LT4256-3IGN datasheet(7/20 Pages) LINER

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LT4256-3

42563f

PI FU CTIO S

GATE (Pin 13): High Side Gate Drive for the External

N-Channel MOSFET. An internal charge pump guarantees

at least 10V of gate drive for VCC supply voltages above

20V and 4.5V of gate drive for VCC supply voltages

between 10.8V and 20V. The rising slope of the voltage on

GATE is set by an external capacitor connected from GATE

to GND and an internal 30

µA pull-up current source from

the charge pump output.

If the current limit is reached, the GATE voltage is adjusted

to maintain a constant voltage across the sense resistor

while the timing capacitor starts to charge. If the TIMER

voltage ever exceeds 4.65V, GATE is pulled low.

GATE is also pulled to GND whenever UV is pulled low; the

VCC supplyvoltagedropsbelowtheexternallyprogrammed

undervoltage threshold, above the overvoltage threshold

or below the internal UVLO threshold (9.8V).

GATE is clamped internally to a maximum voltage of 11.6V

(typ) above VOUT under normal operating conditions.

Driving this pin beyond the clamp voltage may damage the

part. GATE is also clamped to 2V (typ) below VOUT. When

the gate is commanded off due to a fault condition, it is

discharged quickly by a 62mA (typ) capable switch until

GATE is 2V (typ) below VOUT. When GATE is below VOUT

by 2V, the 62mA is reduced to 130

µA to protect the

LT4256-3 against damage if VOUT has large capacitance.

A Zener diode is needed between the gate and source of the

external MOSFET to protect its gate oxide under instanta-

neous short-circuit conditions. See Applications Informa-

tion.

SENSE (Pin 15): Current Limit Sense Input. A sense

resistor is placed in the supply path between VCC and

SENSE. The current limit circuit regulates the voltage

across the sense resistor (VCC – SENSE) to 55mV while in

current limit when FB is 2V or higher. If FB drops below

2V, the regulated voltage across the sense resistor de-

creases linearly and stops at 14mV when FB is 0V. The

OPEN output also uses SENSE to detect when the output

current is less than (3mV)/R5.

To defeat current limit, connect SENSE to VCC.

VCC (Pin 16): Input Supply Voltage. The positive supply

input ranges from 10.8V to 80V for normal operation. ICC

is typically 1.8mA. An internal circuit disables the LT4256-3

for inputs less than 9.8V (typ).

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