AD421 datasheet(13/14 Pages) AD

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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

from VCC to battery voltage without a glitch when VCC power is

lost. The IRFF9113 acts as a current source during normal

operation and provides a constant charging current to the supercap

or Nicad. The loss of VCC drops the IRFF9113’s gate voltage to

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

supercap. Then with VCC and the load present adjust the 100 k

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

DN25D

VCC

DRIVE

LOOP

RTN

COM

AD421*

VLOOP

IRFF9113

SUPERCAP

VCC

GND

MICRO/

MEMORY

*ADDITIONAL CIRCUITRY OMITTED FOR CLARITY

4.7 F

0.1 F

2.2 F

Figure 19. Battery Backup Circuit

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