IMP705/6/7/8, 8 13L
Application Information
Monitoring Voltages Other Than V
CC
The IMP705-708 can monitor voltages other than V
CC
using the
Power Fail circuitry. If a resistive divider is connected from the
voltage to be monitored to the Power Fail input, PFI, the PFO
(output) will go LOW if the divider voltage goes below its 1.25V
reference. Should hysteresis be desired, connect a resistor (equal
to approximately 10 times the sum of the two resistors in the
divider) between the PFI and PFO pins. A capacitor between PFI
and GND will reduce circuit sensitivity to input high-frequency
noise. If it is desired to assert a RESET in addition to the PFO flag,
this may be achieved by connecting the PFO output to MR.
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.
+5V
V
CC
= 5V
V
CC
MR
+
V
CC
MR
V
IN
= 12V
1M
9
IMP70x
PFO
RP
IMP70x
PFO
PFI
130k
9
RESET
GND
To Processor
12V Threshold
~10.87V
RN
—
PFI
RESET
GND
5
−
1.25 1.25
−
V
TRIP
=
RP
RN
705_07.eps
Negative Input
Voltage
705_09.eps
Figure 4. Monitoring Voltages Other Than V
CC
Figure 5. Monitoring a Negative Voltage
©
1999 IMP, Inc.
408-432-9100/www.impweb.com
7
IMP [ IMP, INC ]
