6 of 15

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

Low Power µP Supervisor Circuits

ASM705 / 706 / 707 / 708

ASM813L

rev 1.2

July 2004

Monitoring a Negative Voltage

The Power-Fail circuitry can also monitor a negative supply

rail. When the negative rail is OK, PFO will be LOW, and

when the negative rail is failing (not negative enough), PFO

goes HIGH (the opposite of when positive voltages are

monitored). To trigger a reset, these outputs need to be

inverted: adding the resistors and transistor as shown

achieves this. The RESET output will then have the same

sense as for positive voltages: good = HIGH, bad = LOW. It

should be noted that this circuit’s accuracy depends on the

PFI

MR

PFO

ASM70X

RESET

GND

To

µP

Figure 5: Monitoring a negative voltage

2N3904

=

1.25 - VTRIP

Negative Input Voltage

+