SP690A Datasheet, PDF (12/16 Pages) Sipex Corporation – Low Power Microprocessor Supervisory with Battery Switch-Over

+5V
VCC
PFI
VIN
R1
R3
*C1 R2
PFO
*optional
GND
connect to µP
VTRIP =
1.25
R2
R1 + R2
1.25
R2
=
VL
- 1.25
R1
+
5.0 - 1.25
R3
VH =
1.25
R2 || R3
PFO
R1 + R2 || R3
+5V
0V
VIN
0V
VL VTRIP VH
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_BATT input of
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_OUT through the diode (D1 in Figure 13)
when V_CC is close to the reset threshold.
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 18
shows how to add hysteresis to the power-fail
comparator. Select the ratio of R₁ and R₂ such
that PFI sees 1.25V when V_IN falls to its trip
point (V_TRIP). R₃ adds the hysteresis. It will
typically be an order of magnitude greater (about
10 times) than R₁ or R₂. The current through R₁
and R₂ should be at least 1µA to ensure that the
25nA (max) PFI input current does not shift the
trip point. R₃ 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
19. 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.
Date: 11/29/04
SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over
12
© Copyright 2004 Sipex Corporation