Low-Power Battery Backup

Circuits in Small µDFN Packages

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11

BATTON Indicator

(MAX16035/MAX16039 Only)

BATTON is a push-pull output that asserts high when in

battery-backup mode. BATTON typically sinks 3.2mA

at a 0.4V saturation voltage. In battery-backup mode,

this terminal sources approximately 10µA from OUT.

Use BATTON to indicate battery-switchover status or to

supply base drive to an external pass transistor for

higher current applications (see Figure 3).

RESETIN Comparator

(MAX16036/MAX16040 Only)

An internal 1.235V reference sets the RESETIN thresh-

old voltage. RESET asserts when the voltage at

RESETIN is below 1.235V. Use the RESETIN function to

monitor a secondary power supply.

Use the following equations to set the reset threshold

Figure 4):

choose a value for R2 and calculate R1.

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

values (up to 1MΩ) can be used for R2 with no signifi-

cant loss in accuracy.

Power-Fail Comparator

The MAX16033–MAX16040 issue an interrupt (nonmask-

able or regular) to the µP when a power failure occurs.

The power line is monitored by two external resistors con-

nected to the power-fail input (PFI). When the voltage at

PFI falls below 1.235V, the power-fail output (PFO) drives

the processor’s NMI input low. An earlier power-fail warn-

ing can be generated if the unregulated DC input of the

regulator is available for monitoring. The MAX16033–

MAX16040 turn off the power-fail comparator and force

age (see Figure 1). The MAX160_ _L devices provide

push-pull PFO outputs. The MAX160_ _P devices provide

open-drain PFO outputs.

2.4V TO 5.5V

CE

(CEIN)

( ) FOR MAX16035 ONLY

A0–A15

µP

CMOS RAM

BATTON

BATT

(CEOUT)

GND

RESET

OUT

0.1µF

ADDRESS

DECODE

MAX16035

MAX16039

RESET

Figure 3. MAX16035/MAX16039 BATTON Driving an External Pass Transistor

MAX16036

MAX16040

R1

RESETIN

R2

Figure 4. Setting RESETIN Voltage for the

MAX16036/MAX16040