9

LT1640L/LT1640H

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

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

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

overheating.

Undervoltage and Overvoltage Detection

The UV (Pin 3) and OV (Pin 2) pins can be used to detect

undervoltage and overvoltage conditions at the power

LT1640L

PWRGD

SENSE

C1

0.033

µF

24V

C4

1

µF

100V

C3

100

µF

100V

Q1

IRF530

R2

10

Ω

5%

R8

510k

5%

R3

10k

5%

C2

3.3nF

100V

R4

562k

1%

R7

1M

5%

R5

9.09k

1%

R6

10k

1%

Q3

ZVN3310

Q2

2N2222

D1

1N4148

R1

0.02

Ω

5%

4

3

2

OV

GND

– 48V

UV

56

8

7

1

GATE

DRAIN

1640 F09a

3

2

+

Figure 9. Automatic Restart After Current Fault

1640 F09b