LT3757 datasheet(7/38 Pages) AD

7 / 38 page

LT3757/LT3757A

Rev. G

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PIN FUNCTIONS

VC (Pin 1): Error Amplifier Compensation Pin. Used to

stabilize the voltage loop with an external RC network.

FBX (Pin 2): Positive and Negative Feedback Pin. Receives

the feedback voltage from the external resistor divider

across the output. Also modulates the frequency during

start-up and fault conditions when FBX is close to GND.

SS (Pin 3): Soft-Start Pin. This pin modulates compensa-

tion pin voltage (VC) clamp. The soft-start interval is set

with an external capacitor. The pin has a 10µA (typical)

pull-up current source to an internal 2.5V rail. The soft-

start pin is reset to GND by an undervoltage condition

at SHDN/UVLO, an INTVCC undervoltage or overvoltage

condition or an internal thermal lockout.

RT (Pin 4): Switching Frequency Adjustment Pin. Set the

frequency using a resistor to GND. Do not leave this pin

open.

SYNC (Pin 5): Frequency Synchronization Pin. Used to

synchronize the switching frequency to an outside clock.

If this feature is used, an RT resistor should be chosen

to program a switching frequency 20% slower than the

SYNC pulse frequency. Tie the SYNC pin to GND if this

feature is not used. SYNC is ignored when FBX is close

to GND.

SENSE (Pin 6): The Current Sense Input for the Control

Loop. Kelvin connect this pin to the positive terminal of

the switch current sense resistor in the source of the

N-channel MOSFET. The negative terminal of the current

sense resistor should be connected to GND plane close

to the IC.

GATE (Pin 7): N-Channel MOSFET Gate Driver Output.

Switches between INTVCC and GND. Driven to GND when

IC is shut down, during thermal lockout or when INTVCC

is above or below the OV or UV thresholds, respectively.

INTVCC (Pin 8): Regulated Supply for Internal Loads and

Gate Driver. Supplied from VIN and regulated to 7.2V (typi-

cal). INTVCC must be bypassed with a minimum of 4.7µF

capacitor placed close to pin. INTVCC can be connected

directly to VIN, if VIN is less than 17.5V. INTVCC can also

be connected to a power supply whose voltage is higher

than 7.5V, and lower than VIN, provided that supply does

not exceed 17.5V.

SHDN/UVLO (Pin 9): Shutdown and Undervoltage Detect

Pin. An accurate 1.22V (nominal) falling threshold with

externally programmable hysteresis detects when power

is okay to enable switching. Rising hysteresis is generated

by the external resistor divider and an accurate internal

TYPICAL PERFORMANCE CHARACTERISTICS

Gate Drive Rise

and Fall Time vs INTVCC

Typical Start-Up Waveforms

TA = 25°C, unless otherwise noted.

FBX Frequency Foldback

Waveforms During Overcurrent

INTVCC (V)

3757 G19

CL = 3300pF

RISE TIME

FALL TIME

2ms/DIV

VOUT

5V/DIV

IL1A + IL1B

5A/DIV

3757 G20

VIN = 12V

PAGE 31 CIRCUIT

50µs/DIV

PAGE 31 CIRCUIT

VOUT

10V/DIV

VSW

20V/DIV

IL1A + IL1B

5A/DIV

3757 G21

VIN = 12V

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