SP703 SIPEX [Sipex Corporation], SP703 Datasheet - Page 12

• Manufacturer Part Number: SP703
• Description: Low Power Microprocessor Supervisory with Battery Switch-Over
• Manufacturer: SIPEX [Sipex Corporation]

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
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
V
SP703/704DS/07
OUT
CC
+5V
0V
+5V
PFO
is close to the reset threshold.
0V
through the diode (D1 in Figure 9) when
GND
V
CC
1.25
PFO
PFI
R
V
2
L
V
V
=
H
V
TRIP
=
V
TRIP
L
=
- 1.25
R
R
V
R
1
3
H
1
R
R
+ R
1.25
1.25
1
2
R
connect to P
+ R
|| R
+
2
2
*C
|| R
5.0 - 1.25
2
3
1
SP703/704 Low Power Microprocessor Supervisory
R
3
BATT
*optional
3
V
IN
R
R
V
input
2
1
IN
12
Operation Without a Backup Power
Source
If a backup power source is not used, ground
V
no need to switch over to any backup power
source, V
direct connection to V
drops across the switch which may push V
below V
Replacing the Backup Battery
The backup battery can be removed while V
remains valid, without danger of triggering
RESET/RESET. As long as V
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
Figure 14 shows how to add hysteresis to the
power-fail comparator. Select the ratio of R
and R
to its trip point (V
It will typically be an order of magnitude greater
(about 10 times) than R
through R
ensure that the 25nA (max) PFI input current
does not shift the trip point. R
than 10K 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
BATT
CC
voltage, and the resistors, R1 and R2.
2
and connect V
such that PFI sees 1.25V when V
CC
OUT
1
.
and R
does not need to be switched. A
IN
TRIP
2
should be at least 1 A to
is close to its trip point.
OUT
). R
CC
© Copyright 2000 Sipex Corporation
eliminates any voltage
1
to V
3
or R
adds the hysteresis.
CC
CC
3
should be larger
2
. Since there is
stays above the
. The current
IN
falls
OUT
CC
1