IMP705/6/7/8, 813L
Application Information
Monitoring Voltages Other Than VCC
Monitoring a Negative Voltage
The IMP705-708 can monitor voltages other than V using the The Power-Fail circuitry can also monitor a negative supply rail.
CC
Power Fail circuitry. If a resistive divider is connected from the When the negative rail is OK, PFO will be LOW, and when the
voltage to be monitored to the Power Fail input, PFI, the PFO negative rail is failing (not negative enough), PFO goes HIGH
(output) will go LOW if the divider voltage goes below its 1.25V (the opposite of when positive voltages are monitored). To trigger
reference. Should hysteresis be desired, connect a resistor (equal a reset, these outputs need to be inverted: adding the resistors
to approximately 10 times the sum of the two resistors in the and transistor as shown achieves this. The RESET output will
divider) between the PFI and PFO pins. A capacitor between PFI then have the same sense as for positive voltages: good = HIGH,
and GND will reduce circuit sensitivity to input high-frequency bad = LOW. It should be noted that this circuit’s accuracy depends
noise. If it is desired to assert a RESET in addition to the PFO flag, on the VCC line, the PFI threshold tolerance, and the resistors.
this may be achieved by connecting the PFO output to MR.
+5V
+
V
= 5V
CC
V
V
CC
CC
MR
MR
V
= 12V
1M
IN
IMP70x
IMP70x
PFO
PFO
RP
RN
PFI
RESET
GND
PFI
RESET
GND
To Processor
130k
12V Threshold
~10.87V
5 −1.25 1.25 − V
TRIP
—
=
RP
RN
Negative Input
Voltage
705_09.eps
705_07.eps
Figure 4. Monitoring Voltages Other Than VCC
Figure 5. Monitoring a Negative Voltage
©
1999 IMP, Inc.
408-432-9100/www.impweb.com
7
IMP [ IMP, INC ]
