LT1640L datasheet(10/16 Pages) LINER

10 / 16 page

LT1640L/LT1640H

1640lhfb

APPLICATIONS INFORMATION

Under some conditions, a short circuit at the output can

cause the input supply to dip below the UV threshold,

resetting the circuit breaker immediately.

The LT1640 then cycles on and off repeatedly until the

short is removed. This can be minimized by adding a

deglitching delay to the UV pin with a capacitor from UV to

VEE. This capacitor forms an RC time constant with the

resistors at UV, allowing the input supply to recover before

the UV pin resets the circuit breaker.

A circuit that automatically resets the circuit breaker after

a current fault is shown in Figure 9.

Transistors Q2 and Q3 along with R7, R8, C4 and D1 form

a programmable one-shot circuit. Before a short occurs,

the GATE pin is pulled high and Q3 is turned on, pulling

node 2 to VEE. Resistor R8 turns off Q2. When a short

occurs, the GATE pin is pulled low and Q3 turns off. Node

2 starts to charge C4 and Q2 turns on, pulling the UV pin

low and resetting the circuit breaker. As soon as C4 is fully

charged, R8 turns off Q2, UV goes high and the GATE

starts to ramp up. Q3 turns back on and quickly pulls node

2 back to VEE. Diode D1 clamps node 3 one diode drop

below VEE. The duty cycle is set to 10% to prevent Q1 from

overheating.

VEE

VDD

LT1640L

PWRGD

SENSE

150nF

25V

µF

100V

100

µF

100V

IRF530

Ω

510k

18k

3.3nF

100V

562k

19.1k

10k

562k

ZVN3310

2N2222

1N4148

0.02

Ω

– 48V

GATE

DRAIN

1640 F09a

GND

(SHORT PIN)

GND

* DIODES INC. SMAT70A

1N4148

Figure 9. Automatic Restart After Current Fault

1640 F09b

NODE 2

50V/DIV

GATE

2V/DIV

1s/DIV

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