9

FN3182.8

April 5, 2006

Applications

Single Supply Fault Monitor

Figure 12 shows an over/under voltage fault monitor for a

single supply. The overvoltage trip point is centered around

5.5V and the undervoltage trip point is centered around 4.5V.

Both have some hysteresis to prevent erratic output ON and

OFF conditions. The two outputs are connected in a wired

OR configuration with a pullup resistor to generate a power

OK signal.

Multiple Supply Fault Monitor

The ICL7665S can simultaneously monitor several supplies

when connected as shown in Figure 13. The resistors are

two input voltage are at the desired low voltage detection

voltage of both supplies being monitored. The trip voltage of

one supply while the other supply is at the nominal voltage

will be different that the trip voltage when both supplies are

below their nominal voltages.

The other side of the ICL7665S can be used to detect the

absence of negative supplies. The trip points for OUT1

depend on both the negative supply voltages and the actual

voltage of the +5V supply.

TABLE 1. SET-POINT EQUATIONS

NO HYSTERESIS

HYSTERESIS PER FIGURE 10A

HYSTERESIS PER FIGURE 11

OUT1

SET1

SET2

OUT2

V+

HYST1

HYST2

FIGURE 11. AN ALTERNATIVE HYSTERESIS CIRCUIT

HYST1

HYST2

V+

OUT1

OUT2

324k

Ω

13M

Ω

5%

100k

Ω

249k

Ω

7.5M

Ω

5%

+5V SUPPLY

POWER

OK

100k

Ω

1M

Ω

V+

OPEN VOLTAGE

DETECTOR

OPEN VOLTAGE

DETECTOR

FIGURE 12. FAULT MONITOR FOR A SINGLE SUPPLY

ICL7665S