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Figure 19. Adding Hysteresis to the Power Fail Comparator
ADM69x
Monitoring the Status of the Battery
Figure 18. Rechargeable Battery
Adding Hysteresis to the Power Fail Comparator
For increased noise immunity, hysteresis may be added to the
power fail comparator. Since the comparator circuit is nonin-
verting, hysteresis can be added simply by connecting a resistor be-
tween the
PFO
output and the PFI input as shown in Figure 19.
The power fail comparator can be used to monitor the status of
the backup battery instead of the power supply if desired. This
is shown in Figure 20. The PFI input samples the battery volt-
age and generates an active low
PFO
signal when the battery
voltage drops below a chosen threshold. It may be necessary to
apply a test load in order to determine the loaded battery volt-
age. This can be done under processor control using
CE
OUT.
Since
CE
OUT
is forced high during the battery backup mode, the
test load will not be applied to the battery while it is in use, even
if the microprocessor is not powered.
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