February 2005
rev 1.5
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
V
CC
line, the PFI threshold tolerance, and the resistors.
R
1
V
CC
+
ASM705 / 706 / 707 / 708
ASM813L
R
3
V
CC
MR
R
4
ASM70x
PFI
PFO
2N3904
R
2
GND
Negative Input Voltage
RESET
To
µP
V
CC
- 1.25
R
1
=
1.25 - VTRIP
R
2
Figure 5: Monitoring a negative voltage
Low Power µP Supervisor Circuits
Notice: The information in this document is subject to change without notice
6 of 16
ALSC [ ALLIANCE SEMICONDUCTOR CORPORATION ]
