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LT1308A/LT1308B High Current, Micropower Single Cell, 600kHz DC/DC Converters
FEATURES

DESCRIPTIO
5V at 1A from a Single Li-Ion Cell 5V at 800mA in SEPIC Mode from Four NiCd Cells Fixed Frequency Operation: 600kHz Boost Converter Outputs up to 34V Starts into Heavy Loads Automatic Burst ModeTM Operation at Light Load (LT1308A) Continuous Switching at Light Loads (LT1308B) Low VCESAT Switch: 300mV at 2A Pin-for-Pin Upgrade Compatible with LT1308 Lower Quiescent Current in Shutdown: 1A (Max) Improved Accuracy Low-Battery Detector Reference: 200mV 2% Available in 8-Lead SO and 14-Lead TSSOP Packages
The LT (R)1308A/LT1308B are micropower, fixed frequency step-up DC/DC converters that operate over a 1V to 10V input voltage range. They are improved versions of the LT1308 and are recommended for use in new designs. The LT1308A features automatic shifting to power saving Burst Mode operation at light loads and consumes just 140A at no load. The LT1308B features continuous switching at light loads and operates at a quiescent current of 2.5mA. Both devices consume less than 1A in shutdown. Low-battery detector accuracy is significantly tighter than the LT1308. The 200mV reference is specified at 2% at room and 3% over temperature. The shutdown pin enables the device when it is tied to a 1V or higher source and does not need to be tied to VIN as on the LT1308. An internal VC clamp results in improved transient response and the switch voltage rating has been increased to 36V, enabling higher output voltage applications. The LT1308A/LT1308B are available in the 8-lead SO and the 14-lead TSSOP packages.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

GSM/CDMA Phones Digital Cameras LCD Bias Supplies Answer-Back Pagers GPS Receivers Battery Backup Supplies Handheld Computers
TYPICAL APPLICATIO
L1 4.7H
Converter Efficiency
D1 5V 1A 95 90 85 V IN = 3.6V V IN = 4.2V
+
Li-Ion CELL
VIN C1 47F SHUTDOWN LBI LT1308B SHDN VC 47k 100pF
SW LBO FB GND R2 100k
EFFICIENCY (%)
R1* 309k
+
80 75 70 65 60 55 V IN = 1.5V
C2 220F
C1: AVX TAJC476M010 C2: AVX TPSD227M006 D1: IR 10BQ015
L1: MURATA LQH6C4R7 *R1: 887k FOR VOUT = 12V
1308A/B F01a
50 1 10 100 LOAD CURRENT (mA) 1000
1308A/B F01b
Figure 1. LT1308B Single Li-Ion Cell to 5V/1A DC/DC Converter
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V IN = 2.5V
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LT1308A/LT1308B ABSOLUTE AXI U RATI GS (Note 1)
Operating Temperature Range Commercial ............................................ 0C to 70C Extended Commerial (Note 2) ........... - 40C to 85C Industrial ........................................... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C VIN, SHDN, LBO Voltage ......................................... 10V SW Voltage ............................................... - 0.4V to 36V FB Voltage ....................................................... VIN + 1V VC Voltage ................................................................ 2V LBI Voltage ................................................. - 0.1V to 1V Current into FB Pin .............................................. 1mA
PI CO FIGURATIO
TOP VIEW VC 1 FB 2 SHDN 3 GND 4
8 7 6 5
LBO LBI VIN SW
S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 125C, JA = 190C/W
ORDER I FOR ATIO
LEAD FREE FINISH LT1308ACS8#PBF LT1308AIS8#PBF LT1308BCS8#PBF LT1308BIS8#PBF LT1308ACF#PBF LT1308BCF#PBF LEAD BASED FINISH LT1308ACS8 LT1308AIS8 LT1308BCS8 LT1308BIS8 LT1308ACF LT1308BCF TAPE AND REEL
LT1308ACS8#TRPBF LT1308AIS8#TRPBF LT1308BCS8#TRPBF LT1308BIS8#TRPBF LT1308ACF#TRPBF LT1308BCF#TRPBF TAPE AND REEL LT1308ACS8#TR LT1308AIS8#TR LT1308BCS8#TR LT1308BIS8#TR LT1308ACF#TR LT1308BCF#TR
Consult LTC Marketing for parts specified with wider operating temperature ranges. 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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TOP VIEW VC FB SHDN GND GND GND GND 1 2 3 4 5 6 7 14 LBO 13 LBI 12 VIN 11 VIN 10 SW 9 8 SW SW
F PACKAGE 14-LEAD PLASTIC TSSOP (Note 6) TJMAX = 125C, JA = 80C/W
NOT RECOMMENDED FOR NEW DESIGNS
Contact Linear Technology for Potential Replacement
PART MARKING 1308A 1308AI 1308B 1308BI LT1308ACF LT1308BCF PART MARKING 1308A 1308AI 1308B 1308BI LT1308ACF LT1308BCF
PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO 8-Lead Plastic SO 8-Lead Plastic SO 14-Lead Plastic TSSOP 14-Lead Plastic TSSOP PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO 8-Lead Plastic SO 8-Lead Plastic SO 14-Lead Plastic TSSOP 14-Lead Plastic TSSOP
TEMPERATURE RANGE 0C to 70C -40C to 85C 0C to 70C -40C to 85C 0C to 70C 0C to 70C TEMPERATURE RANGE 0C to 70C -40C to 85C 0C to 70C -40C to 85C 0C to 70C 0C to 70C
LT1308A/LT1308B
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER IQ Quiescent Current
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. Commercial Grade 0C to 70C. VIN = 1.1V, VSHDN = VIN, unless otherwise noted.
CONDITIONS Not Switching, LT1308A Switching, LT1308B VSHDN = 0V (LT1308A/LT1308B)
MIN
TYP 140 2.5 0.01
MAX 240 4 1 1.24 80 0.4 0.2 1
UNITS A mA A V nA %/V %/V V mhos V/V
VFB IB
Feedback Voltage FB Pin Bias Current Reference Line Regulation Minimum Input Voltage (Note 3) 1.1V VIN 2V 2V VIN 10V I = 5A VIN = 1.2V Duty Cyle = 30% (Note 4) ISW = 2A (25C, 0C), VIN = 1.5V ISW = 2A (70C), VIN = 1.5V VIN = 2.5V, Circuit of Figure 1 VSHDN = 1.1V VSHDN = 6V VSHDN = 0V
1.20
1.22 27 0.03 0.01 0.92 60 100

gm AV fOSC
Error Amp Transconductance Error Amp Voltage Gain Switching Frequency Maximum Duty Cycle Switch Current Limit Switch VCESAT Burst Mode Operation Switch Current Limit (LT1308A) Shutdown Pin Current

500 82 2
600 90 3 290 330 400
700 4.5 350 400
kHz % A mV mV mA

2 20 0.01 196 194 200 200 0.1 0.01 33 3000
5 35 0.1 204 206 0.25 0.1 100 10
A A A mV mV V A nA V/V A
LBI Threshold Voltage LBO Output Low LBO Leakage Current LBI Input Bias Current (Note 5) Low-Battery Detector Gain Switch Leakage Current VSW = 5V
ISINK = 50A VLBI = 250mV, VLBO = 5V VLBI = 150mV

0.01
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. Industrial Grade - 40C to 85C. VIN = 1.2V, VSHDN = VIN, unless otherwise noted.
SYMBOL PARAMETER IQ Quiescent Current CONDITIONS Not Switching, LT1308A Switching, LT1308B VSHDN = 0V (LT1308A/LT1308B) (Note 3) 1.1V VIN 2V 2V VIN 10V I = 5A

MIN
TYP 140 2.5 0.01
MAX 240 4 1 1.25 80 0.4 0.2 1
UNITS A mA A V nA %/V %/V V mhos V/V
VFB IB
Feedback Voltage FB Pin Bias Current Reference Line Regulation Minimum Input Voltage
1.19
1.22 27 0.05 0.01 0.92 60 100
gm AV
Error Amp Transconductance Error Amp Voltage Gain
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LT1308A/LT1308B
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER fOSC Switching Frequency Maximum Duty Cycle Switch Current Limit Switch VCESAT Burst Mode Operation Switch Current Limit (LT1308A) Shutdown Pin Current
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. Industrial Grade - 40C to 85C. VIN = 1.2V, VSHDN = VIN, unless otherwise noted.
CONDITIONS

MIN 500 82 2
TYP 600 90 3 290 330 400
MAX 750 4.5 350 400
UNITS kHz % A mV mV mA
Duty Cyle = 30% (Note 4) ISW = 2A (25C, - 40C), VIN = 1.5V ISW = 2A (85C), VIN = 1.5V VIN = 2.5V, Circuit of Figure 1 VSHDN = 1.1V VSHDN = 6V VSHDN = 0V

2 20 0.01 196 193 200 200 0.1 0.01 33 3000
5 35 0.1 204 207 0.25 0.1 100 10
A A A mV mV V A nA V/V A
LBI Threshold Voltage
LBO Output Low LBO Leakage Current LBI Input Bias Current (Note 5) Low-Battery Detector Gain Switch Leakage Current
ISINK = 50A VLBI = 250mV, VLBO = 5V VLBI = 150mV VSW = 5V

0.01
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 LT1308ACS8, LT1308ACF, LT1308BCS8 and LT1308BCF are designed, characterized and expected to meet the industrial temperature limits, but are not tested at - 40C and 85C. I grade devices are guaranteed over the -40C to 85C operating temperature range. Note 3: Bias current flows into FB pin.
Note 4: Switch current limit guaranteed by design and/or correlation to static tests. Duty cycle affects current limit due to ramp generator (see Block Diagram). Note 5: Bias current flows out of LBI pin. Note 6: Connect the four GND pins (Pins 4-7) together at the device. Similarly, connect the three SW pins (Pins 8-10) together and the two VIN pins (Pins 11, 12) together at the device.
TYPICAL PERFORMANCE CHARACTERISTICS
LT1308B 3.3V Output Efficiency
95 90 85
EFFICIENCY (%)
V IN = 1.8V
V IN = 2.5V
EFFICIENCY (%)
75 70 65 60 55 50 1
V IN = 1.2V
EFFICIENCY (%)
80
100 10 LOAD CURRENT (mA)
4
UW
1308A/B G01
LT1308A 3.3V Output Efficiency
95 90 85 80 75 70 65 60 55 50 1000 1 100 10 LOAD CURRENT (mA) 1000
1308A/B G02
LT1308A 5V Output Efficiency
95 90 V IN = 4.2V V IN = 3.6V
V IN = 1.8V
V IN = 2.5V
85 80 75 70 65 60 55 50 1
V IN = 1.2V
V IN = 1.5V
V IN = 2.5V
10 100 LOAD CURRENT (mA)
1000
1308A/B G03
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LT1308A/LT1308B TYPICAL PERFORMANCE CHARACTERISTICS
LT1308B 12V Output Efficiency
90 85 80 V IN = 5V
SWITCH VCESAT (mV)
CURRENT LIMIT (A)
V IN = 3.3V
EFFICIENCY (%)
75 70 65 60 55 50 1 100 10 LOAD CURRENT (mA) 1000
1308A/B G04
SHDN Pin Bias Current vs Voltage
50
SHDN PIN CURRENT (A)
40
BIAS CURRENT (nA)
25C 85C
40 30 FB 20
VREF (mV)
30
20
10
0 0 2 6 8 4 SHDN PIN VOLTAGE (V) 10
1308 G07
Oscillator Frequency vs Temperature
800 750
QUIESCENT CURRENT (A)
700
FREQUENCY (kHz)
650 600 550 500 450 400 -50 -2.5 0 25 50 TEMPERATURE (C) 75 100
140 130 120 110 100 -50 -25 0 25 50 TEMPERATURE (C) 75 100
1308 * G11
VFB (V)
UW
- 40C
1308 * G10
Switch Current Limit vs Duty Cycle
4.0
500
Switch Saturation Voltage vs Current
3.5
400 85C 300 25C -40C
3.0
200
2.5
100
2.0
0
20
60 40 DUTY CYCLE (%)
80
100
1308 * G05
0
0
1.0 0.5 1.5 SWITCH CURRENT (A)
2.0
1308 G06
FB, LBI Bias Current vs Temperature
80 70 60 LBI 50 200 199 198 197 196 -25 0 25 50 TEMPERATURE (C) 75 100
1308 * G08
Low Battery Detector Reference vs Temperature
203 202 201
10 0 -50
195 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1308 * G09
LT1308A Quiescent Current vs Temperature
180 170 160 150
Feedback Pin Voltage vs Temperature
1.25 1.24 1.23 1.22 1.21 1.20 1.19 1.18 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1308 * G12
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LT1308A/LT1308B
PIN FUNCTIONS
VC (Pin 1/Pin 1): Compensation Pin for Error Amplifier. Connect a series RC from this pin to ground. Typical values are 47k and 100pF. Minimize trace area at VC. FB (Pin 2/Pin 2): Feedback Pin. Reference voltage is 1.22V. Connect resistive divider tap here. Minimize trace area at FB. Set VOUT according to: VOUT = 1.22V(1 + R1/R2). SHDN (Pin 3/Pin 3): Shutdown. Ground this pin to turn off switcher. To enable, tie to 1V or more. SHDN does not need to be at VIN to enable the device. GND (Pin 4/Pins 4, 5, 6, 7): Ground. Connect directly to local ground plane. Ground plane should enclose all components associated with the LT1308. PCB copper connected to these pins also functions as a heat sink. For the TSSOP package, connect all pins to ground copper to get the best heat transfer. This keeps chip heating to a minimum.
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(SO/TSSOP)
SW (Pin 5/Pins 8, 9, 10): Switch Pins. Connect inductor/ diode here. Minimize trace area at these pins to keep EMI down. For the TSSOP package, connect all SW pins together at the package. VIN (Pin 6/Pins 11, 12): Supply Pins. Must have local bypass capacitor right at the pins, connected directly to ground. For the TSSOP package, connect both VIN pins together at the package. LBI (Pin 7/Pin 13): Low-Battery Detector Input. 200mV reference. Voltage on LBI must stay between -100mV and 1V. Low-battery detector does not function with SHDN pin grounded. Float LBI pin if not used. LBO (Pin 8/Pin 14): Low-Battery Detector Output. Open collector, can sink 50A. A 220k pull-up is recommended. LBO is high impedance when SHDN is grounded.
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LT1308A/LT1308B
BLOCK DIAGRA S
VIN VIN 6 R5 40k R6 40k VIN Q4 2VBE SHDN
VOUT R1 (EXTERNAL) FB R2 (EXTERNAL)
FB 2
R4 140k
RAMP GENERATOR
600kHz OSCILLATOR *HYSTERESIS IN LT1308A ONLY
Figure 2a. LT1308A/LT1308B Block Diagram (SO-8 Package)
VIN Q4 2VBE SHDN
VIN 11 VIN 12 R5 40k R6 40k
VIN
+
gm
VC 1 LBI ERROR AMPLIFIER BIAS
VOUT R1 (EXTERNAL) FB R2 (EXTERNAL)
-
FB 2 Q1 Q2 x10 R3 30k R4 140k RAMP GENERATOR
+
*
ENABLE 200mV
-
COMPARATOR FF R Q S DRIVER
600kHz OSCILLATOR *HYSTERESIS IN LT1308A ONLY
Figure 2b. LT1308A/LT1308B Block Diagram (TSSOP Package)
+
+ +
+
+ +
-
-
W
Q1
+
gm
VC 1 LBI ERROR AMPLIFIER BIAS
SHUTDOWN
3
-
Q2 x10 R3 30k
+
*
ENABLE 200mV
7
+ -
A4
LBO 8
-
A1 COMPARATOR
SW FF R Q S DRIVER 5 Q3
A2
+
A=3 0.03
-
4 GND
1308 BD2a
SHUTDOWN
3
13
+ -
A4
LBO 14 SW SW SW 8 9 10
A1
Q3
A2
+
A=3 0.03
-
4 5 6 7
1308 BD2b
GND GND GND GND
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LT1308A/LT1308B
APPLICATIONS INFORMATION
OPERATION The LT1308A combines a current mode, fixed frequency PWM architecture with Burst Mode micropower operation to maintain high efficiency at light loads. Operation can be best 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 140A. 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 400mA 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 LT1308A. 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 1mA or less. If the output load increases sufficiently, A1's output remains high, resulting in continuous operation. When the LT1308A 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 flip-flop 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 1V or greater. It need not be tied to VIN as on the LT1308. The LT1308B differs from the LT1308A in that there is no hysteresis in comparator A1. Also, the bias point on A1 is set lower than on the LT1308B 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 Figure 3. The top two traces in Figure 3 shows an LT1308A/LT1308B circuit, using the components indicated in Figure 1, set to a 5V output. Input voltage is 3V. Load current is stepped from 50mA to 800mA for both circuits. Low frequency Burst Mode operation voltage ripple is observed on Trace A, while none is observed on Trace B. At light loads, the LT1308B 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 LT1308A output ripple. Further, the LT1308B can be forced into micropower mode, where IQ falls from 3mA to 200A by sinking 40A or more out of the VC pin. This stops switching by causing A1's output to go low.
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TRACE A: LT1308A VOUT, 100mV/DIV AC COUPLED TRACE B: LT1308B VOUT, 100mV/DIV AC COUPLED 800mA ILOAD 50mA VIN = 3V 200s/DIV (CIRCUIT OF FIGURE 1)
1308 F03
Figure 3. LT1308A Exhibits Burst Mode Operation Output Voltage Ripple at 50mA Load, LT1308B Does Not
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LT1308A/LT1308B
APPLICATIONS INFORMATION
Waveforms for a LT1308B 5V to 12V boost converter using a 10F ceramic output capacitor are pictured in Figures 4 and 5. In Figure 4, the converter is operating in continuous mode, delivering a load current of approximately 500mA. The top trace is the output. The voltage increases as inductor current is dumped into the output capacitor during the switch off time, and the voltage decreases when the switch is on. Ripple voltage is in this case due to capacitance, as the ceramic capacitor has little ESR. The middle trace is the switch voltage. This voltage alternates between a VCESAT and VOUT plus the diode drop. The lower trace is the switch current. At the beginning of the switch cycle, the current is 1.2A. At the end of the switch on time, the current has increased to 2A, at which point the switch turns off and the inductor current flows into the output capacitor through the diode. Figure 5 depicts converter waveforms at a light load. Here the converter operates in discontinuous mode. The inductor current reaches zero during the switch off time, resulting in some ringing at the switch node. The ring frequency is set by switch capacitance, diode capacitance and inductance. This ringing has little energy, and its sinusoidal shape suggests it is free from harmonics. Minimizing the copper area at the switch node will prevent this from causing interference problems.
VOUT 100mV/DIV VSW 10V/DIV MULTIPLE VIAs ISW 1A/DIV 500ns/DIV GND C2 VOUT
Figure 4. 5V to 12V Boost Converter Waveforms in Continuous Mode. 10F Ceramic Capacitor Used at Output
VOUT 20mV/DIV
VSW 10V/DIV ISW 500mA/DIV 500ns/DIV
Figure 5. Converter Waveforms in Discontinuous Mode
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LAYOUT HINTS The LT1308A/LT1308B switch current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layout. Figure 6 shows recommended component placement for an SO-8 package boost (step-up) converter. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor C1 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. The negative terminal of output capacitor C2 should tie close to the ground pin(s) of the LT1308A/LT1308B. Doing this reduces dI/dt in the ground copper which keeps high frequency spikes to a minimum. The DC/DC converter ground should tie to the PC board ground plane at one place only, to avoid introducing dI/dt in the ground plane.
GROUND PLANE
LBI LBO
C1
+
VIN
R1 R2 SHUTDOWN
1 2 3 4 LT1308A LT1308B
8 7 6 5 L1
+
D1
1308 F04
Figure 6. Recommended Component Placement for SO-8 Package Boost Converter. Note Direct High Current Paths Using Wide PC Traces. Minimize Trace Area at Pin 1 (VC) and Pin 2 (FB). Use Multiple Vias to Tie Pin 4 Copper to Ground Plane. Use Vias at One Location Only to Avoid Introducing Switching Currents into the Ground Plane
Figure 7 shows recommended component placement for a boost converter using the TSSOP package. Placement is similar to the SO-8 package layout.
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LT1308A/LT1308B
APPLICATIONS INFORMATION
GROUND PLANE LBI LBO C1
+
VIN
R1 R2 SHUTDOWN
1 2 3 4 5 LT1308A LT1308B
14 13 12 11 10 9 8 L1
MULTIPLE VIAs
6 7
+
GND C2 VOUT
D1
Figure 7. Recommended Component Placement for TSSOP Boost Converter. Placement is Similar to Figure 4.
GROUND PLANE C1 LBI LBO
+
R1 R2 SHUTDOWN
1 2 3 4 LT1308A LT1308B
MULTIPLE VIAs GND
C3
+
Figure 9. Recommended Component Placement for SEPIC
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A SEPIC (Single-Ended Primary Inductance Converter) schematic is shown in Figure 8. This converter topology produces a regulated output over an input voltage range that spans (i.e., can be higher or lower than) the output. Recommended component placement for an SO-8 package SEPIC is shown in Figure 9.
L1A CTX10-2 C2 4.7F CERAMIC D1
VIN 3V TO 10V
+
VIN C1 47F SHUTDOWN LT1308B SHDN VC 47k 680pF
SW L1B R1 309k FB GND R2 100k VOUT 5V 500mA
+
C3 220F 6.3V
C1: AVX TAJC476M016 C2: TAIYO YUDEN EMK325BJ475(X5R) C3: AVX TPSD227M006
1308 F07
D1: IR 10BQ015 L1: COILTRONICS CTX10-2
1308A/B F08
Figure 8. SEPIC (Single-Ended Primary Inductance Converter) Converts 3V to 10V Input to a 5V/500mA Regulated Output
VIN
8 7 6 5 L1A L1B
C2
D1
VOUT
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LT1308A/LT1308B
APPLICATIONS INFORMATION
SHDN PIN The LT1308A/LT1308B SHDN pin is improved over the LT1308. The pin does not require tying to VIN to enable the device, but needs only a logic level signal. The voltage on the SHDN pin can vary from 1V to 10V independent of VIN. Further, floating this pin has the same effect as grounding, which is to shut the device down, reducing current drain to 1A or less. LOW-BATTERY DETECTOR The low-battery detector on the LT1308A/LT1308B features improved accuracy and drive capability compared to the LT1308. The 200mV reference has an accuracy of 2% and the open-collector output can sink 50A. The LT1308A/ LT1308B 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 reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward. Figure 10 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 11.
5V R1 LBI R2 100k VIN LT1308A LT1308B LBO
195 200 VLBI (mV) 205
1308 F12
+ -
200mV INTERNAL REFERENCE GND
100k TO PROCESSOR
VBAT
R1 =
VLB - 200mV 2A
1308 F10
Figure 10. Setting Low-Battery Detector Trip Point
200k 2N3906 VBAT VREF 200mV 10k LBO
VIN LT1308A LT1308B GND
1308 F11
+
10F
LBI
Figure 11. Accessing 200mV Reference
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A cross plot of the low-battery detector is shown in Figure 12. The LBI pin is swept with an input which varies from 195mV to 205mV, and LBO (with a 100k pull-up resistor) is displayed.
VLBO 1V/DIV
Figure 12. Low-Battery Detector Input/Output Characteristic
START-UP The LT1308A/LT1308B can start up into heavy loads, unlike many CMOS DC/DC converters that derive operating voltage from the output (a technique known as "bootstrapping"). Figure 13 details start-up waveforms of Figure 1's circuit with a 20 load and VIN of 1.5V. Inductor current rises to 3.5A as the output capacitor is charged. After the output reaches 5V, inductor current is about 1A. In Figure 14, the load is 5 and input voltage is 3V. Output voltage reaches 5V in 500s after the device is enabled. Figure 15 shows start-up behavior of Figure 5's SEPIC circuit, driven from a 9V input with a 10 load. The output reaches 5V in about 1ms after the device is enabled.
VOUT 2V/DIV
IL1 1A/DIV VSHDN 5V/DIV 1ms/DIV
1308 F13
Figure 13. 5V Boost Converter of Figure 1. Start-Up from 1.5V Input into 20 Load
11
LT1308A/LT1308B
APPLICATIONS INFORMATION
VOUT 1V/DIV
IL1 2A/DIV VSHDN 5V/DIV 500s/DIV
1308 F14
Figure 14. 5V Boost Converter of Figure 1. Start-Up from 3V Input into 5 Load
VOUT 2V/DIV
ISW 2A/DIV VSHDN 5V/DIV 500s/DIV
1308 F15
Figure 15. 5V SEPIC Start-Up from 9V Input into 10 Load
Soft-Start In some cases it may be undesirable for the LT1308A/ LT1308B to operate at current limit during start-up, e.g.,
VIN 5V
+
C1 47F
SHUTDOWN
C4 33nF R4 33k Q1 R3 33k 11.3k RC 47k CC 100pF
SOFT-START COMPONENTS C1: AVX TAJ476M010 C2: TAIYO YUDEN TMK432BJ106MM D1: IR 10BQ015 L1: MURATA LQH6C4R7 Q1: 2N3904
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4.
1308abfa
12
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when operating from a battery composed of alkaline cells. The inrush current may cause sufficiency internal voltage drop to trigger a low-battery indicator. A programmable soft-start can be implemented with 4 discrete components. A 5V to 12V boost converter using the LT1308B is detailed in Figure 16. C4 differentiates VOUT, causing a current to flow into R3 as VOUT increases. When this current exceeds 0.7V/33k, or 21A, current flows into the base of Q1. Q1's collector then pulls current out the VC pin, creating a feedback loop where the slope of VOUT is limited as follows: VOUT 0.7V = t 33k * C 4 With C4 = 33nF, VOUT/t is limited to 640mV/ms. Start-up waveforms for Figure 16's circuit are pictured in Figure 17. Without the soft-start circuit implemented, the inrush current reaches 3A. The circuit reaches final output voltage in approximately 250s. Adding the soft-start components reduces inductor current to less than 1A, as detailed in Figure 18, while the time required to reach final output voltage increases to about 15ms. C4 can be adjusted to achieve any output slew rate desired.
L1 4.7H
D1
VOUT 12V 500mA
VIN SHDN LT1308B
SW
100k 10k FB
330pF C2 10F
VC
GND
1308 F16
LT1308A/LT1308B
APPLICATIONS INFORMATION
12V VOUT 5V/DIV 5V
IL1 1A/DIV VSHDN 10V/DIV 50s/DIV
1308 F17
Figure 17. Start-Up Waveforms of Figure 16's Circuit without Soft-Start Components
12V VOUT 5V
IL1 1A/DIV VSHDN 10V/DIV 5ms/DIV
1308 F18
Figure 18. Start-Up Waveforms of Figure 16's Circuit with Soft-Start Components Added
COMPONENT SELECTION Diodes We have found ON Semiconductor MBRS130 and International Rectifier 10BQ015 to perform well. For applications where VOUT exceeds 30V, use 40V diodes such as MBRS140 or 10BQ040. Height limited applications may benefit from the use of the MBRM120. This component is only 1mm tall and offers performance similar to the MBRS130. Inductors Suitable inductors for use with the LT1308A/LT1308B must fulfill two requirements. First, the inductor must be able to handle current of 2A steady-state, as well as support transient and start-up current over 3A without inductance decreasing by more than 50% to 60%. Second, the DCR of the inductor should have low DCR, under 0.05
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so that copper loss is minimized. Acceptable inductance values range between 2H and 20H, with 4.7H best for most applications. Lower value inductors are physically smaller than higher value inductors for the same current capability. Table 1 lists some inductors we have found to perform well in LT1308A/LT1308B application circuits. This is not an exclusive list.
Table 1
VENDOR Murata Sumida Coiltronics Coilcraft PART NO. LQH6C4R7 CDRH734R7 CTX5-1 LPO2506IB-472 VALUE 4.7H 4.7H 5H 4.7H PHONE NO. 770-436-1300 847-956-0666 561-241-7876 847-639-6400
Capacitors Equivalent Series Resistance (ESR) is the main issue regarding selection of capacitors, especially the output capacitors. The output capacitors specified for use with the LT1308A/ LT1308B circuits have low ESR and are specifically designed for power supply applications. Output voltage ripple of a boost converter is equal to ESR multiplied by switch current. The performance of the AVX TPSD227M006 220F tantalum can be evaluated by referring to Figure 3. When the load is 800mA, the peak switch current is approximately 2A. Output voltage ripple is about 60mVPP, so the ESR of the output capacitor is 60mV/2A or 0.03. Ripple can be further reduced by paralleling ceramic units. Table 2 lists some capacitors we have found to perform well in the LT1308A/LT1308B application circuits. This is not an exclusive list.
Table 2
VENDOR AVX AVX Taiyo Yuden Taiyo Yuden SERIES TPS TPS X5R X5R PART NO. TPSD227M006 TPSD107M010 LMK432BJ226 TMK432BJ106 VALUE 220F, 6V PHONE NO. 803-448-9411
100F, 10V 803-448-9411 22F, 10V 10F, 25V 408-573-4150 408-573-4150
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LT1308A/LT1308B
APPLICATIONS INFORMATION
Ceramic Capacitors Multilayer ceramic capacitors have become popular, due to their small size, low cost, and near-zero ESR. Ceramic capacitors can be used successfully in LT1308A/LT1308B designs provided loop stability is considered. A tantalum capacitor has some ESR and this causes an "ESR zero" in the regulator loop. This zero is beneficial to loop stability. Ceramics do not have appreciable ESR, so the zero is lost when they are used. However, the LT1308A/LT1308B have external compensation pin (VC) so component values can be adjusted to achieve stability. A phase lead capacitor can also be used to tune up load step response to optimum levels, as detailed in the following paragraphs. Figure 19 details a 5V to 12V boost converter using either a tantalum or ceramic capacitor for C2. The input capacitor has little effect on loop stability, as long as minimum capacitance requirements are met. The phase lead capacitor CPL parallels feedback resistor R1. Figure 20 shows load step response of a 50mA to 500mA load step using a 47F tantalum capacitor at the output. Without the phase lead capacitor, there is some ringing, suggesting the phase margin is low. CPL is then added, and response to the same load step is pictured in Figure 21. Some phase margin is restored, improving the response. Next, C2 is replaced by a 10F, X5R dielectric, ceramic capacitor.
VIN 5V L1 4.7H D1 VOUT 12V 500mA
VIN SHDN LT1308B
SW CPL 330pF C2
R3 10k FB GND
R1 100k
VC
+
C1 47F 47k 100pF
R2 11.3k
C1: AVX TAJC476M010 C2: AVX TPSD476M016 (47F) OR TAIYO YUDEN TMK432BJ106MM (10F) D1: IR 10BQ015 L1: MURATA LQH6C4R7
1308 F19
Figure 19. 5V to 12V Boost Converter
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Without CPL, load step response is pictured in Figure 22. Although the output settles faster than the tantalum case, there is appreciable ringing, again suggesting phase margin is low. Figure 23 depicts load step response using the 10F ceramic output capacitor and CPL. Response is clean and no ringing is evident. Ceramic capacitors have the added benefit of lowering ripple at the switching frequency due to their very low ESR. By applying CPL in tandem with the series RC at the VC pin, loop response can be tailored to optimize response using ceramic output capacitors.
VOUT 500mV/DIV
IL1 1A/DIV LOAD CURRENT 500mA 50mA 200s/DIV
1308 F20
Figure 20. Load Step Response of LT1308B 5V to 12V Boost Converter with 47F Tantalum Output Capacitor
VOUT 500mV/DIV
IL1 1A/DIV LOAD CURRENT 500mA 50mA 200s/DIV
1308 F21
Figure 21. Load Step Response with 47F Tantalum Output Capacitor and Phase Lead Capacitor CPL
VOUT 1V/DIV
IL1 1A/DIV LOAD CURRENT 500mA 50mA 200s/DIV
1308 F22
Figure 22. Load Step Response with 10F X5R Ceramic Output Capacitor
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LT1308A/LT1308B
APPLICATIONS INFORMATION
VOUT 500mV/DIV VOUT VIN = 4.2V VOUT VIN = 3.6V IL1 1A/DIV LOAD CURRENT 500mA 50mA 200s/DIV
1308 F23
Figure 23. Load Step Response with 10F X5R Ceramic Output Capacitor and CPL
GSM AND CDMA PHONES The LT1308A/LT1308B are suitable for converting a single Li-Ion cell to 5V for powering RF power stages in GSM or CDMA phones. Improvements in the LT1308A/LT1308B error amplifiers allow external compensation values to be reduced, resulting in faster transient response compared to the LT1308. The circuit of Figure 24 (same as Figure 1, printed again for convenience) provides a 5V, 1A output from a Li-Ion cell. Figure 25 details transient response at the LT1308A operating at a VIN of 4.2V, 3.6V and 3V. Ripple voltage in Burst Mode operation can be seen at 10mA load. Figure 26 shows transient response of the LT1308B under the same conditions. Note the lack of Burst Mode ripple at 10mA load.
L1 4.7H D1 5V 1A SW LT1308B SHDN VC 47k 100pF C1: AVX TAJC476M010 C2: AVX TPSD227M006 D1: IR 10BQ015 L1: MURATA LQH6N4R7 FB GND R2 100k R1 309k
VOUT VIN = 4.2V VOUT VIN = 3.6V VOUT VIN = 3V ILOAD 1A 10mA
+
Li-Ion CELL
VIN C1 47F SHUTDOWN
+
Figure 24. Li-Ion to 5V Boost Converter Delivers 1A
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VOUT VIN = 3V ILOAD 1A 10mA VOUT TRACES = 200mV/DIV 200s/DIV
1308 F25
Figure 25. LT1308A Li-Ion to 5V Boost Converter Transient Response to 1A Load Step
VOUT TRACES = 200mV/DIV
100s/DIV
1308 F26
Figure 26. LT1308B Li-Ion to 5V Boost Converter Transient Response to 1A Load Step
C2 220F
1308A/B F24
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LT1308A/LT1308B
TYPICAL APPLICATIO S
Triple Output TFTLCD Bias Supply
D2 C4 1F 0.22F 0.22F VOFF -9V 10mA D3 C5 1F D4 0.22F C6 1F VON 27V 15mA
VIN 5V 3 C1 4.7F 220k 100pF
C1:TAIYO-YUDEN JMK212BJ475MG C2, C3:TAIYO-YUDEN LMK325BJ106MN C4, C5, C6:TAIYO-YUDEN EMK212BJ105MG D1: MBRM120 D2,D3,D4: BAT54S L1: TOKO 817FY-4R7M
ILOAD
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L1 4.7H 6 VIN SHDN LT1308B 1 VC FB GND 4 5 SW
D1 AVDD 10V 500mA 76.8k C2, C3 10F x2
2
10.7k
1308 TA02
TFTLCD Bias Supply Transient Response
AVDD 500mV/DIV
VON 500mV/DIV VOFF 500mV/DIV 800mA 200mA 100s/DIV
1308abfa
LT1308A/LT1308B
TYPICAL APPLICATIO S
40nF EL Panel Driver
T1 1:12 D2 4 D3
3.3V REGULATED 100k Q1 150k 47k
3.3k
C1: AVX TAJC476M010 C2: VITRAMON VJ225Y105KXCAT D1: BAT54 D2, D3: BAV21
High Voltage Supply 350V at 1.2mA
10nF 250V D3 D2 10nF 250V VOUT 350V 1.2mA
VIN 2.7V TO 6V
T1 1:12
+
C1 47F
3
4 10nF 250V
1
6 D4
VIN SHUTDOWN SHDN LT1308A
SW
VC 47k 100pF 10nF D1, D2, D3: BAV21 200mA, 250V D4: MBR0540 T1: MIDCOM 31105R LP = 1.5H GND
FB 34.8k
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D1
VBAT 3V TO 6V
+
3 C1 47F 1 D1
6
1F VIN LBO 2M 324k LBI VC 22nF 49.9k 100pF 47pF 10k SHDN GND SHUTDOWN Q2 400V LT1308A SW FB 17k C2 1F 200V EL PANEL 40nF 4.3M
1308 TA03
Q1: MMBT3906 Q2: ZETEX FCX458 T1: MIDCOM 31105
SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output
VIN 3V TO 10V
L1A CTX10-2
C2 4.7F CERAMIC
D1
+
VIN C1 47F SHUTDOWN LT1308B SHDN VC 47k
SW L1B R1 309k FB GND R2 100k VOUT 5V 500mA
+
10M
680pF C1: AVX TAJC476M016 C2: TAIYO YUDEN EMK325BJ475(X5R) C3: AVX TPSD227M006
1308 TA04
C3 220F 6.3V
D1: IR 10BQ015 L1: COILTRONICS CTX10-2
1308A/B TA05
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17
LT1308A/LT1308B
PACKAGE DESCRIPTION
S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.189 - .197 (4.801 - 5.004) NOTE 3 8 7 6 5
.050 BSC
.245 MIN
.030 .005 TYP RECOMMENDED SOLDER PAD LAYOUT
.010 - .020 x 45 (0.254 - 0.508) .008 - .010 (0.203 - 0.254) 0- 8 TYP
.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)
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.045 .005
.160 .005
.228 - .244 (5.791 - 6.197)
.150 - .157 (3.810 - 3.988) NOTE 3
1
2
3
4
.053 - .069 (1.346 - 1.752)
.004 - .010 (0.101 - 0.254)
.014 - .019 (0.355 - 0.483) TYP
.050 (1.270) BSC
SO8 0303
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LT1308A/LT1308B
PACKAGE DESCRIPTION
F Package 14-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1650)
4.90 - 5.10* (.193 - .201) 1.05 0.10 14 13 12 11 10 9 8
6.60 0.10
0.45 0.05
RECOMMENDED SOLDER PAD LAYOUT 4.30 - 4.50** (.169 - .177)
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
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
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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4.50 0.10
6.40 (.252) BSC
0.65 BSC 1234567 1.10 (.0433) MAX
0 - 8
0.25 REF
0.65 (.0256) BSC 0.19 - 0.30 (.0075 - .0118) TYP
0.05 - 0.15 (.002 - .006)
F14 TSSOP 0204
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LT1308A/LT1308B
TYPICAL APPLICATIO
2.7V TO 4.2V VIN C1 47F SHUTDOWN
C1: AVX TAJC476M010 C2: AVX TPSD107M016 D1: IR 10BQ015
RELATED PARTS
PART NUMBER LT1302 LT1304 LT1316 LTC(R)1474 LTC1516 LTC1522 LT1610 LT1611 LT1613 LT1615 LT1617 LTC1682 LT1949 LT1949-1 DESCRIPTION High Output Current Micropower DC/DC Converter 2-Cell Micropower DC/DC Converter Burst Mode Operation DC/DC with Programmable Current Limit Micropower Step-Down DC/DC Converter 2-Cell to 5V Regulated Charge Pump Micropower, 5V Charge Pump DC/DC Converter Single-Cell Micropower DC/DC Converter Inverting 1.4MHz Switching Regulator in 5-Lead SOT-23 1.4MHz Switching Regulator in 5-Lead SOT-23 Micropower Step-Up DC/DC in 5-Lead SOT-23 Micropower Inverting DC/DC Converter in SOT-23 Doubler Charge Pump with Low Noise LDO 600kHz, 1A Switch PWM DC/DC Converter 1.1MHz, 1A Switch DC/DC Converter COMMENTS 5V/600mA from 2V, 2A Internal Switch, 200A IQ 5V/200mA, Low-Battery Detector Active in Shutdown 3.3V at 75mA from One Cell, MSOP Package 1.5V Minimum, Precise Control of Peak Current Limit 100A IQ, Operate with VIN as Low as 1.5V 94% Efficiency, 10A IQ, 9V to 5V at 250mA 12A IQ, No Inudctors, 5V at 50mA from 3V Input Regulated 5V 4% Output, 20mA from 3V Input 3V at 30mA from 1V, 1.7MHz Fixed Frequency - 5V at 150mA from 5V Input, Tiny SOT-23 package 5V at 200mA from 4.4V Input, Tiny SOT-23 package 20A IQ, 36V, 350mA Switch VIN = 1V to 15V; VOUT to -34V Adjustable or Fixed 3.3V, 5V Outputs, 60VRMS Output Noise 1.1A, 0.5, 30V Internal Switch, VIN as Low as 1.5V 1.1MHz Version of LT1949
LT1307/LT1307B Single Cell, Micropower, 600kHz PWM DC/DC Converters LT1317/LT1317B Micropower, 600kHz PWM DC/DC Converters
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 FAX: (408) 434-0507 www.linear-tech.com
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Li-Ion to 12V/300mA Step-Up DC/DC Converter
L1 4.7H D1 12V 300mA SW LT1308B SHDN VC 47k 330pF L1: MURATA LQH6C4R7
1308A/B TA01
+
Li-Ion CELL
R1 887k FB GND R2 100k
+
C2 100F
1308abfa LT 0807 REV A * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1999

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