Operating Temperature Range Commercial (Note 1) ......................... - 20C to 70C Industrial ........................................... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C VIN, SHDN, LBO Voltage ......................................... 12V SW Voltage ............................................................. 30V FB Voltage ....................................................... VIN + 1V VC Voltage ................................................................ 2V LBI Voltage ............................................ 0V VLBI 1V Current into FB Pin .............................................. 1mA
PACKAGE/ORDER I FOR ATIO
ORDER PART NUMBER
TOP VIEW VC FB SHDN GND 1 2 3 4 8 7 6 5 LBO LBI VIN SW
LT1307CMS8 LT1307BCMS8
MS8 PACKAGE 8-LEAD PLASTIC MSOP
TJMAX = 125C, JA = 160C/W
MS8 PART MARKING BU BF
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
Commercial Grade 0C to 70C. VIN = 1.1V, VSHDN = VIN, TA = 25C, LT1307/LT1307B unless otherwise noted.
SYMBOL IQ PARAMETER Quiescent Current CONDITIONS Not Switching (LT1307) Not Switching (LT1307B) VSHDN = 0V VFB = VREF 1V VIN 2V (25C, 0C) 1V VIN 2V (70C) 2V VIN 5V
q q q q q
VFB IB
Feedback Voltage FB Pin Bias Current (Note 2) Reference Line Regulation
Minimum Input Voltage Input Voltage Range gm AV fOSC Error Amp Transconductance Error Amp Voltage Gain Switching Frequency I = 5A 25C, 0C 70C
q q q
2
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TOP VIEW VC 1 FB 2 SHDN 3 GND 4 N8 PACKAGE 8-LEAD PDIP 8 7 6 5 LBO LBI VIN SW
ORDER PART NUMBER LT1307CN8 LT1307CS8 LT1307IS8 LT1307BCS8 LT1307BIS8 S8 PART MARKING 1307 1307I 1307B 1307BI
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 125C, JA = 100C/W (N8) TJMAX = 125C, JA = 120C/W (S8)
MIN
TYP 50 1.0 1
MAX 90 1.5 3 1.24 60 1.1 1.5 0.8 1 5 65
UNITS A mA A V nA %/V %/V %/V V V mhos V/V V/V
1.20
1.22 27 0.6
q
0.3 0.92 1 25 35 30 550 35 100 600
750
kHz
LT1307/LT1307B
ELECTRICAL CHARACTERISTICS
Commercial Grade 0C to 70C. VIN = 1.1V, VSHDN = VIN, TA = 25C unless otherwise noted.
SYMBOL PARAMETER Maximum Duty Cycle Switch Current Limit (Note 3) Switch VCESAT Burst Mode Operation Switch Current Limit (LT1307 Only) Shutdown Pin Current LBI Threshold Voltage LBO Output Low LBO Leakage Current LBI Input Bias Current (Note 4) Low-Battery Detector Gain Switch Leakage Current Reverse Battery Current ISINK = 10A VLBI = 250mV, VLBO = 5V VLBI = 150mV 1M Load (25C, 0C) 1M Load (70C) VSW = 5V (Note 5)
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CONDITIONS 25C, 0C 70C DC = 40% DC = 75% ISW = 500mA (25C, 0C) ISW = 500mA (70C) L = 10H L = 22H VSHDN = VIN VSHDN = 0V
q q q q q q q
MIN 80 76
TYP 84 0.6 0.5 295 100 50 2.5 - 1.5
MAX
UNITS % %
1.25 350 400
A A mV mV mA mA
4.0 - 2.5 210 0.25 0.1 25
A A mV V A nA V/V V/V
190
200 0.1 0.01 5
1000 500
3000 0.01 750 3
A mA
Commercial Grade TA = - 20C, VIN = 1.1V, VSHDN = VIN, unless otherwise noted (Note 1).
SYMBOL IQ PARAMETER Quiescent Current CONDITIONS VFB = 1.3V, Not Switching (LT1307) VFB = 1.3V, Not Switching (LT1307B) VSHDN = 0V 1.195 I = 5A 25 35 500 80 ISW = 500mA, VIN = 1.2V VSHDN = VIN VSHDN = 0V 186 MIN TYP 50 1.1 1 1.22 35 100 600 84 250 2.5 - 1.5 200 350 4.0 - 2.5 210 750 MAX 100 1.6 3 1.245 65 UNITS A mA A V mhos V/V kHz % mV A A mV
VFB gm AV fOSC
Feedback Voltage Error Amp Transconductance Error Amp Voltage Gain Switching Frequency Maximum Duty Cycle Switch VCESAT Shutdown Pin Current LBI Threshold Voltage
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LT1307/LT1307B
ELECTRICAL CHARACTERISTICS
Industrial Grade - 40C to 85C. VIN = 1.1V, VSHDN = VIN, LT1307/LT1307B unless otherwise noted.
SYMBOL IQ PARAMETER Quiescent Current CONDITIONS VFB = 1.3V, Not Switching (LT1307) VFB = 1.3V, Not Switching (LT1307B) VSHDN = 0V VFB = VREF 1V VIN 2V (- 40C) 1V VIN 2V (85C) 2V VIN 5V - 40C 85C
q q q q q q
MIN
TYP 50 1 1
MAX 100 1.8 3 1.245 100 1.1 3.2 0.8 1.2 1.0 5 65
UNITS A mA A V nA %/V %/V %/V V V V mhos V/V V/V
VFB IB
Feedback Voltage FB Pin Bias Current (Note 2) Reference Line Regulation
1.195 10
1.22 27 0.6
q
0.3 1.1 0.8
Minimum Input Voltage Input Voltage Range gm AV fOSC Error Amp Transconductance Error Amp Voltage Gain Switching Frequency Maximum Duty Cycle Switch Current Limit (Note 3) Switch VCESAT Burst Mode Operation Switch Current Limit (LT1307 Only) Shutdown Pin Current LBI Threshold Voltage LBO Output Low LBO Leakage Current LBI Input Bias Current (Note 4) Low-Battery Detector Gain Switch Leakage Current
I = 5A - 40C 85C
q
25 35 30
35
q
500 80 75
600 84 80 0.6 0.5 250 330 100 50
750
kHz % %
- 40C 85C DC = 40% DC = 75% ISW = 500mA, VIN = 1.2V (- 40C) ISW = 500mA (85C) L = 10H L = 22H VSHDN = VIN VSHDN = 0V ISINK = 10A VLBI = 250mV, VLBO = 5V VLBI = 150mV 1M Load (- 40C) 1M Load (85C) VSW = 5V
q q q q q q q q
1.25 350 400
A A mV mV mA mA
2.5 - 1.5 186 200 0.1 0.1 5 1000 400 6000 0.01
4.0 - 2.5 210 0.25 0.3 30
A A mV V A nA V/V V/V
3
A
The q denotes specifications which apply over the full operating temperature range. Note 1: Specifications for commercial (C) grade devices are guaranteed but not tested at - 20C. MS8 package devices are designed for and intended to meet commercial temperature range specifications but are not tested at - 20C or 0C.
Note 2: Bias current flows into FB pin. Note 3: Switch current limit guaranteed by design and/or correlation to static tests. Duty cycle affects current limit due to ramp generator. Note 4: Bias current flows out of LBI pin. Note 5: The LT1307 will withstand continuous application of 1.6V applied to the GND pin while VIN and SW are grounded.
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LT1307/LT1307B TYPICAL PERFORMANCE CHARACTERISTICS
5V Output Efficiency, Circuit of Figure 1 (LT1307)
90
90 80
80
EFFICIENCY (%)
EFFICIENCY (%)
60 50 40 30 20
EFFICIENCY (%)
70
VIN = 1.00V VIN = 1.25V VIN = 1.5V
60
50 0.1
10 1 LOAD CURRENT (mA)
Quiescent Current vs Temperature
80 70
FEEDBACK BIAS CURRENT (nA)
QUIESCENT CURRENT (A)
50 40 30 20 10 0 -50 -25 50 25 TEMPERATURE (C) 0 75 100
LBI BIAS CURRENT (nA)
60
Quiescent Current in Shutdown
10 10
QUIESCENT CURRENT (A)
8
SHUTDOWN PIN CURRENT (A)
6
12
VCESAT (mV)
4
2
0 0 1 3 2 INPUT VOLTAGE (V) 4 5
UW
LT1307 * G01 LT1307 * TPC04 LT1307 * TPC07
3.3V Output Efficiency, Circuit of Figure 1 (LT1307B)
90 80 70 VIN = 1.25V VIN = 1V
5V Output Efficiency, Circuit of Figure 1 (LT1307B)
VIN = 1.5V
70
VIN = 1V 60 50 40 30 20 10 VIN = 1.25V
VIN = 1.5V
100 200
10 0.1
1 10 LOAD CURRENT (mA)
100
1307 G02
0.1
1 10 LOAD CURRENT (mA)
100
LT1307 * TPC03
Feedback Bias Current vs Temperature
50 VIN = 1.1V 40
LBI Bias Current vs Temperature
16 14 12 10 8 6 4 2
30
20
10
0 -50
-25
0 25 50 TEMPERATURE (C)
75
100
0 -50
-25
0 50 25 TEMPERATURE (C)
75
100
LTC1307 * TPC05
LT1307 * TPC06
Shutdown Pin Bias Current vs Input Voltage
500
Switch VCESAT vs Current
TA = 25C
16
400
300
8
200
4
100
0 0 1 3 2 INPUT VOLTAGE (V) 4 5
0
0
100
500 200 300 400 SWITCH CURRENT (mA)
600
LT1307 * TPC08
LT1307 * TPC09
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LT1307/LT1307B TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage vs Temperature
1.230 1.225
210 208
REFERENCE VOLTAGE (mV)
FEEDBACK VOLTAGE (V)
1.220 1.215 1.210 1.205 1.200 -50
FREQUENCY (kHz)
-25
0 25 50 TEMPERATURE (C)
Transient Response (LT1307)
VOUT 200mV/DIV AC COUPLED IL 200mA/DIV ILOAD 55mA 5mA VIN = 1.25V VOUT = 3.3V 500s/DIV
1307 G13
Load Regulation (LT1307)
VOUT 50mV/DIV DC COUPLED OFFSET ADDED
VIN = 1V VOUT = 3.3V
ILOAD 20mA/DIV
Load Regulation (LT1307)
VOUT 50mV/DIV AC COUPLED VSW 5V/DIV IL 100mA/DIV VIN = 1.15V VOUT = 5V ILOAD 10mA/DIV
1307 G19
VOUT 50mV/DIV DC COUPLED OFFSET ADDED
6
UW
75
LT1307 * TPC10
LBI Reference vs Temperature
900
Oscillator Frequency vs Input Voltage
206 204 202 200 198 196 194 192
800 85C 700
25C
-40C 600
500
100
190 -50
400
-25 25 50 0 TEMPERATURE (C) 75 100
1
2
3 INPUT VOLTAGE (V)
4
5
LT1307 * TPC12
LT1307 * TPC11
Transient Response (LT1307B)
VOUT 200mV/DIV AC COUPLED IL 200mA/DIV ILOAD 55mA 5mA VIN = 1.25V VOUT = 3.3V 500s/DIV
1307 G14
Load Regulation (LT1307)
VOUT 50mV/DIV DC COUPLED OFFSET ADDED
VIN = 0.92V VOUT = 3.3V
ILOAD 10mA/DIV
1307 G15
Load Regulation (LT1307)
Load Regulation (LT1307)
VOUT 50mV/DIV DC COUPLED OFFSET ADDED
VOUT 50mV/DIV DC COUPLED OFFSET ADDED
1307 G16
VIN = 1.15V VOUT = 3.3V
ILOAD 20mA/DIV
1307 G17
VIN = 1V VOUT = 5V
ILOAD 10mA/DIV
1307 G18
Circuit Operation, L = 10H (LT1307)
VOUT 50mV/DIV AC COUPLED VSW 5V/DIV IL 100mA/DIV VIN = 1.25V VOUT = 5V ILOAD = 1.5mA 100s/DIV
1307 G20
Circuit Operation, L = 22H (LT1307)
VIN = 1.25V VOUT = 5V ILOAD = 1.5mA
100s/DIV
1307 G21
LT1307/LT1307B
PIN FUNCTIONS
VC (Pin 1): Compensation Pin for Error Amplifier. Connect a series RC from this pin to ground. Typical values are 100k and 680pF. Minimize trace area at VC. FB (Pin 2): Feedback Pin. Reference voltage is 1.22V. Connect resistor divider tap here. Minimize trace area at FB. Set VOUT according to: VOUT = 1.22V(1 + R1/R2). SHDN (Pin 3): Shutdown. Ground this pin to turn off switcher. Must be tied to VIN (or higher voltage) to enable switcher. Do not float the SHDN pin. GND (Pin 4): Ground. Connect directly to local ground plane. SW (Pin 5): Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to keep EMI down. VIN (Pin 6): Supply Pin. Must have 1F ceramic bypass capacitor right at the pin, connected directly to ground. LBI (Pin 7): Low-Battery Detector Input. 200mV reference. Voltage on LBI must stay between ground and 700mV. LBO (Pin 8): Low-Battery Detector Output. Open collector, can sink 10A. A 1M pull-up is recommended.
BLOCK DIAGRAM
VIN 6
VOUT R1 (EXTERNAL) FB R2 (EXTERNAL)
FB 2
RAMP GENERATOR
600kHz OSCILLATOR
*HYSTERESIS IN LT1307 ONLY
Figure 2. LT1307/LT1307B Block Diagram
+
+ +
-
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VIN R5 40k R6 40k
+
gm
VC 1 LBI
SHDN SHUTDOWN 3
-
Q1 Q2 x10 R3 30k R4 140k ERROR AMPLIFIER
+
* BIAS ENABLE 200mV
7
+ -
A4
LBO 8
-
A1
COMPARATOR FF R A2 S Q DRIVER
SW 5 Q3
+
A=3 0.15
-
4 GND
1307 F02
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LT1307/LT1307B
APPLICATIONS INFORMATION
OPERATION The LT1307 combines a current mode, fixed frequency PWM architecture with Burst Mode micropower operation to maintain high efficiency at light loads. Operation can best be understood by referring to the block diagram in Figure 2. Q1 and Q2 form a bandgap reference core whose loop is closed around the output of the converter. When VIN is 1V, the feedback voltage of 1.22V, along with an 80mV drop across R5 and R6, forward biases Q1 and Q2's base collector junctions to 300mV. Because this is not enough to saturate either transistor, FB can be at a higher voltage than VIN. When there is no load, FB rises slightly above 1.22V, causing VC (the error amplifier's output) to decrease. When VC reaches the bias voltage on hysteretic comparator A1, A1's output goes low, turning off all circuitry except the input stage, error amplifier and lowbattery detector. Total current consumption in this state is 50A. As output loading causes the FB voltage to decrease, A1's output goes high, enabling the rest of the IC. Switch current is limited to approximately 100mA initially after A1's output goes high. If the load is light, the output voltage (and FB voltage) will increase until A1's output goes low, turning off the rest of the LT1307. Low frequency ripple voltage appears at the output. The ripple frequency is dependent on load current and output capacitance. This Burst Mode operation keeps the output regulated and reduces average current into the IC, resulting in high efficiency even at load currents of 100A or less. If the output load increases sufficiently, A1's output remains high, resulting in continuous operation. When the LT1307 is running continuously, peak switch current is controlled by VC to regulate the output voltage. The switch is turned on at the beginning of each switch cycle. When the summation of a signal representing switch current and a ramp generator (introduced to avoid subharmonic oscillations at duty factors greater than 50%) exceeds the VC signal, comparator A2 changes state, resetting the flipflop and turning off the switch. Output voltage increases as switch current is increased. The output, attenuated by a resistor divider, appears at the FB pin, closing the overall loop. Frequency compensation is provided by an external series RC network connected between the VC pin and ground. Low-battery detector A4's open collector output (LBO) pulls low when the LBI pin voltage drops below 200mV. There is no hysteresis in A4, allowing it to be used as an amplifier in some applications. The entire device is disabled when the SHDN pin is brought low. To enable the converter, SHDN must be at VIN or at a higher voltage. The LT1307B differs from the LT1307 in that there is no hysteresis in comparator A1. Also, the bias point on A1 is set lower than on the LT1307 so that switching can occur at inductor current less than 100mA. Because A1 has no hysteresis, there is no Burst Mode operation at light loads and the device continues switching at constant frequency. This results in the absence of low frequency output voltage ripple at the expense of efficiency. The difference between the two devices is clearly illustrated in Figures 3 and 4. The top two traces in Figure 3 show an LT1307/LT1307B circuit, using the components indicated in Figure 1, set to a 5V output. Input voltage is 1.25V. Load current is stepped from 1mA to 41mA for both circuits. Low frequency Burst Mode operation voltage ripple is observed on Trace A, while none is observed on
LT1307 VOUT TRACE A 500mV/DIV AC COUPLED LT1307B VOUT 500mV/DIV AC COUPLED IL 41mA 1mA VIN = 1.25V VOUT = 5V 1ms/DIV
1307 F03
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TRACE B
Figure 3. LT1307 Exhibits Burst Mode Operation Ripple at 1mA Load, LT1307B Does Not
LT1307 VOUT TRACE A 200mV/DIV AC COUPLED LT1307B VOUT 200mV/DIV AC COUPLED IL 45mA 5mA VIN = 1.5V VOUT = 5V 500s/DIV
1307 F04
TRACE B
Figure 4. At Higher Loading and a 1.5V Supply, LT1307 Again Exhibits Burst Mode Operation Ripple at 5mA Load, LT1307B Does Not
LT1307/LT1307B
APPLICATIONS INFORMATION
Trace B. Similarly, Figure 4 details the two circuits with a load step from 5mA to 45mA with a 1.5V input. The LT1307B also can be used in lower current applications where a clean, low ripple output is needed. Figure 5 details transient response of a single cell to 3.3V converter, using an inductor value of 100H. This high inductance minimizes ripple current, allowing the LT1307B to regulate without skipping cycles. As the load current is stepped from 5mA to 10mA, the output voltage responds cleanly. Note that the VC pin loop compensation has been made more conservative (increased C, decreased R).
VOUT 100mV/DIV AC COUPLED
OUTPUT NOISE VOLTAGE (dBmVRMS)
IL 20mA/DIV IL 10mA 5mA VIN = 1.25V VOUT = 3.3V 1ms/DIV
1307 F05
Figure 5. Increasing L to 100H, Along with RC = 36k, CC = 20nF and COUT = 10F, Low Noise Performance of LT1307B Can Be Realized at Light Loads of 5mA to 10mA
OUTPUT NOISE VOLTAGE (dBmVRMS)
At light loads, the LT1307B will begin to skip alternate cycles. The load point at which this occurs can be decreased by increasing the inductor value. However, output ripple will continue to be significantly less than the LT1307 output ripple. Further, the LT1307B can be forced into micropower mode, where IQ falls from 1mA to 50A by pulling down VC to 0.3V or less externally. DC/DC CONVERTER NOISE CONSIDERATIONS Switching regulator noise is a significant concern in many communications systems. The LT1307 is designed to keep noise energy out of the sensitive 455kHz band at all load levels while consuming only 60W to 100W at no load. At light load levels, the device is in Burst Mode, causing low frequency ripple to appear at the output. Figure 6 details spectral noise directly at the output of Figure 1's circuit in a 1kHz to 1MHz bandwidth. The converter supplies a 5mA load from a 1.25V input. The Burst Mode fundamental at 5.1kHz and its harmonics are
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quite evident, as is this particular device's 575kHz switching frequency (nominal switching frequency is 600kHz). Note, however, the absence of significant energy at 455kHz. Figure 7's plot reduces the frequency span from 255kHz to 655kHz with a 455kHz center. Burst Mode low frequency ripple creates sidebands around the 575kHz switching fundamental. These sidebands have low signal amplitude at 455kHz, measuring - 55dBmVRMS. As load current is further reduced, the Burst Mode frequency decreases. This spaces the sidebands around the switching frequency closer together, moving spectral energy further
40 30 20 10 0 -10 -20 -30 -40 -50 -60 1 10 100 FREQUENCY (kHz) 1000
1307 F06
RBW = 100Hz
Figure 6. Spectral Noise Plot of 3.3V Converter Delivering 5mA Load. Burst Mode Fundamental at 5.1kHz is 23dBmVRMS or 14mVRMS
-20 -25 -30 -35 -40 -45 -50 -55 -60 -65 -70 255 455 FREQUENCY (kHz) 655
1307 F07
RBW = 100Hz
Figure 7. Span Centered at 455kHz Shows - 55dBmVRMS (1.8VRMS) at 455kHz. Burst Mode Creates Sidebands 5.1kHz Apart Around the Switching Frequency Fundamental of 575kHz
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LT1307/LT1307B
APPLICATIONS INFORMATION
away from 455kHz. Figure 8 shows the noise spectrum of the converter with the load increased to 20mA. The LT1307 shifts out of Burst Mode operation, eliminating low frequency ripple. Spectral energy is present only at the switching fundamental and its harmonics. Noise voltage measures - 5dBmVRMS or 560VRMS at the 575kHz switching frequency, and is below - 60dBmVRMS for all other frequencies in the range. By combining Burst Mode with fixed frequency operation, the LT1307 keeps noise away from 455kHz.
0
RBW = 100Hz
OUTPUT NOISE VOLTAGE (dBmVRMS)
-10 -20 -30 -40 -50 -60 -70 -80 -90 455 FREQUENCY (kHz) 655
1307 F08
-100 255
Figure 8. With Converter Delivering 20mA, Low Frequency Sidebands Disappear. Noise is Present Only at the 575kHz Switching Frequency
0
OUTPUT VOLTAGE NOISE (dBmVRMS)
-10 -20 -30 -40 -50 -60 -70 -80 -90 -100 205 455 FREQUENCY (kHz) 705
LT1307 * F09
Figure 9. LT1307B at 5mA Load Shows No Audio Components or Sidebands About Switching Frequency, 333kHz Fundamental Amplitude is -10dBmV, or 316VRMS
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To eliminate the low frequency noise of Figure 6, the LT1307 can be replaced with the LT1307B. Figure 9 details the spectral noise at the output of Figure 1's circuit using an LT1307B at 5mA load. Although spectral energy is present at 333kHz due to alternate pulse skipping, all Burst Mode operation spectral components are gone. Alternate pulse skipping can be eliminated by increasing inductance. FREQUENCY COMPENSATION Obtaining proper values for the frequency compensation network is largely an empirical, iterative procedure, since variations in input and output voltage, topology, capacitor value and ESR, and inductance make a simple formula elusive. As an example, consider the case of a 1.25V to 3.3V boost converter supplying 50mA. To determine optimum compensation, the circuit is built and a transient load is applied to the circuit. Figure 10 shows the setup.
10H MBR0520L VOUT VIN SW SHDN LT1307 VC FB GND R 590k C 50 10F*
1M
66
1F 1.25V
3300
1307 * F10
*CERAMIC
Figure 10. Boost Converter with Simulated Load
Figure 11a details transient response without compensation components. Although the output ripple voltage at a 1mA load is low, allowing the error amplifier to operate wideband results in excessive ripple at a 50mA load. Some kind of loop stabilizing network is obviously required. A 100k/22nF series RC is connected to the VC pin, resulting in the response pictured in Figure 11b. The output settles in about 7ms to 8ms. This may be acceptable, but we can do better. Reducing C to 2nF gives Figure 11c's response. This is clearly in the right direction. After another order of magnitude reduction, Figure 11d's response shows some
LT1307/LT1307B
APPLICATIONS INFORMATION
VOUT 200mV/DIV AC COUPLED VOUT 200mV/DIV AC COUPLED
IL 51mA 1mA 5ms/DIV
1307 F11a
Figure 11a. VC Pin Left Unconnected. Output Ripple Voltage is 300mVP-P Under Load
VOUT 200mV/DIV AC COUPLED
IL 51mA 1mA 1ms/DIV
1307 F11a
Figure 11c. Reducing C to 2nF Speeds Up Response, Although Still Overdamped
VOUT 200mV/DIV AC COUPLED
IL 51mA 1mA 1ms/DIV
1307 F11b
Figure 11e. A 100k/680pF RC Provides Optimum Settling Time with No Ringing
underdamping. Now settling time is about 300s. Increasing C to 680pF results in the response shown in Figure 11e. This response has minimum settling time with no overshoot or underdamping. Converters using a 2-cell input need more capacitance at the output. This added capacitance moves in the output
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IL 51mA 1mA 5ms/DIV
1307 F11b
Figure 11b. Inclusion of a 100k/22nF Series RC on VC Pin Results in Overdamped Stable Response
VOUT 200mV/DIV AC COUPLED
IL 51mA 1mA 500s/DIV
1307 F11b
Figure 11d. A 100k/200pF Series RC Shows Some Underdamping
pole, requiring added C at the VC pin network to prevent loop oscillation. Observant readers will notice R has been set to 100k for all the photos in Figure 11. Usable R values can be found in the 10k to 500k range, but after too many trips to the resistor bins, 100k wins.
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LT1307/LT1307B
APPLICATIONS INFORMATION
LAYOUT HINTS The LT1307 switches current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layouts. Figure 12 shows recommended component placement. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor CIN must be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from CIN to VIN will cause problems such as inability to regulate or oscillation. A 1F ceramic bypass capacitor is the only input capacitance required provided the battery has a low inductance path to the circuit. The battery itself provides the bulk capacitance the device requires for proper operation. If the battery is located some distance from the circuit, an additional input capacitor may be required. A 100F aluminum electrolytic unit works well in these cases. This capacitor need not have low ESR.
RC CC KEEP TRACES OR LEADS SHORT! 1 2 R1 R2 3 4 LT1307 8 7 6 5 CIN D COUT
1306 F12
AA CELL
L
VOUT
GROUND
Figure 12. Recommended Component Placement. Traces Carrying High Current Are Direct. Trace Area at FB Pin and VC Pin is Kept Low. Lead Length to Battery Should Be Kept Short
OPERATION FROM A LABORATORY POWER SUPPLY If a lab supply is used, the leads used to connect the circuit to the supply can have significant inductance at the LT1307's switching frequency. As in the previous situation, an electrolytic capacitor may be required at the circuit in order to reduce the AC impedance of the input sufficiently. An alternative solution would be to attach the circuit directly to the power supply at the supply terminals, without the use of leads. The power supply's output capacitance will then provide the bulk capacitance the LT1307 circuit requires.
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COMPONENT SELECTION Inductors Inductors appropriate for use with the LT1307 must possess three attributes. First, they must have low core loss at 600kHz. Most ferrite core units have acceptable losses at this switching frequency. Inexpensive iron powder cores should be viewed suspiciously, as core losses can cause significant efficiency penalties at 600kHz. Second, the inductor must handle current of 500mA without saturating. This places a lower limit on the physical size of the unit. Molded chokes or chip inductors usually do not have enough core to support 500mA current and are unsuitable for the application. Lastly, the inductor should have low DCR (copper wire resistance) to prevent efficiency-killing I2R losses. Linear Technology has identified several inductors suitable for use with the LT1307. This is not an exclusive list. There are many magnetics vendors whose components are suitable for use. A few vendor's components are listed in Table 1.
Table 1. Inductors Suitable for Use with the LT1307
PART LQH3C100 DO1608-103 CD43-100 CD54-100 CTX32CT-100 VALUE 10H 10H 10H 10H 10H MAX DCR 0.57 0.16 0.18 0.10 0.50 MFR Murata-Erie Coilcraft Sumida Sumida Coiltronics HEIGHT (mm) 2.0 3.0 3.2 4.5 2.2 Best Efficiency 1210 Footprint COMMENT Smallest Size
Capacitors For single cell applications, a 10F ceramic output capacitor is generally all that is required. Ripple voltage in Burst Mode can be reduced by increasing output capacitance. For 2- and 3-cell applications, more than 10F is needed. For a typical 2-cell to 5V application, a 47F to 100F low ESR tantalum capacitor works well. AVX TPS series (100% surge tested) or Sprague (don't be vague--ask for Sprague) 594D series are both good choices for low ESR capacitors. Alternatively, a 10F ceramic in parallel with a low cost (read high ESR) electrolytic capacitor, either tantalum or aluminum, can be used instead. For through hole applica-
LT1307/LT1307B
APPLICATIONS INFORMATION
tions where small size is not critical, Panasonic HFQ series aluminum electrolytic capacitors have been found to perform well.
Table 2. Vendor Telephone Numbers
VENDOR Coilcraft Marcon Murata-Erie Sumida Tokin AVX Sprague Coiltronics COMPONENTS Inductors Capacitors Inductors, Capacitors Inductors Capacitors Capacitors Capacitors Inductors TELEPHONE (708) 639-6400 (708) 913-9980 (404) 436-1300 (847) 956-0666 (408) 432-8020 (207) 282-5111 (603) 224-1961 (407) 241-7876
Q1
1307 F13
Diodes Most of the application circuits on this data sheet specify the Motorola MBR0520L surface mount Schottky diode. This 0.5A, low drop diode complements the LT1307 quite well. In lower current applications, a 1N4148 can be used, although efficiency will suffer due to the higher forward drop. This effect is particularly noticeable at low output voltages. For higher voltage output applications, such as LCD bias generators, the extra drop is a small percentage of the output voltage so the efficiency penalty is small. The low cost of the 1N4148 makes it attractive wherever it can be used. In through hole applications the 1N5818 is the all around best choice. SHUTDOWN PIN The LT1307 has a Shutdown pin (SHDN) that must be grounded to shut the device down or tied to a voltage equal or greater than VIN to operate. The shutdown circuit is shown in Figure 13. Note that allowing SHDN to float turns on both the startup current (Q2) and the shutdown current (Q3) for VIN > 2VBE. The LT1307 doesn't know what to do in this situation and behaves erratically. SHDN voltage above VIN is allowed. This merely reverse-biases Q3's base emitter junction, a benign condition.
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VIN Q3 R2 400k SHDN 200k START-UP CURRENT Q2 SHUTDOWN CURRENT
Figure 13. Shutdown Circuit
LOW-BATTERY DETECTOR The LT1307's low-battery detector is a simple PNP input gain stage with an open collector NPN output. The negative input of the gain stage is tied internally to a 200mV 5% reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward. Figure 14 details hookup. R1 and R2 need only be low enough in value so that the bias current of the LBI pin doesn't cause large errors. For R2, 100k is adequate. The 200mV reference can also be accessed as shown in Figure 15.
3.3V R1 LBI R2 100k VIN LT1307 1M LBO TO PROCESSOR
+ -
200mV INTERNAL REFERENCE GND
1307 F14
R1 =
VLB - 200mV 2A
Figure 14. Setting Low-Battery Detector Trip Point
200k 2N3906 VREF 200mV 10k LBO
VIN LT1307
+
10F
LBI GND
1307 F15
Figure 15. Accessing 200mV Reference
13
LT1307/LT1307B
APPLICATIONS INFORMATION
REVERSE BATTERY CONSIDERATIONS The LT1307 is built on a junction-isolated bipolar process. The p-type substrate is connected to the GND pin of the LT1307. Substrate diodes, normally reverse-biased, are present on the SW pin and the VIN pin as shown in Figure 16. When the battery polarity is reversed, these diodes conduct, as illustrated in Figure 17. With a single AA or AAA cell, several hundred milliamperes flow in the circuit. The LT1307 can withstand this current without damage. In laboratory tests, the LT1307 performed without degradation after sustaining polarity reversal for the life of a single AA alkaline cell. When using a 2- or 3-cell supply, an external protection diode is recommended as shown in Figure 18. When the battery polarity is reversed, the 1N4001 conducts, limiting reverse voltage across the LT1307 to a single diode drop. This arrangement will quickly deplete the cells' energy, but it does prevent the LT1307 from excessive power dissipation and potential damage.
1.5V VIN 1 CELL D1 LT1307 D2 SW
1 CELL
Q1 GND
1307 F16
Figure 16. LT1307 Showing Internal Substrate Diodes D1 and D2. In Normal Operation Diodes are Reverse-Biased
2 OR 3 CELLS
Figure 18. 1N4001 Diode Protects LT1307 from Excessive Power Dissipation When a 2- or 3-Cell Battery is Used
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- 1.5V CURRENT FLOW VIN D1 LT1307 D2 SW
Q1 GND
1307 F17
Figure 17. When Cell Is Reversed Current Flows through D1 and D2
1N4001
VIN LT1307 GND
SW
1307 F18
LT1307/LT1307B
TYPICAL APPLICATIONS N
Externally Controlled Burst Mode Operation
L1 10H VOUT 1F CERAMIC 300k VIN VC LT1307B LBO SHDN R1 10M LBI GND R2 499k C2* 10F CERAMIC SW FB R5 590k R3 698k R4 1M MBR0520
2 CELLS
100k M1 2N7002 1nF
GROUND = HIGH POWER/LOW NOISE FLOAT = Burst Mode OPERATION
This circuit overcomes the limitation of load-based transitioning between Burst Mode operation and constant switching mode by adding external control. If M1's gate is grounded by an external open-drain signal, the converter functions normally in constant switching mode, delivering 3.3V. Output noise is low, however efficiency at loads less than 1mA is poor due to the 1mA supply current of the LT1307B. If M1's gate is allowed to float, the low-battery
VOUT 500mV/DIV
IL
10mA 100A 0.2s/DIV
1307 F20
This photo details output voltage as the circuit is switched between the two modes. Load current is 100A in Burst Mode operation; 10mA in constant switching mode.
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VOUT 3.3V 200mA
+ C1
100F
SHUTDOWN
1307 F19
3.0V IN LOW-POWER Burst Mode OPERATION C1 =AVX TPSC107K006R0150 L1 = COILCRAFT DO1608-103 SUMIDA CD43-100 *C2 OPTIONAL: REDUCES OUTPUT RIPPLE CAUSED BY C1'S ESR
detector now drives the VC pin. R3 and R2 set the output to 3V by allowing M1's gate to go to VOUT until the output voltage drops below 3V. R1 adds hysteresis, resulting in low-frequency Burst Mode operation ripple voltage at the output. By pulling the VC pin below a VBE, quiescent current of the LT1307B drops to 60A, resulting in acceptable efficiency at loads in the 100A range.
VOUT 100mV/DIV
IL100mA 10mA
2ms/DIV
1307 F21
This photo shows transient response in constant switching mode with a 10mA to 100mA stepped load. Output ripple at the switching frequency can be reduced considerably by adding a 10F ceramic capacitor in parallel with the 100F tantalum.
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LT1307/LT1307B
TYPICAL APPLICATIONS N
Low Cost 2-Cell to 5V
VIN 1.4V TO 3.3V L1 10H C1* 220F 6.3V 0.1F 100k 4700pF
1307 TA02
+
C1, C2: PANASONIC ECA0JFQ221 (DIGI-KEY P5604-ND) L1: SUMIDA CD43-100
VIN 2.1V TO 4.8V
L1: COILTRONICS CTX10-1 OR 2 MURATA ERIE LQH3C100 EFFICIENCY 70% TO 73%
Constant Current NiCd Battery Charger with Overvoltage Protection for Acknowledge-Back Pagers
L1 10H 2.2F CERAMIC
VIN 1.8V TO 1V 1F
1 CELL AA OR AAA 47k 2200pF L1: COILTRONICS CTX10-1
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1N5818 5V 100mA C2 220F 6.3V
+
VIN LT1307 SHDN GND 323k FB SW 1M 0.1F
Step-Up/Step-Down Converter
L1 10H 2.2F CERAMIC MBR0520 3.3V 100mA 10F CERAMIC
*
SW
1F CERAMIC 3 CELLS 100k
VIN LT1307 VC SHDN
1.02M FB GND 608k
*
L1*
1000pF SHDN
1307 TA03
MBR0520L
3 VIN VC
*
2
15mA
SW FB LT1307
1M OVERVOLTAGE 323k PROTECTION
*1
4 -100mV 280k 3V 6.7 30k
1F CERAMIC 3 CELLS NiCd
LBO SHDN 1 = CHARGE 0 = SHUTDOWN
LBI GND
200mV 1nF
1307 TA04
LT1307/LT1307B
TYPICAL APPLICATIONS N
Single Cell Powered Constant Current LED Driver
VIN 100k Q1 2N3906 AA CELL VIN LBO LT1307B VC SHDN LBI GND 40mA R2 22k R1 5.1
1307 TA05
L1:MURATA-ERIE LQH3C100K04 D1:1N4148 VIN C1, C2:CERAMIC D2, D3:LUMEX SSL-X100133SRC/4 "MEGA-BRITE" RED LED OR PANASONIC LNG992CF9 HIGH BRIGHTNESS BLUE LED
VIN 3V TO 5.5V
+
SHUTDOWN 1N4148 47k 2000pF D1: MOTOROLA MBR0520L L1: MURATA-ERIE LQH3C100K04
VIN 1V TO 5V
T1 1:12 1F CERAMIC 3
1
*
SHUTDOWN
100k
VIN SW SHDN FB LT1307 VC GND
1000pF
1307 TA06
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*
L1 10H
D1
D2 SW FB NC C1 1F CERAMIC
+ C2 1F CERAMIC
C3 22F 100k
ON/OFF
Flash Memory VPP Supply
L1 10H 1F TANTALUM 0.33F VIN SW SHDN LT1307 VC GND FB 232k 1%
1307 TA09
D1
12V/30mA FROM 3V 12V/60mA FROM 5V ~250mVP-P RIPPLE 10pF 2M 1% 0.33F CERAMIC x2
High Voltage Flyback Converter
OPTIONAL DOUBLER 0.01F 1N4148 T1: DALE LPE3325-A190, n = 12 (605) 665-9301 4 VOUT = 1.22V 1 + 6 2VOUT 0.1F
R1 VOUT R2 240k 1%
0.1F
MAXIMUM DUTY CYCLE: 80% FOR FLYBACK, VOUT = DC n(VIN - VSW) 1 - DC FOR 1VIN, MAXIMUM VOUT = 0.8 12(1 - 0.2) 37V 1 - 0.8 FOR 2VIN, MAXIMUM VOUT 85V. HIGHER VOLTAGES ACHIEVED WITH CAPACITIVE DOUBLER OR TRIPLER NO SNUBBER REQUIRED WITH SPECIFIED TRANSFORMER AND VIN < 5V
()
R1 R2
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LT1307/LT1307B
TYPICAL APPLICATIONS N
Single Cell CCFL Power Supply
1.5V
PACKAGE DESCRIPTION
0.007 (0.18)
0 - 6 TYP SEATING PLANE
0.025 (0.65) TYP * DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
0.021 0.004 (0.53 0.01)
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6 T1 4 5 3 2 1.5V
10
47pF 3kV
1
100 1.5V Q1
C1 0.1F
Q2
CCFL
D1
L1 33H
1F CERAMIC 1 CELL
VIN
SW 10k
1N4148
LT1307B SHDN FB GND VC 0.1F
1N4148 1k 0.1F 10k DIMMING
1307 TA08
1 = OPERATE 0 = SHUTDOWN
C1: WIMA MKP-20 D1: MOTOROLA MBR0520L L1: SUMIDA CD54-330 T1: COILTRONICS CTX110611 Q1, Q2: ZETEX FZT-849
Dimensions in inches (millimeters) unless otherwise noted.
MS8 Package 8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 0.004* (3.00 0.10) 8 76 5
0.040 0.006 (1.02 0.15)
0.006 0.004 (0.15 0.10)
0.012 (0.30)
0.192 0.004 (4.88 0.10)
0.118 0.004** (3.00 0.10)
1
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MSOP08 0596
LT1307/LT1307B
PACKAGE DESCRIPTION
0.300 - 0.325 (7.620 - 8.255)
0.009 - 0.015 (0.229 - 0.381)
0.065 (1.651) TYP 0.005 (0.127) MIN 0.100 0.010 (2.540 0.254) 0.125 (3.175) MIN 0.018 0.003 (0.457 0.076) 0.015 (0.380) MIN
(
+0.025 0.325 -0.015 8.255 +0.635 -0.381
)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.016 - 0.050 0.406 - 1.270 *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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.
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Dimensions in inches (millimeters) unless otherwise noted.
N8 Package 8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
0.045 - 0.065 (1.143 - 1.651)
0.130 0.005 (3.302 0.127) 8
0.400* (10.160) MAX 7 6 5
0.255 0.015* (6.477 0.381)
1
2
3
4
N8 0695
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 8 7 6 5
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1
2
3
4
0.053 - 0.069 (1.346 - 1.752)
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
SO8 0695
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LT1307/LT1307B
TYPICAL APPLICATION
LCD Bias Generator
D1 -VOUT 0.1F D2 1F
1, 2 OR 3 CELLS
L1: 3.3H (1 CELL) 4.7 H (2 CELLS) 10 H (3 CELLS) SUMIDA CD43 MURATA-ERIE LQH3C COILCRAFT D01608 C1:1 F FOR +OUTPUT 0.01 F FOR - OUTPUT D1 TO D3: MBR0530 OR 1N4148
100k PWM IN 3.3V, 0% TO 100%
RELATED PARTS
PART NUMBER LTC(R)1163 LTC1174 LT1302 LT1304 LTC1440/1/2 LTC1516 LT1521 DESCRIPTION Triple High Side Driver for 2-Cell Inputs Micropower Step-Down DC/DC Converter High Output Current Micropower DC/DC Converter 2-Cell Micropower DC/DC Converter Ultralow Power Single/Dual Comparators with Reference 2-Cell to 5V Regulated Charge Pump Micropower Low Dropout Linear Regulator COMMENTS 1.8V Minimum Input, Drives N-Channel MOSFETs 94% Efficiency, 130A IQ, 9V to 5V at 300mA 5V/600mA from 2V, 2A Internal Switch, 200A IQ Low-Battery Detector Active in Shutdown 2.8A IQ, Adjustable Hysteresis 12A IQ, No Inductors, 5V at 50mA from 3V Input 500mV Dropout, 300mA Current, 12A IQ
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
+
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L1
D3 VOUT 16V TO 24V 5mA FROM 1 CELL 15mA FROM 2 CELLS 35mA FROM 3 CELLS 10pF FB GND 1M 215k
1307 TA07
VIN 1F LT1307 VC 100k SHDN
SW 3.3M
C1
4700pF
SHUTDOWN
3.3F
LT/GP 1196 7K * PRINTED IN THE USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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