+5V
V
IN
V
CC
PFI
R
3
PFO
*optional
GND
V
TRIP
=
connect to
µP
1.25
R
2
R
1
+ R
2
*C
1
R
2
R
1
Operation Without a Backup Power
Source
If a backup power source is not used, ground
V
BATT
and connect V
OUT
to V
CC
. Since there is
no need to switch over to any backup power
source, V
OUT
does not need to be switched. A
direct connection to V
CC
eliminates any voltage
drops across the switch which may push V
OUT
below V
CC
.
Replacing the Backup Battery
The backup battery can be removed while V
CC
remains valid, without danger of triggering
RESET/RESET. As long as V
CC
stays above the
reset threshold, battery-backup mode cannot be
entered.
Adding Hysteresis to the Power-Fail
Comparator
Hysteresis adds a noise margin to the power-fail
comparator and prevents repeated triggering
of PFO when V
IN
is close to its trip point.
Figure 14
shows how to add hysteresis to the
power-fail comparator. Select the ratio of R
1
and R
2
such that PFI sees 1.25V when V
IN
falls
to its trip point (V
TRIP
). R
3
adds the hysteresis.
It will typically be an order of magnitude greater
(about 10 times) than R
1
or R
2
. The current
through R
1
and R
2
should be at least 1µA to
ensure that the 25nA (max) PFI input current
does not shift the trip point. R
3
should be larger
than 10KΩ so it does not load down the PFO pin.
Capacitor C1 adds additional noise rejection.
Monitoring a Negative Voltage
The power-fail comparator can be used to
monitor a negative supply rail using the circuit
of
Figure 15.
When the negative rail is valid,
PFO is low. When the negative supply voltage
drops, PFO goes high. This circuit's accuracy
is affected by the PFI threshold tolerance, the
V
CC
voltage, and the resistors, R1 and R2.
1.25
R
2
=
V
L
- 1.25
+ 5.0 - 1.25
R
3
R
1
1.25
R
2
|| R
3
R
1
+ R
2
|| R
3
V
H
=
PFO
+5V
0V
0V
V
L
V
TRIP
V
H
V
IN
Figure 14. Adding Hysteresis to the POWER-FAIL
Comparator
Allowable Backup Power-Source
Batteries
Lithium batteries work very well as backup
batteries due to very low self-discharge rate and
high energy density. Single lithium batteries
with open-circuit voltages of 3.0V to 3.6V are
ideal. Any battery with an open-circuit voltage
less than the minimum reset threshold plus 0.3V
can be connected directly to the V
BATT
input
of this series with no additional circuitry; see
Figure 8.
However, batteries with open-circuit
voltages that are greater than this value cannot
be used for backup, as current is sourced into
V
OUT
through the diode
(D1 in Figure 9)
when
V
CC
is close to the reset threshold.
SP703/704DS/07
SP703/704 Low Power Microprocessor Supervisory
© Copyright 2000 Sipex Corporation
12
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