5

FN3183.4

July 22, 2005

Applications for the ICL7673 include volatile semiconductor

memory storage systems, real-time clocks, timers, alarm

systems, and over/under the voltage detectors. Other

systems requiring DC power when the master AC line supply

fails can also use the ICL7673.

A typical application, as illustrated in Figure 12, would be a

microprocessor system requiring a 5V supply. In the event of

primary supply failure, the system is powered down, and a

3V battery is employed to maintain clock or volatile memory

clock or volatile memory. The ICL7673 will sense the main

event of main supply failure, the circuit will sense that the

Figure 11 illustrates the use of external PNP power

transistors to increase the power switching capability of the

circuit. In this application the output current is limited by the

beta and thermal characteristics of the power transistors.

If hysteresis is desired for a particular low power application,

Figure 12. For high power applications hysteresis can be

applied as shown in Figure 13.

The ICL7673 can also be used as a clipping circuit as

illustrated in Figure 14. With high impedance loads the

circuit output will be nearly equal to the greater of the two

input signals.

GND

Pbar

8

2

1

6

+5V OR +3V

STATUS

INDICATOR

LITHIUM

BATTERY

GND

+5V

PRIMARY

SUPPLY

4

+

-

FIGURE 7. ICL7673 BATTERY BACKUP CIRCUIT

GND

8

2

1

+5V OR +3V

RECHARGEABLE

BATTERY

GND

+5V

PRIMARY

SUPPLY

4

+

-

FIGURE 8. APPLICATION REQUIRING RECHARGEABLE

BATTERY BACKUP

FIGURE 9. POWER SUPPLY FOR LOW POWER PORTABLE AC TO DC SYSTEMS

FIGURE 10. TYPICAL MICROPROCESSOR MEMORY APPLICATION

FUSE

120/240

VAC

BRIDGE

RECTIFIER

BATTERY

STACK

GND

1

8

2

4

2

8

4

6

STEPDOWN

TRANSFORMER

+

-

ICL7673

BATTERY

BACK-UP

ICL7663

REGULATOR

POWER

FAIL

DETECTOR

MICROPROCESSOR

VOLATILE

RAM

INTERRUPT SIGNAL

ICL7673

BACKUP CIRCUIT

+5V

MAIN

POWER

+

-

ICL7673