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LT1432-3.3 3.3V High Efficiency Step-Down Switching Regulator Controller
FEATURES
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DESCRIPTIO
Accurate Preset 3.3V Output Up to 87% Efficiency Optional Burst ModeTM Operation for Light Loads Can Be Used with Many LTC Switching ICs Accurate Ultra-Low-Loss Current Limit Operates with Inputs from 4.5V to 30V Shutdown Mode Draws Only 15A Uses Small 30H Inductor
The LT (R)1432-3.3 is a control chip designed to operate with the LT1171/LT1271 family of switching regulators to make a very high efficiency 3.3V step-down (buck) switching regulator. A minimum of external components is needed. Included is an accurate current limit which uses only 60mV sense voltage and uses "free" PC board trace material for the sense resistor. Logic controlled electronic shutdown mode draws only 15A battery current. The switching regulator operates down to 4.5V input. The LT1432-3.3 has a logic controlled Burst Mode operation to achieve high efficiency at very light load currents (0mA to 100mA) such as memory keep-alive. In normal switching mode, the standby power loss is about 30mW, limiting efficiency at light loads. In Burst Mode operation, standby loss is reduced to approximately 11mW. Output current in this mode is typically in the 5mA to 100mA range. The LT1432-3.3 is available in 8-pin SO and PDIP packages. The LT1171/LT1271 is also available in surface mount DD packages.
APPLICATI
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Laptop and Palmtop Computers Portable Data-Gathering Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATI
D2 1N5818 VIN
+
VSW C1 330F 35V C6 0.02F C5 0.03F D1 MBR330p LT1271 FB VC
VIN
10H 3A 100F 16V
+
GND
D2**
OPTIONAL OUTPUT FILTER
+
VC VIN LT1432-3.3 DIODE V+ VLIM VOUT GND
x
C2 390F 16V
VOUT*** 3.3V 3A
EFFICIENCY (%)
C3 10F TANT
+
L1 30H
R2* 0.013
MODE LOGIC 220pF
MODE
<0.3V = NORMAL MODE >2.5V = SHUTDOWN OPEN = Burst Mode OPERATION
*R2 IS MADE FROM PC BOARD COPPER TRACES. **OPTIONAL CONNECTION FOR D2. FOR CIRCUITS WHICH DO NOT USE EFFICIENCY IS HIGHER, BUT MINIMUM V IN INCREASES. SEE APPLICATION Burst Mode OPERATION, C5 MAY INFORMATION SECTION. BE PARALLEL WITH A 680, 0.1F IN SERIES TO GIVE WIDE PHASE MARGIN ***MAXIMUM CURRENT IS DETERMINED BY THE CHOICE OF LT1071 FAMILY MAIN SWITCHER IC. WITH DIFFERENT SWITCHING ICs AND SEE APPLICATION INFORMATION SECTION. OUTPUT CAPACITORS
LT1432-3.3 TA01
Figure 1. High Efficiency 5V Buck Converter
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Efficiency
100 LT1271, L = 30H VIN = 7V 90 D2 CONNECTED TO OUTPUT 80 D2 CONNECTED TO INPUT 70 Burst Mode OPERATION (USE mA SCALE) 60 0 0 1A 20mA 2A 40mA 3A 60mA
LT1432-3.3 TA02
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LT1432-3.3 ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW VLIM 1 VOUT 2 VIN 3 V+ 4 8 7 6 5 MODE GND VC DIODE
VIN Pin .................................................................... 30V V + Pin ..................................................................... 40V VC ........................................................................... 35V VLIM and VOUT Pins ................................................... 7V Diode Pin Voltage ................................................... 30V Mode Pin Current (Note 2) ..................................... 1mA Operating Temperature Range .................... 0C to 70C Storage Temperature Range ................ -65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LT1432CN8-3.3 LT1432CS8-3.3
N8 PACKAGE 8-LEAD PDIP
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 100C, JA = 150C/W (N8) TJMAX = 100C, JA = 170C/W (S8)
Consult factory for Military and Industrial grade parts.
ELECTRICAL CHARACTERISTICS
VC = 4V, VIN = 4V, V + = 8V, VDIODE = Open, VLIM = VOUT, VMODE = 0V, TJ = 25C Device is in standard test loop unless otherwise noted.
PARAMETER Regulated Output Voltage Output Voltage Line Regulation Input Supply Current (Note 1) Quiescent Output Load Current Mode Pin Current Mode Pin Threshold Voltage (Normal to Burst) VC Pin Saturation Voltage VC Pin Maximum Sink Current VC Pin Source Current Current Limit Sense Voltage (Note 3) VLIM Pin Current Supply Current in Shutdown Burst Mode Operation Output Ripple Burst Mode Operation Average Output Voltage Clamp Diode Forward Voltage Start-up Drive Current Restart Time Delay Transconductance, Output to VC Pin VMODE = 0V (Current Is Out of Pin) VMODE = 3.3V (Shutdown) IMODE = 10A (Out of Pin) VOUT = 3.6V (Forced) VOUT = 3.6V (Forced) VOUT = 3.0V (Forced) Device in Current Limit Loop Device in Current Limit Loop (Current Is Out of Pin) VMODE > 3V, VIN < 30V, VC and V + = 0V Device in Burst Test Circuit Device in Burst Test Circuit IF = 1mA, All Other Pins Open VOUT = 1.5V (Forced), VIN = 4V to 26V, V + = VIN - 1V, VC = VIN - 1.5V (Note 4) IC = 150A to 250A
q q q q q q q q q q q
CONDITIONS VC Current = 220A VIN = 4V to 30V VIN = 4V to 30V, V + = VIN + 5V, VC = VIN + 1V
q q q
MIN 3.24
TYP 3.30 5 0.3 0.9 30 15
MAX 3.36 20 0.5 1.2 50 30 1.5 0.45 1.5 100 64 70 60 3.45 0.65
UNITS V mV mA mA A A V V mA A mV A A mVp-p V V mA
0.6
0.9 0.25
0.45 35 56 30
0.8 60 60 45 15 100
3.15 30 0.7 2700
3.30 0.5 45 1.2 3600
10 5000
mho
2
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ms
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LT1432-3.3
ELECTRICAL CHARACTERISTICS
Operating parameters in standard circuit configuration. VIN = 7V, IOUT = 0, unless otherwise noted. These parameters guaranteed where indicated, but not tested.
PARAMETER Burst Mode Operation Quiescent Input Supply Current Burst Mode Operation Output Ripple Voltage Normal Mode Equivalent Input Supply Current Normal Mode Minimum Operating Input Voltage Burst Mode Operation Minimum Operating Input Voltage Efficiency Load Regulation IOUT = 0 IOUT = 50mA Extrapolated from IOUT = 20mA 100mA < IOUT < 1.5A 5mA < IOUT < 50mA Normal Mode IOUT = 0.5A Burst Mode Operation IOUT = 25mA Normal Mode 50mA < IOUT < 2A Burst Mode Operation 0 < IOUT < 50mA 4.1 86 70 5 30 15 CONDITIONS MIN TYP 1.6 80 120 3.0 4.5 V % % mV mV MAX 2.2 UNITS mA mVp-p mVp-p mA V
The q denotes specifications which apply over the full operating temperature range. Note 1: Does not include current drawn by the power IC. See operating parameters in standard circuit. Note 2: Breakdown voltage on the Mode pin is 7V. External current must be limited to value shown.
Note 3: Current limit sense voltage temperature coefficient is +0.33%/C to match TC of copper trace material. Note 4: VOUT pin switched from 3.6V to 3.0V.
EQUIVALE T SCHE ATIC
D2 VIN VSW LT1271 VC VIN FB GND VLIM 60mV 1 2 VOUT OPTIONAL CONNECTION OF D2 3.3V
3
VIN
S3*
* S3 IS CLOSED ONLY DURING START-UP. ** S1 AND S2 ARE SHOWN IN NORMAL MODE. REVERSE FOR Burst Mode OPERATION.
S2**
MODE CONTROL 7 8 MODE GND
LT1432-3.3 F02
Figure 2
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+
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V+ 6 VC 5 DIODE
-
S1**
3
LT1432-3.3
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage
100 TJ = 25C LT1271, L = 50H 90
EFFICIENCY (%)
EFFICIENCY (%)
DIODE TO OUTPUT
INPUT VOLTAGE (V)
80
DIODE TO INPUT IOUT = 1A
70
60 0 5 10 15 20 INPUT VOLTAGE (V) 25 30
Minimum Input Voltage - Normal Mode (Diode to Output)
9.0 TJ = 25C 8.0
5.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
7.0 LT1171 6.0 DIODE TO INPUT 5.0 LT1271
4.5
LT1271 LT1270
INPUT VOLTAGE (V)
4.0
0
1
3 4 2 OUTPUT CURRENT (A)
Shutdown Current vs Input Voltage
50 TJ = 25C 40
CURRENT (A)
30 40
30
CURRENT (A)
VIN = 30V
SENSE VOLTAGE (mV)
20
10
0
0
5
10 15 20 INPUT VOLTAGE (V)
4
UW
LT1432-3.3 G01
Efficiency vs Load Current
95 TJ = 25C VIN = 7V
Minimum Input Voltage to Start - Normal Mode (Diode to Input)
6.5 TJ = 25C 6.0
90
LT1270 85 LT1271 80
5.5 DIODE TO INPUT 5.0 LT1270 4.5 LT1171 LT1271
75 0 0.5 1.0 1.5 2.0 LOAD CURRENT (A) 2.5 3.0
4.0
0
1
3 4 2 OUTPUT CURRENT (A)
5
LT1432-3.3 G02
LT1432-3.3 G03
Minimum Running Voltage - Normal Mode*
5.5 TJ = 25C
Burst Mode Operation Minimum Input Voltage
5.5 TJ = 25C
LT1171
5.0
4.5
4.0
3.5
4.0
3.0
5
0
1
3 4 2 OUTPUT CURRENT (A)
5
3.5
0
10
30 40 20 LOAD CURRENT (mA)
50
LT1432-3.3 G04
LT1432-3.3 G05
LT1432-3.3 G06
*SEE MINIMUM INPUT VOLTAGE TO START
Battery Current in Shutdown*
80
Current Limit Sense Voltage*
70
20 VIN = 6V 10
60
50
25
30
0
40
0 25 50 TEMPERATURE (C) 75 100
LT1432-3.3 G08
0
50 75 25 JUNCTION TEMPERATURE (C)
100
LT1432-3.3 G9
LT1432-3.3 G07
*DOES NOT INCLUDE LT1271 SWITCH LEAKAGE.
* TEMPERATURE COEFFICIENT OF SENSE VOLTAGE IS DESIGNED TO TRACK COPPER RESISTANCE.
LT1432-3.3
TYPICAL PERFOR A CE CHARACTERISTICS
Incremental Battery Current * in Burst Mode Operation
2.0 TJ = 25C
INCREMENTAL FACTOR (mA/mA)
1.5
BATTERY CURRENT (mA)
4
TRANSCONDUCTANCE (mho)
1.0
0.5
0
0
5
15 20 10 BATTERY VOLTAGE (V)
* TO CALCULATE TOTAL BATTERY CURRENT IN Burst Mode OPERATION, MULTIPLY LOAD CURRENT BY INCREMENTAL FACTOR AND ADD NO-LOAD CURRENT.
Line Regulation
40 TJ = 25C 20
OUTPUT CHANGE (mV)
OUTPUT CHANGE (mV)
CURRENT (A)
0
NORMAL MODE
Burst Mode OPERATION -20
-40
0
5
10 INPUT VOLTAGE (V)
15
Restart Load Current
40 VOUT = 4.5V 4
V+ PIN CURRENT (mA)
30
TIME DELAY (ms)
CURRENT (mA)
20
10
0
0
50 75 25 JUNCTION TEMPERATURE (C)
UW
LT1432-3.3 G10
LT1432-3.3 G13
No Load Battery Current in Burst Mode Operation
5 TJ = 25C
4000 5000
Transconductance - VOUT to VC Current
Gm = I(VC PIN) VOUT
3
3000
2
1
2000
25
0
0
5
15 20 10 BATTERY VOLTAGE (V)
25
1000
0
50 75 25 JUNCTION TEMPERATURE (C)
100
LT1432-3.3 G11
LT1432-3.3 G12
Burst Mode Operation Load Regulation
25 TJ = 25C VIN = 7V 0
Mode Pin Current
60 TJ = 25C 40
20
-25
0
-50
-20 MODE DRIVE MUST SINK 30A AT 0V
-75
20
-40
0 20 60 80 40 LOAD CURRENT (mA) 100
0
2
6 8 4 MODE PIN VOLTAGE (V)
10
LT1432-3.3 G14
LT1432-3.3 G15
Restart Time Delay
5
Start-up Switch Characteristics
TJ = 25C 0
3
NOTE VERTICAL AND HORIZONTAL SCALE CHANGES AT 0,0
-20
2
-40
1
-60
100
0
-80
0 50 75 25 JUNCTION TEMPERATURE (C) 100
-2
-1
10 0 V + TO VIN VOLTAGE
20
30
LT1432-3.3 G16
LT1432-3.3 G16
LT1432-3.3 G18
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LT1432-3.3
APPLICATI
S I FOR ATIO
More applications information on the LT1432-3.3 is available in the LT1432 data sheet. Basic Circuit Description The LT1432-3.3 is a dedicated 3.3V buck converter driver chip intended to be used with an IC switcher from the LT1171/ LT1271 family. This family of current mode switchers includes current ratings from 1.25A to 10A, and switching frequencies from 40kHz to 100kHz as shown in the table below.
DEVICE LT1270A LT1270 LT1170 LT1070 LT1269 LT1271 LT1171 LT1071 LT1172 LT1072 SWITCH CURRENT 10A 8A 5A 5A 4A 4A 2.5A 2.5A 1.25A 1.25A FREQUENCY 60kHz 60kHz 100kHz 40kHz 100kHz 60kHz 100kHz 40kHz 100kHz 40kHz OUTPUT CURRENT IN BUCK CONVERTER 7.5A 6A 3.75A 3.75A 3A 3A 1.8A 1.8A 0.9A 0.9A
The maximum load current which can be delivered by these chips in a buck converter is approximately 75% of their switch current rating. This is partly due to the fact that buck converters must operate at very high duty cycles when input voltage is low. The current mode nature of the LT1271 family requires an internal reduction of peak current limit at high duty cycles, so these devices are rated at only 80% of their full current rating when duty cycle is 80%. A second factor is inductor ripple current, half of which subtracts from maximum available load current. The LT1271 family was originally intended for topologies which have the negative side of the switch grounded, such as boost converters. It has an extremely efficient quasi-saturating NPN switch which mimics the linear resistive nature of a MOSFET but consumes much less die area. Driver losses are kept to a minimum with a patented adaptive antisat drive that maintains a forced beta of 40 over a wide range of switch currents. This family is attractive for high efficiency buck converters because of the low switch loss, but to operate as a positive buck converter, the GND pin of the IC must be floated to act as the switch output node. This requires a floating power supply for the chip and some means for level shifting the feedback signal. The LT1432-3.3 performs these functions as well as adding
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current limiting, micropower shutdown, and dual mode operation for high conversion efficiency with both heavy and very light loads. The circuit in Figure 1 is a basic 3.3V positive buck converter which can operate with input voltage from 4.5V to 30V. The power switch is located between the VSW pin and GND pin on the LT1271. Its current and duty cycle are controlled by the voltage on the VC pin with respect to the GND pin. This voltage ranges from 1V to 2V as switch current increases from zero to full-scale. Correct output voltage is maintained by the LT1432-3.3 which has an internal reference and error amplifier (see Equivalent Schematic in Figure 2). The amplifier output is level shifted with an internal open collector NPN to drive the VC pin of the switcher. The normal resistor divider feedback to the switcher feedback pin cannot be used because the feedback pin is referenced to the GND pin, which is switching up and down. The Feedback pin (FB) is simply bypassed with a capacitor. This forces the switcher VC pin to swing high with about 200A sourcing capability. The LT1432-3.3 VC pin then sinks this current to control the loop. Transconductance from the regulator output to the VC pin current is controlled to approximately 3600mhos by local feedback around the LT1432-3.3 error amplifier (S2 closed in Figure 2). This is done to simplify frequency compensation of the overall loop. A word of caution about the FB pin bypass capacitor (C6): this capacitor value is very non-critical, but the capacitor must be connected directly to the GND pin or tab of the switcher to avoid differential spikes created by fast switch currents flowing in the external PCB traces. This is also true for the frequency compensation capacitor C5. C5 forms the dominant loop pole. A floating power supply for the switcher is generated by D2 and C3 which peak detect the input voltage during switch off time. This is different than the 5V version of the LT1432 which connects the anode of the diode to the output rather than the input. The output connection is more efficient because the floating voltage is a constant 5V (or 3.3V), independent of input voltage, but in the case of the 3.3V circuit, minimum required input voltage for starting is several volts higher (see the Typical Performance Characteristics curves). When the diode is connected to the input, the suggested type is a
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LT1432-3.3
APPLICATI
S I FOR ATIO
Schottky 1N5818. Diode type is more critical for the output connection because the high capacitance of Schottky diodes creates narrow output spikes. These spikes will be eliminated if a secondary output filter is used or if there is sufficient lead length between the regulator output and the load bypass capacitors. Low capacitance diodes like the 1N4148 do not create large spikes, but their high forward resistance requires even higher input voltage to start. D1, L1 and C2 act as the conventional catch diode and output filter of the buck converter. These components should be selected carefully to maintain high efficiency and acceptable output ripple. See the original LT1432 (5V) data sheet for detailed discussions of these parts. Current limiting is performed by R2. Sense voltage is only 60mV to maintain high efficiency. This also reduces the value of the sense resistor enough to utilize a printed circuit board trace as the sense resistor. The sense voltage has a positive temperature coefficient of 0.33%/C to match the temperature coefficient of copper. The basic regulator has three different operating modes, defined by the Mode pin drive. Normal operation occurs when the Mode pin is grounded. A low quiescent current Burst Mode operation can be initiated by floating the Mode pin. Input supply current is typically 1.3mA in this mode, and output ripple voltage is 100mVp-p. Pulling the Mode pin above 2.5V forces the entire regulator into micropower shutdown where it typically draws less than 20A. Burst Mode Operation Burst Mode operation is initiated by allowing the Mode pin to float, where it will assume a DC voltage of approximately 1V. If AC pickup from surrounding logic lines is likely, the Mode pin should be bypassed with a 200pF capacitor. Burst Mode operation is used to reduce quiescent operating current when the regulator output current is very low, as in sleep mode in a lap-top computer. In this mode, hysteresis is added to the error amplifier to make it switch on and off, rather than maintain a constant amplifier output. This forces the switching IC to either provide a rapidly increasing current or to go into full micropower shutdown. Current is delivered to the output capacitor in pulses of higher amplitude and low duty cycle rather than a continuous stream of low amplitude
LOAD CURRENT - mA (NORMAL MODE)
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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pulses. This maximizes efficiency at light load by eliminating quiescent current in the switching IC during the period between bursts. The result of pulsating currents into the output capacitor is that output ripple amplitude increases and ripple frequency becomes a function of load current. The typical output ripple in Burst Mode operation is 100mVp-p, and ripple frequency can vary from 50Hz to 2kHz. This is not normally a problem for the logic circuits which are kept alive during sleep mode. Some thought must be given to proper sequencing between normal mode and Burst Mode operation. A heavy (>100mA) load in Burst Mode operation can cause excessive output ripple, and an abnormally light load (10mA to 30mA, see Figure 3) in normal mode can cause the regulator to revert to a quasi-Burst Mode operation that also has higher output ripple. The worst condition is a sudden, large increase in load current (>100mA) during this quasi-Burst Mode operation or just after a switch from Burst Mode operation to normal mode. This can cause the output to sag badly while the regulator is establishing normal mode operation (100s). To avoid problems, it is suggested that the power-down sequence consist of reducing load current to below 100mA, but greater than the minimum for normal mode, then switching to Burst Mode operation, followed by a reduction of load current to the final sleep value. Power-up would consist of increasing the load current to the minimum for
50 NORMAL MODE TJ = 25C 40 30 20 DIODE TO OUTPUT (1N5818) DIODE TO INPUT (1N5818) 10 0 4 5 7 6 INPUT VOLTAGE (V) 8 9
LT1432-3.3 * F03
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Figure 3. Minimum Normal Mode Load Current
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LT1432-3.3
APPLICATI
S I FOR ATIO
normal mode, then switching to normal mode, pausing for 1ms, followed by return to full load. If this sequence is not possible, an alternative is to increase the output capacitor to > 680F. This modification will often allow the power-down sequence to consist of simultaneous turn-off of load current and switch to Burst Mode operation. Power-up is accomplished by switching to normal mode and simultaneously increasing load current to the lowest possible value (30mA to 500mA), followed by a short pause and return to full load current.
PACKAGE DESCRIPTIO
0.300 - 0.325 (7.620 - 8.255)
Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead Plastic DIP
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
0.009 - 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
0.053 - 0.069 (1.346 - 1.752)
0.014 - 0.019 (0.355 - 0.483) *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
0.016 - 0.050 0.406 - 1.270
RELATED PARTS
PART NUMBER
LTC1148 LT1432 LT1507
DESCRIPTION
High Efficiency Step-Down Switching Regulator Controller High Efficiency Synchronous Step-Down Switching Regulator 1.5A, 500kHz Step-Down Switching Regulator
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
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Full Shutdown When the Mode pin is driven high, full shutdown of the regulator occurs. Regulator input current will then consist of the LT1432 shutdown current (15A) plus the switch leakage of the switching IC (1A to 25A). Mode input current (15A at 5V) must also be considered. Start-up from shutdown can be in either normal or Burst Mode operation, but one should always check start-up overshoot, especially if the output capacitor or frequency compensation components have been changed.
0.255 0.015* (6.477 0.381) 1 2 3 4
N8 0695
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S8 Package 8-Lead Plastic SOIC
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0.189 - 0.197* (4.801 - 5.004) 7 6 5
0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197) 0.050 (1.270) BSC 1 2 3 4 0.150 - 0.157** (3.810 - 3.988)
SO8 0695
COMMENTS
5V Regulated Output Voltage Adjustable and Fixed 5V or 3.3V Outputs Fixed Frequency PWM for Low Input Voltages from 4.5V to 12V
LT/GP 0895 2K REV A * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1992

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