11

LTC1642A

1642af

APPLICATIO S I FOR ATIO

Automatic Restart After the Circuit Breaker Opens

The LTC1642A will automatically attempt to restart itself

after the circuit breaker opens if the FAULT output is tied

to the ON pin. The circuit is shown in Figure 5. Diode D1

blocks the weak FAULT pull-up current source from unbal-

ancing the R6-R5 divider.

During a continuous current limit such as a load short,

Q1’s duty cycle is equal to the circuit breaker timer period,

divided by the sum of the circuit breaker and system timer

periods:

Short - Circuit Duty Cycle

=

+

C

CC

4

410

1

•

The duty cycle is 9% for the Figure 5 circuit. Waveforms

during a load short are shown in Figure 6.

Undervoltage Lockout

An internal undervoltage lockout circuit holds the charge

it turns off the charge pump and clears overvoltage and

current limit faults.

For higher lockout thresholds tie the ON pin to a resistor

(1+R6/R5) • 1.22V.

D1

1N4148

D2

1N4705

18V

GATE

ON

14

SENSE

15

4

FAULT

6

BRK TMR

2

16

R2

0.015

Ω

Q1

FDS6630A

GND

LTC1642A

8

RST TMR

3

R5

60.4k

1%

R6

464k

1%

R10

30k

1642a F05

R3

100

Ω

R4

330

Ω

C2

0.047

µF

+

C1

0.33

µF

C4

0.33

µF

ALL RESISTORS

±5% UNLESS NOTED

12V

2.5A

Figure 5. Automatic Restart Circuit

Figure 6. Automatic Retry Following a Load Short

20V/DIV

10V/DIV

1V/DIV

1V/DIV

40ms/DIV

1642a F06

D1

1N4705

18V

GATE

14

SENSE

15

ON

4

16

R2

0.015

Ω

Q1

FDS6630A

GND

LTC1642A

8

RST TMR

3

R6

464k

1%

R5

60.4k

1%

12V

2.5A

1642a F07

R3

100

Ω

R4

330

Ω

C2

0.047

µF

+

C1

0.33

µF

ALL RESISTORS

±5% UNLESS NOTED

UNDERVOLTAGE

LOCKOUT

THRESHOLD = 10.7V

Figure 7. Setting a Higher Undervoltage Lockout