Ensuring a Valid
RESET
Output
Down to V
CC
= 0V
When V
CC
falls below 1V, the MAX703/MAX704 RESET
output no longer sinks current; it becomes an open cir-
cuit. High-impedance CMOS logic inputs can drift to
undetermined voltages if left as open circuits. If a pull-
down resistor is added to the RESET pin as shown in
Figure 6, any stray charge or leakage currents will flow
to ground, holding RESET low. Resistor value R1 is not
critical. It should be about 100kΩ, which is large
enough not to load RESET and small enough to pull
RESET to ground.
Replacing the Backup Battery
The backup battery can be removed while V
CC
remains
valid without triggering a reset. As long as V
CC
stays
above the reset threshold, battery-backup mode cannot
be entered. This is an improvement on switchover ICs
that initiate a reset when V
CC
and V
BATT
are at or near
the same voltage level (regardless of the reset thresh-
old voltage). If the voltage on the unconnected V
BATT
pin floats up toward V
CC
, this condition alone cannot
initiate a reset when using the MAX703/MAX704.
Adding Hysteresis to the
Power-Fail Comparator
Hysteresis adds a noise margin to the power-fail com-
parator and prevents repeated triggering of PFO when
V
IN
is near the power-fail comparator trip point. Figure 7
shows how to add hysteresis to the power-fail com-
parator. Select the ratio of R1 and R2 so that PFI sees
1.25V when V
IN
falls to the desired trip point (V
TRIP
).
Resistor R3 adds hysteresis. It will typically be an order
of magnitude greater 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 to prevent it
from loading 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 voltage using Figure 8’s circuit. When
the negative supply is valid, PFO is low. When the neg-
ative supply voltage droops, PFO goes high. This cir-
cuit’s accuracy is affected by the PFI threshold
tolerance, the V
CC
voltage, and resistors R1 and R2.
MAX703/MAX704
Low-Cost Microprocessor Supervisory
Circuits with Battery Backup
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