14

LTC1642A

1642af

APPLICATIO S I FOR ATIO

The undervoltage monitor behaves differently if FB is

above its threshold when the GATE begins ramping:

RESET goes high as soon as the GATE ramp begins.

2.5V internal undervoltage lockout threshold.

To disable the undervoltage monitor, tie FB to REF and

ground RESET.

D1

1N4705

18V

GATE

ON

14

SENSE

15

FB

7

RESET

5

4

16

R2

0.015

Ω

Q1

FDS6630A

GND

LTC1642A

8

RST TMR

3

1642a F12

R3

100

Ω

R4

330

Ω

C2

0.047

µF

R9

95.3k

1%

R8

12.4k

1%

+

C1

0.33

µF

ALL RESISTORS

±5% UNLESS NOTED.

12V

2.5A

Figure 12. Undervoltage Monitoring Circuitry

Figure 13. Supply Monitor Waveforms

REFERENCE CURRENT

100

µA

1mA

10mA

1642a F14

0.1

0.2

0.4

1.0

2.0

4.0

10.0

Figure 14. Minimum REF Compensation vs REF Current

Reference

The LTC1642A’s internal voltage reference is buffered and

brought out to the REF pin. The buffer amplifier should be

compensated with a capacitor connected between REF

and ground. If no DC current is drawn from REF, 0.1

µF

ensures an adequate phase margin, but the minimum

compensation increases if REF sources a substantial DC

current, as shown in Figure 14.

250ms/DIV

1642a F13

20V/DIV

1V/DIV

10V/DIV