10

LTC1642

APPLICATIO S I FOR ATIO

Undervoltage Monitor

The LTC1642 will assert RESET if a monitored voltage falls

below a programmable minimum. When the monitored

voltage has exceeded its minimum for at least one system

timing cycle, RESET goes high. The monitoring circuitry

comprises an internal 1.23V bandgap reference, an inter-

nal precision voltage comparator and an external resistive

divider to monitor the output supply voltage. The circuit is

shown in Figure 12, and typical waveforms in Figure 13.

When the voltage at the FB pin rises above its reset

threshold (1.23V), the comparator output goes low and a

timing cycle starts (times 1 and 5). Following the cycle

RESET is pulled high.

At time 2 the voltage at FB drops below the comparator’s

threshold and RESET is pulled low. If the FB pin rises

above the reset threshold for less than a timing cycle the

RESET output will remain low (time 3 to time 4). The 15

µA

resistor without forcing current back into the supply.

Reference

The LTC1642’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.

Uncommitted Comparator

The uncommitted comparator has an open drain output.

The comparator has 3mV of hysteresis: the output goes

high when the differential input voltage exceeds 1.5mV

and goes low when the differential input is less than

–1.5mV.

GATE

ON

14

SENSE

15

FB

7

RESET

5

4

16

R2

0.010

Ω

Q1

FDR9410A

GND

LTC1642

8

RST TMR

3

1642 F12

R3

100

Ω

R4

330

Ω

C2

0.047

µF

R7

95.3k

1%

R6

12.4k

1%

+

C1

0.33

µF

ALL RESISTORS

±5% UNLESS NOTED.

12V

2.5A

Figure 12. Undervoltage Monitoring Circuitry

1642 F13

RST TMR

V1

V2

V2

V1

5

4

3

2

1

RESET

V1

Figure 13. Supply Monitor Waveforms

REFERENCE CURRENT

100

µA

1mA

10mA

1642 F14

0.1

0.2

0.4

1.0

2.0

4.0

10.0

Figure 14. Minimum REF Compensation vs REF Current