ASM690A/692A

ASM802L/802M

April 2008

ASM805L

rev 1.7

µP Power Supply Supervisor With Battery Backup Switch

6 of 15

Notice: The information in this document is subject to change without notice.

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.

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

reasonable and should be larger than 10k

Ω 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.

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