Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
connected directly to the V
BATT
input of the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L with
no additional circuitry (see the
Typical Operating Circuit).
However, batteries with open-circuit voltages that are
greater
cannot
be used for backup, as current is sourced
into the substrate through the diode (D1 in Figure 3) when
V
CC
is close to the reset threshold.
ICs where battery-backup mode is entered whenever
V
BATT
gets close to V
CC
, an unconnected V
BATT
pin
accumulates leakage charge and triggers
RESET/RESET in error.
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 close to its trip point. Figure 6 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 than R
1
or R
2
(about 10 times either 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.
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. In other switchover
+5V
Monitoring a Negative Voltage
The power-fail comparator can be used to monitor a
negative supply rail using the circuit of Figure 7. When
the negative rail is good (a negative voltage of large
magnitude),
PFO
is low. When the negative rail is
degraded (a negative voltage of lesser magnitude),
PFO
goes high. This circuit’s accuracy is affected by
the PFI threshold tolerance, the V
CC
line, and the resis-
tors.
R
1
V
CC
PFI
R
2
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
PFO
Interfacing to µPs with
Bidirectional Reset Pins
µPs with bidirectional reset pins, such as the Motorola
68HC11
series,
can
contend
with
the
MAX690A/MAX692A/MAX802L/MAX802M
RESET
out-
put. If, for example, the
RESET
output is driven high
and the µP wants to pull it low, indeterminate logic lev-
els may result. To correct this, connect a 4.7kΩ resis-
tor between the
RESET
output and the µP reset I/O, as
in Figure 8. Buffer the
RESET
output to other system
components
0V
GND
V-
+5V
PFO
0V
V
TRIP
V-
=
NOTE:
1
V
TRIP
IS NEGATIVE
R
R2
5
−
1.25
1.25 - VTRIP
Figure 7. Monitoring a Negative Voltage
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9
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