ASM705 / 706 / 707 / 708
ASM813L
February 2005
rev 1.5
Monitoring a Negative Voltage
VCC
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
+
R1
R3
VCC
MR
R4
2N3904
ASM70x
PFO
PFI
R2
VCC line, the PFI threshold tolerance, and the resistors.
RESET
To µP
- 1.25
GND
V
1.25 - VTRIP
CC
=
Negative Input Voltage
R
2
R
1
Figure 5: Monitoring a negative voltage
6 of 16
Low Power µP Supervisor Circuits
Notice: The information in this document is subject to change without notice
ALSC [ ALLIANCE SEMICONDUCTOR CORPORATION ]
