Microprocessor Supervisory Circuits

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9

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

Operation

Without a Backup Power Source

eliminates any voltage drops across the switch which

Replacing the Backup Battery

valid, without danger of triggering RESET/RESET. As

backup mode cannot be entered. In other switchover

PFO

GND

MAX690A

MAX692A

MAX802L

MAX802M

MAX805L

+5V

PFI

+5V

0V

PFO

V-

0V

V-

Figure 7. Monitoring a Negative Voltage

5

1.25

R1

1.25 - VTRIP

R2

−

=

ICs where battery-backup mode is entered whenever

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

hysteresis to the power-fail comparator. Select the

that the 25nA (max) PFI input current does not shift the

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 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

tors.

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 4.7k

Ω resis-

tor between the RESET output and the µP reset I/O, as

in Figure 8. Buffer the RESET output to other system

components