Detailed Description

The Typical Operating Circuit shows a typical connection

for the MAX6361–MAX6364 family. OUT powers the stat-

ic random-access memory (SRAM). OUT is internally

than the reset threshold. OUT can supply up to 150mA

an internal MOSFET connects the backup battery to

OUT. The on-resistance of the MOSFET is a function of

backup-battery voltage and is shown in the Battery to

Out On-Resistance vs. Temperature graph in the Typical

Operating Characteristics section.

Backup-Battery Switchover

In a brownout or power failure, it may be necessary to

preserve the contents of the RAM. With a backup bat-

tery installed at BATT, the MAX6361–MAX6364 auto-

falls. The MAX6363 has a BATT ON output that goes

high when in battery-backup mode. These devices

require two conditions before switching to battery-

backup mode:

Table 1 lists the status of the inputs and outputs in bat-

tery-backup mode. The device will not power up if the

only voltage source is on BATT. OUT will only power up

Manual Reset Input (MAX6361 Only)

Many µP-based products require manual reset capabili-

ty, allowing the operator, a test technician, or external

logic circuitry to initiate a reset. For the MAX6361, a logic

low on MR asserts reset. Reset remains asserted while

returns high. MR has an internal 20k

Ω pull-up resistor to

els or with open-drain/collector outputs. Connect a nor-

mally open momentary switch from MR to GND to create

a manual reset function; external debounce circuitry is

not required. If MR is driven from long cables or the

device is used in a noisy environment, connect a 0.1µF

capacitor from MR to GND to provide additional noise

immunity.

Watchdog Input (MAX6362 Only)

The watchdog monitors µP activity through the input

WDI. If the µP becomes inactive, the reset output is

asserted in pulses. To use the watchdog function, con-

nect WDI to a bus line or µP I/O line. A change of state

(high to low or low to high) within the watchdog timeout

the watchdog timer. If WDI remains high or low for longer

than the watchdog timeout period, the internal watchdog

timer runs out and a reset pulse is triggered for the reset

whenever reset asserts or the WDI sees a rising or falling

edge within the watchdog timeout period. If WDI remains

in a high or low state for an extended period of time, a

reset pulse asserts after every watchdog timeout period

Reset In (MAX6364 Only)

RESET IN is compared to an internal 1.235V reference.

If the voltage at RESET IN is less than 1.235V, reset is

asserted. The RESET IN comparator may be used as

an undervoltage detector to signal a failing power sup-

ply. It can also be used as a secondary power-supply

reset monitor.

power supply, use the following equation (see Typical

Operating Circuit):

choose a value for R2 and calculate R1:

Since the input current at RESET IN is 25nA (max), large

values (up to 1M

Ω) can be used for R2 with no signifi-

cant loss in accuracy. For example, in the Typical

SOT23, Low-Power µP Supervisory Circuits

with Battery Backup

6

_______________________________________________________________________________________

Table 1. Input and Output Status in

Battery-Backup Mode

PIN

STATUS

Disconnected from OUT

OUT

Connected to BATT

BATT

Connected to OUT. Current drawn from

RESET/RESET

Asserted

BATT ON

High state

MR, RESET IN,

WDI

Inputs ignored

VV

R

R

RTH

REF

=+

⎛

⎝⎜

⎞

⎠⎟

1

2

1

RR

V

V

RTH

REF

12

1

/

=

[]