MAX805LESA Maxim, MAX805LESA Datasheet - Page 9

• Manufacturer Part Number: MAX805LESA
• Description: Microprocessor Supervisory Circuits
• Manufacturer: Maxim

connected directly to the V
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
If a backup power source is not used, ground V
and connect V
switch over to any backup power source, V
not need to be switched. A direct connection to V
eliminates any voltage drops across the switch which
may push V
The backup battery can be removed while V
valid, without danger of triggering RESET/RESET. As
long as V
backup mode cannot be entered. In other switchover
Figure 7. Monitoring a Negative Voltage
CC
PFO
+5V
NOTE: V
0V
is close to the reset threshold.
+5V
5 1.25
R 1
R
TRIP
R
CC
1
2
1.25 - V TRIP
IS NEGATIVE
OUT
stays above the reset threshold, battery-
Without a Backup Power Source
R 2
OUT
below V
Replacing the Backup Battery
V-
_______________________________________________________________________________________
to V
V
V-
TRIP
PFI
CC
CC
. Since there is no need to
.
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
Microprocessor Supervisory Circuits
V
GND
CC
BATT
PFO
0V
input of the
Operation
CC
OUT
remains
does
BATT
CC
ICs where battery-backup mode is entered whenever
V
accumulates
RESET/RESET in error.
Hysteresis adds a noise margin to the power-fail com-
parator and prevents repeated triggering of PFO when
V
hysteresis to the power-fail comparator. Select the
ratio of R
falls to its trip point (V
will typically be an order of magnitude greater than R
or R
through R
that the 25nA (max) PFI input current does not shift the
trip point. R
load down the PFO pin. Capacitor C1 adds additional
noise rejection.
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
tors.
µPs with bidirectional reset pins, such as the Motorola
68HC11
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
IN
BATT
is close to its trip point. Figure 6 shows how to add
2
gets close to V
(about 10 times either R
1
1
and R
and R
3
series,
should be larger than 10kΩ so it does not
Monitoring a Negative Voltage
leakage
2
2
such that PFI sees 1.25V when V
should be at least 1µA to ensure
TRIP
CC
Adding Hysteresis to the
can
Bidirectional Reset Pins
, an unconnected V
Power-Fail Comparator
Interfacing to µPs with
). R
charge
3
contend
adds the hysteresis. It
1
CC
or R
line, and the resis-
and
2
). The current
with
triggers
BATT
the
pin
IN
9
1