calculate the amount of capacitance required to allow

the shutdown routine to complete before reset is

asserted:

reset threshold.

Output Supply Voltage

BATT to the µP, RAM, and other external circuitry. At

capacitor to ground.

Battery-Backup Mode

Battery-backup mode preserves the contents of RAM in

the event of a brownout or power failure. With a backup

battery installed at BATT, the MAX801/MAX808 automati-

Two conditions are required for switchover to battery-

inputs and outputs during battery-backup mode.

BATT is designed to conduct up to 20mA to OUT dur-

ing battery backup. The PMOS switch on-resistance is

approximately 12

Ω. Figure 4 shows the two series pass

elements (between the BATT input and OUT) that

normal operation without causing a reset.

8-Pin µP Supervisory Circuits

with ±1.5% Reset Accuracy

_______________________________________________________________________________________

9

GND

TO

μP NMI

4.5V to 5.5V

LOWLINE

MAX801

MAX808

REGULATOR

Figure 3. Using LOWLINE to Provide a Power-Fail Warning to

the µP

MAX801

MAX808

P

PP

OUT

BATT

0.1

μF

CONTROL

CIRCUITRY

RESET

Logic low

3

3

RESET

Logic high; the open-circuit

—

PIN

5

GND

LOWLINE

Ground—0V reference for

all signals

4

4

WDI

WDI is ignored and goes

high impedance.

—

6

BATT

Supply current is 1µA max for

≤ 2.8V.

7

7

CE IN

CE OUT

High impedance (MAX808)

6

—

Logic high. The open-circuit

output voltage is equal to

5

—

OUT

OUT is connected to BATT

through two internal PMOS

switches in series.

NAME

8

8

Battery switchover

for active switchover.

1

1

Logic low

2

2

MAX801 MAX808

STATUS

Table 1. Input and Output Status in

Battery-Backup Mode