10

FN3182.8

April 5, 2006

Combination Low Battery Warning and Low

Battery Disconnect

When using rechargeable batteries in a system, it is

important to keep the batteries from being over discharged.

The circuit shown in Figure 14 provides a low battery

warning and also disconnects the low battery from the rest of

the system to prevent damage to the battery. OUT1 is used

to shutdown the ICL7663S when the battery voltage drops to

the value where the load should be disconnected. As long as

drops below 1.3V, OUT1 goes high shutting off the

ICL7663S. OUT2 is used for low battery warning. When

and the low battery warning goes off. The trip voltage for low

battery warning can be set higher than the trip voltage for

shutdown to give advance low battery warning before the

battery is disconnected.

Power Fail Warning and Powerup/Powerdown

Reset

Figure 15 shows a power fail warning circuit with

powerup/powerdown reset. When the unregulated DC input

is above the trip point, OUT1 is low. When the DC input

drops below the trip point, OUT1 shuts OFF and the power

fail warning goes high. The voltage on the input of the 7805

7.3V, this circuit will provide a certain amount of warning

before the 5V output begins to drop.

The ICL7665S OUT2 is used to prevent a microprocessor

from writing spurious data to a CMOS battery backup

memory by causing OUT2 to go low when the 7805 5V

output drops below the ICL7665S trip point.

FIGURE 13. MULTIPLE SUPPLY FAULT MONITOR

HYST1

HYST2

V+

OUT1

OUT2

274k

Ω

100k

Ω

22

POWER

OK

301

+5V

-15V

-5V

M

Ω

k

Ω

787

k

Ω

+5V

1M

Ω

1.02M

Ω

49.9k

Ω

+5V

+15V

HYST1

SET1

SET2

HYST2

V+

OUT1

OUT2

GND

+

-

LOW BATTERY SHUTDOWN

1M

Ω

V+

OUT1

SHUTDOWN

OUT2

V+

GND

1M

Ω

LOW BATTERY WARNING

SENSE

100

Ω

+5V

1A

ICL7663S

ICL7665S

FIGURE 14. LOW BATTERY WARNING AND LOW BATTERY DISCONNECT

ICL7665S