HN705/HN706
Monitoring Voltages Other Than V
CC
The HN705/706 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
PFI
130k
9
HN70x
PFO
RP
HN70x
PFO
RESET
GND
To Processor
RN
PFI
RESET
GND
5
−
1.25 1.25
−
V
TRIP
=
RP
RN
12V Threshold
~10.87V
—
Negative Input
Voltage
Figure 5. Monitoring Voltages Other Than V
CC
Figure 6. Monitoring a Negative Voltage
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HN [ NANJING HONANO ELECTRONIC CO., LTD. ]
