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

ASM690A / 692A

ASM802L / 802M

ASM805L

µP Power Supply Supervisor With Battery Backup Switch

rev 1.0

October 2003

Operation without a Backup Power Source

due to the ON-resistance of the PMOS switch.

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.

PFI

PFO

GND

R1

R2

ASM690A

ASM692A

ASM802L

ASM802M

ASM805L

A

B

+5V

PFO

0V

A

5R2

R1 R2

+

-------------------- 1.25V

<

=

B

5R2

R1 R2

+

-------------------- 1.25V

>

=

Figure 6: Simple Voltage divider sets PFI trip point

+5V

PFI

PFO

GND

+5V

To µP

ASM690A

ASM692A

ASM802L

ASM802M

ASM805L

C1*

* Optional

+5V

PFO

0V

0V

Figure 7: Hysterisis Added To PFI Pin

V

TRIP

1.25

R2

R2 R2

+

--------------------

⎝⎠

⎜⎟

⎛⎞

-------------------------

=

1.25

||

||

+

------------------------------

⎝⎠

⎛⎞

------------------------------------

=

–

----------------------- 51.25

–

-------------------

+

1.25

----------

=