8

LT1641

APPLICATIO S I FOR ATIO

Figure 9. Short-Circuit Waveforms

TIMER

SENSE

GATE

87

6

54

2

1

3

D1

CMPZ

5248B

GND

PWRGD

PWRGD

FB

R5

10

Ω

5%

ON

R1

49.9k

1%

24V

GND

1641 F10

R2

3.4k

1%

C2

0.68

µF

0.025

Ω

C1

10nF

R3

59k

1%

R4

3.57k

1%

R7

24k

5%

LT1641

Q1

IRF530

Figure 10. Automatic Restart Application

TIMER

The TIMER pin (Pin 5) provides a method for program-

ming the maximum time the chip is allowed to operate in

current limit. When the current limit circuitry is not active,

the TIMER pin is pulled to GND by a 3

µA current source.

After the current limit circuit becomes active, an 80

µApull-

up current source is connected to the TIMER pin and the

voltage will rise with a slope equal to 77

as the current limit circuit remains active. Once the desired

maximum current limit time is set, the capacitor value is:

C(nF) = 62 • t(ms).

If the current limit circuit turns off, the TIMER pin will be

discharged to GND by the 3

µA current source.

Whenever the TIMER pin reaches 1.233V, the internal fault

latch is set. The GATE pin is immediately pulled to GND and

the TIMER pin is pulled back to GND by the 3

µA current

source. The part is not allowed to turn on again until the

voltage at the TIMER pin falls below 0.5V. The fault latch

is cleared by pulling the ON pin low.

The waveform in Figure 9 shows how the output latches off

following a short-circuit. The drop across the sense resis-

tor is held at 12mV as the timer ramps up. Since the output

did not rise bringing FB above 0.5V, the circuit latches off.

Automatic Restart

To force the LT1641 to automatically restart after an

overcurrent fault, the bottom plate of capacitor C1 can be

tied back to the ON pin (Figure 10).