sp690acp Exar Corporation, sp690acp Datasheet - Page 12

• Manufacturer Part Number: sp690acp
• Description: Low Power Microprocessor Supervisory With Battery Switch-over
• Manufacturer: Exar Corporation

Figure 18. 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
this series with no additional circuitry; see
FIGURE 12. However, batteries with open-
circuit voltages that are greater than this value
cannot be used for backup, as current is sourced
into V
when V
Date: 11/29/04
+5V
0V
+5V
PFO
0V
OUT
GND
V
CC
CC
through the diode (D1 in Figure 13)
is close to the reset threshold.
1.25
PFO
PFI
R
SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over
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 µP
+ R
|| R
+
2
2
*C
|| R
5.0 - 1.25
2
3
BATT
1
R
3
*optional
3
V
input of
IN
R
R
V
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
shows how to add hysteresis to the power-fail
comparator. Select the ratio of R
that PFI sees 1.25V when V
point (V
typically be an order of magnitude greater (about
10 times) than R
and R
25nA (max) PFI input current does not shift the
trip point. R
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
19. When the negative rail is valid, PFO is low.
When the negative supply voltage drops, PFO
goes high.
affected by the PFI threshold tolerance, the V
voltage, and the resistors, R1 and R2.
BATT
2
and connect V
should be at least 1µA to ensure that the
CC
TRIP
OUT
.
IN
). R
3
does not need to be switched. A
should be larger than 10KΩ so it
is close to its trip point. Figure 18
This circuit's accuracy is
1
3
or R
adds the hysteresis. It will
OUT
2
CC
. The current through R
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eliminates any voltage
to V
CC
IN
CC
. Since there is
stays above the
falls to its trip
1
and R
2
such
OUT
CC
CC
1