MAX690R(1994) 查看數據表（PDF） - Maxim Integrated

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MAX690R
(Rev.:1994)

3.0V/3.3V Microprocessor Supervisory Circuits

Maxim Integrated 

3.0V/3.3V Microprocessor Supervisory Circuits

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-

matically switch RAM to backup power when VCC

falls.

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

systems) doesn’t always connect VBATT to VOUT when

VBATT is greater than VCC. VBATT connects to VOUT

(through a 140Ω switch) when VCC is below VSW and

VBATT is greater than VCC, or when VCC falls below

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

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

cell) to have a higher voltage than VCC.

Switchover at VSW (2.40V) ensures that battery-backup

mode is entered before VOUT gets too close to the 2.0V

minimum required to reliably retain data in CMOS RAM.

Switchover at higher VCC voltages would decrease

backup-battery life. When VCC recovers, switchover is

deferred until VCC rises above the reset threshold

(VRST) to ensure a stable supply. VOUT is connected to

VCC through a 3Ω PMOS power switch.

Awhloilegic–M—lRo–w

on –M—R–

is low,

asserts

and for

Manual Reset

reset. Reset

tWP (200ms)

raefmtearin–M—s R–asreseturrtends

high.

it can

This input has an

be left open if it is

ninotteurnsaeld7.0–M—µRA–

pull-up

can be

current, so

driven with

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

ofruotmpu–Mts—.R–

Connect

to GND

a

to

normally open momentary switch

create a manual-reset function;

external debounce circuitry is not required.

Table 1. Input and Output Status in

Battery-Backup Mode

PIN NAME

STATUS

VOUT

Connected to VBATT through an internal

140Ω switch

VCC Disconnected from VOUT

PFI

–P—F—O–

The power-fail comparator is disabled when

VCC < VSW

Logic low when VCC < VSW or PFI < VPFT

WDI

–M—R–

–R—E—S—E—T–

The watchdog timer is disabled

Disabled

Low logic

RESET High impedance

VBATT Connected to VOUT

__________Applications Information

These µP supervisory circuits are not short-circuit

protected. Shorting VOUT to ground—excluding power-

up transients such as charging a decoupling

capacitor—destroys the device. Decouple both VCC

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

longer backup period. Since VBATT can exceed VCC

while VCC is above the reset threshold, there are no

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,

connect both VBATT and VOUT to VCC, or use a

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

MAX708T/S/R.

Replacing the Backup Battery

The backup power source can be removed while VCC

remains valid, if VBATT is decoupled with a 0.1µF

cRaEpSaEcTi/t–Ro—rE—tSo—E—gT–ro.

und, wit

As long

hou

as

t danger o

VCC stays

f trigg

above

ering

VSW,

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).

Ibfeatdwdeietinon–P—aFl—nO–oiasendmPaFrgI iansisshdoewsinreidn,

connect a

Figure 4a.

resistor

Select

the ratio of R1 and R2 such that PFI sees 1.237V (VPFT)

when VIN falls to its trip point (VTRIP). R3 adds the

hysteresis and will typically be more than 10 times the

value of R1 or R2. The hysteresis window extends both

above (VH) and below (VL) the original trip point (VTRIP).

™ SuperCap is a trademark of Baknor Industries.

8 _______________________________________________________________________________________

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