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 V
CC
to battery voltage without a glitch when V
CC
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 V
CC
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 V
CC
and the load present adjust the 100 kΩ
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
V
CC
DRIVE
LOOP
RTN
COM
AD421*
V
LOOP
IRFF9113
SUPERCAP
V
CC
GND
MICRO/
MEMORY
*ADDITIONAL CIRCUITRY OMITTED FOR CLARITY
4.7F0.1F
2.2F
Figure 19. Battery Backup Circuit
