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LT1641IS8

Positive High Voltage Hot Swap Controller

Linear Technology 

LT1641

APPLICATIO S I FOR ATIO

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 3µA current source.

After the current limit circuit becomes active, an 80µA pull-

up current source is connected to the TIMER pin and the

voltage will rise with a slope equal to 77µA/CTIMER as long

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 3µA 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 3µA 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).

Figure 9. Short-Circuit Waveforms

VIN

24V

R1

49.9k

1%

RS

0.025Ω

Q1

IRF530

R5

10Ω

5%

C1

10nF

D1

CMPZ R3

5248B 59k

1%

VOUT

CL

R2

3.4k

1%

GND

8

VCC

1

ON

7

SENSE

6

GATE

LT1641

TIMER

5

C2

0.68µF

GND

4

2

FB

3

PWRGD

R7

24k

5%

R4

3.57k

1%

PWRGD

1641 F10

Figure 10. Automatic Restart Application

8

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