MAX692ACSLASHD datasheet(9/12 Pages) MAXIM

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Microprocessor Supervisory Circuits

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connected directly to the VBATT 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

VCC is close to the reset threshold.

Operation

Without a Backup Power Source

If a backup power source is not used, ground VBATT

and connect VOUT to VCC. Since there is no need to

switch over to any backup power source, VOUT does

not need to be switched. A direct connection to VCC

eliminates any voltage drops across the switch which

may push VOUT below VCC.

Replacing the Backup Battery

The backup battery can be removed while VCC remains

valid, without danger of triggering RESET/RESET. As

long as VCC stays above the reset threshold, battery-

backup mode cannot be entered. In other switchover

PFO

VCC

GND

MAX690A

MAX692A

MAX802L

MAX802M

MAX805L

+5V

PFI

+5V

PFO

VTRIP

V-

NOTE: VTRIP IS NEGATIVE

Figure 7. Monitoring a Negative Voltage

1.25

1.25 - VTRIP

−

ICs where battery-backup mode is entered whenever

VBATT gets close to VCC, an unconnected VBATT 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

VIN is close to its trip point. Figure 6 shows how to add

hysteresis to the power-fail comparator. Select the

ratio of R1 and R2 such that PFI sees 1.25V when VIN

falls to its trip point (VTRIP). R3 adds the hysteresis. It

will typically be an order of magnitude greater than R1

or R2 (about 10 times either 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 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 VCC line, and the resis-

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

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