April 2008
rev 1.7
Power-Fail Comparator
The Power Fail feature is an independent voltage
monitoring function that can be used for any number of
monitoring activities. The PFI function can provide an
early sensing of power supply failure by sensing the
voltage of the unregulated DC ahead of the regulated
supply sensing seen by the backup-battery switchover
circuitry. The PFI pin is compared to a 1.25V internal
reference. If the voltage at the PFI pin is less than this
reference voltage, the PFO pin goes low. By sensing the
voltage of the raw DC power supply, the microprocessor
system can prepare for imminent power-loss, especially if
the battery backup supply is not enabled. The input
voltage at the PFI pin results from a simple resistor
voltage divider as shown in Figure 6.
ASM690A/692A
ASM802L/802M
ASM805L
reasonable and should be larger than 10kΩ to avoid
excessive loading on the PFO pin. The calculations for
the correct values of resistors to set the hysteresis
thresholds are given in
Figure 7.
A capacitor can be
added to offer additional noise rejection by low-pass
filtering.
Power Fail Hysteresis
A noise margin can be added to the simple monitoring
circuit of
Figure 6
by adding positive feedback from the
PFO pin. The circuit of
Figure 7
adds this positive
“latching” effect by means of an additional resistor R3
connected between PFO and PFI which helps in pulling
PFI in the direction of PFO and eliminating an indecision
at the trip point. Resistor R3 is normally about 10 times
higher in resistance than R2 to keep the hysteresis band
Monitoring Capabilities Of The Power-fail Input:
Although designed for power supply failure monitoring,
the PFI pin can be used for monitoring any voltage
condition that can be scaled by means of a resistive
divider. An example is the negative power supply monitor
configured in Figure 8. In this case a good negative
supply will hold the PFI pin below 1.25V and the PFO pin
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice.
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PULSECORE [ PulseCore Semiconductor ]
