MAX802_CSLASHD datasheet(8/13 Pages) MAXIM

8 / 13 page

Backup-Battery Switchover

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

necessary to preserve the contents of RAM. With a back-

up battery installed at VBATT, the devices automatically

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.

Manual Reset

A logic low on MR asserts reset. Reset remains asserted

while MR is low, and for tWP (200ms) after MR returns

high. This input has an internal 70µA pullup current, so it

can be left open if it is not used. MR can be driven with

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

outputs. Connect a normally open momentary switch from

MR to GND to create a manual-reset function; external

debounce circuitry is not required.

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 as

possible to the device.

Using a SuperCap as a Backup Power Source

SuperCaps are capacitors with extremely high capaci-

tance 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 a bove 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

capacitor to ground, without danger of triggering RESET/

RESET. As long as VCC stays above VSW, battery-back-

up 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 applications where a

power-supply line is being monitored through an external

voltage divider (see the Monitoring an Additional Power

Supply section).

If additional noise margin is desired, connect a resistor

between PFO and PFI as shown in Figure 4a. 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).

Connecting an ordinary signal diode in series with R3, as

shown in Figure 4b, causes the lower trip point (VL) to

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

The power-fail comparator is disabled when

VCC < VSW

PFO

Logic low when VCC < VSW or PFI < VPFT

WDI

The watchdog timer is disabled

Disabled

RESET

Low logic

RESET

High impedance

VBATT

Connected to VOUT

MAX690T/S/R, MAX704T/S/R,

MAX802T/S/R, MAX804−MAX806T/S/R

3.0V/3.3V Microprocessor

Supervisory Circuits

www.maximintegrated.com

Maxim Integrated │ 8

MAX802_C/D 数据表(PDF) 8 Page - Maxim Integrated Products

MAX802_C/D 数据表(HTML) 8 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13

数据表下载