LTC1642_15 datasheet(7/16 Pages) LINER

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LTC1642

1642fb

OV (Pin 9): Overvoltage Input. When the voltage on OV

exceeds its trip point the GATE pin is pulled low immedi-

ately and the CRWBR timer starts. If OV remains above its

trip point (minus 3mV of hysteresis) long enough for

CRWBR to reach its trip point, then the part turns off until

reset by pulsing the ON pin low. Otherwise, the GATE pin

begins ramping up one RST TMR timing cycle after OV

goes below its trip point. Ground the OV pin to disable

overvoltage protection.

COMPOUT (Pin 10): Uncommitted Comparator’s Open

Drain Output.

COMP + (Pin 11): Uncommitted Comparator’s Noninvert-

ing Input.

COMP – (Pin 12): Uncommitted Comparator’s Inverting

Input.

REF (Pin 13): Reference Voltage Output. The 1.22V

±1%

reference should be bypassed with a 0.1

µF compensation

capacitor. For VCC = 5V it can source 1mA.

GATE (Pin 14): Gate Drive for the External N-Channel

MOSFET. An internal charge pump provides at least 4.5V

of gate drive and sources 25

µA. The pin requires an

external series RC network to ground to compensate the

current limit loop and to limit the ramp rate. A resistor of

100

Ω is also recommended in series with the MOSFET

PI FU CTIO S

gate to suppress high frequency oscillations. GATE is

immediately pulled to ground when the overvoltage com-

parator trips or the input supply is below the undervoltage

lockout trip point. During current limit the GATE voltage is

adjusted to maintain constant load current until the circuit

breaker timer trips. At that point GATE is pulled to ground

until the chip is reset. Clamp the GATE pin with an 18V

zener diode (IN4705) to ground if the supply is 8V

or higher.

SENSE (Pin 15): Current Sense Input. To use the current

limit place a sense resistor in the supply path between VCC

and SENSE. When the drop across the resistor exceeds a

threshold voltage, the GATE pin is adjusted to maintain a

constant load current and the circuit breaker timer is

started. A foldback feature reduces the current limit as the

voltage at FB approaches ground. Short SENSE to VCC to

disable the current limiting.

VCC (Pin 16): Positive Supply Voltage. An internal under-

voltage lockout circuit holds the GATE pin at ground until

VCC exceeds 2.73V. If VCC exceeds 16.5V an internal shunt

regulator protects the chip from VCC and SENSE pin

voltages up to 33V. In this case the GATE pin voltage will

usually be low but this is not guaranteed; use the OV pin

to ensure that the pass device is off. The VCC pin also

provides a high side connection to the SENSE resistor.

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