AD421

–13–

REV. C

Battery Backup

Figure 19 shows an application circuit for the AD421 where the

micro and memory sections of the circuitry are protected against

losing data if the loop is broken. The backup circuit switches

lost. The IRFF9113 acts as a current source during normal

operation and provides a constant charging current to the supercap

zero volts, which allows the battery or supercaps current to flow

through the MOSFETs channel and integral body diode to

provide power for the micro and memory sections. To calibrate

this circuit, connect an ammeter in series with the battery or

Ω

potentiometer for the battery charging current recommended by

the battery or supercap manufacturer.

Nonrechargeable batteries should not be used in this application

due to danger of explosion.

100k

IN3611

IN3611

DN25D

DRIVE

LOOP

RTN

COM

AD421*

IRFF9113

SUPERCAP

GND

MICRO/

MEMORY

*ADDITIONAL CIRCUITRY OMITTED FOR CLARITY

4.7 F

0.1 F

2.2 F

Figure 19. Battery Backup Circuit