Backup-Battery Switchover

In the event of a brownout or power failure, it may be

necessary to preserve the contents of RAM. With a

backup battery installed at VBATT, the devices auto-

falls.

This family of µP supervisors (designed for 3.3V and 3V

(through a 140

Ω switch) when V

1.75V (typ) regardless of the VBATT voltage. This is

done to allow the backup battery (e.g., a 3.6V lithium

minimum required to reliably retain data in CMOS RAM.

Manual Reset

A logic low on

–

M

—

R

–

asserts reset. Reset remains asserted

while

–

M

—

R

–

–

M

—

R

–

returns

high. This input has an internal 70µA pull-up current, so

it can be left open if it is not used.

–

M

—

R

–

can be driven with

TTL or CMOS logic levels, or with open-drain/collector

outputs. Connect a normally open momentary switch

from

–

M

—

R

–

to GND to create a manual-reset function;

external debounce circuitry is not required.

__________Applications Information

These µP supervisory circuits are not short-circuit

up transients such as charging a decoupling

and VBATT pins to ground by placing 0.1µF capacitors

as close to the device as possible.

Using a SuperCap

as a Backup Power Source

SuperCaps™ are capacitors with extremely high

capacitance values (e.g., order of 0.47F) for their size.

Figure 3 shows two ways to use a SuperCap as a

backup power source. The SuperCap may be

connected through a diode to the 3V input (Figure 3a)

or, if a 5V supply is also available, the SuperCap may

be charged up to the 5V supply (Figure 3b) allowing a

special precautions when using these µP supervisors

with a SuperCap.

Operation without a Backup

Power Source

These µP supervisors were designed for battery-

backed applications. If a backup battery is not used,

different µP supervisor such as the MAX706T/S/R or

MAX708T/S/R.

Replacing the Backup Battery

remains valid, if VBATT is decoupled with a 0.1µF

capacitor to ground, without danger of triggering

RESET/

–

R

—

E

—

S

—

E

—

T

–

battery-backup mode cannot be entered.

Adding Hysteresis

to the Power-Fail Comparator

The power-fail comparator has a typical input

hysteresis of 10mV. This is sufficient for most applica-

tions where a power-supply line is being monitored

through an external voltage divider (see the section

Monitoring an Additional Power Supply).

If additional noise margin is desired, connect a resistor

between

–

P

—

F

—

O

–

and PFI as shown in Figure 4a. Select

hysteresis and will typically be more than 10 times the

value of R1 or R2. The hysteresis window extends both

3.0V/3.3V Microprocessor Supervisory Circuits

8

_______________________________________________________________________________________

™ SuperCap is a trademark of Baknor Industries.

PIN NAME

STATUS

Connected to VBATT through an internal

140

Ω switch

PFI

The power-fail comparator is disabled when

–

P

—

F

—

O

–

WDI

The watchdog timer is disabled

–

M

—

R

–

Disabled

–

R

—

E

—

S

—

E

—

T

–

Low logic

RESET

High impedance

VBATT

Table 1. Input and Output Status in

Battery-Backup Mode