Power-Fail Comparator

The power-fail comparator is an uncommitted compara-

tor that has no effect on the other functions of the IC.

Common uses include monitoring supplies other than

5V (see the

Typical Operating Circuit and the

Monitoring a Negative Voltage section) and early

power-fail detection when the unregulated power is

easily accessible (Figure 9b).

Power-Fail Input

PFI is the input to the power-fail comparator. PFI has a

guaranteed input leakage of ±25nA max over tempera-

being restored). If unused, connect this input to

ground.

Power-Fail Output

The Power-Fail Output (PFO) goes low when PFI goes

below 1.25V. It typically sinks 3.2mA with a saturation

voltage of 0.1V. With PFI above 1.25V, PFO is actively

divider to an unregulated supply allows PFO to gener-

ate an NMI as the unregulated power begins to fall

(Figure 9b). If the unregulated supply is inaccessible,

use LOWLINE to generate the NMI. The LOWLINE

threshold is typically 150mV above the reset threshold

(see

LOWLINE Output section).

Microprocessor Supervisory Circuit

12

______________________________________________________________________________________

Table 1. Input and Output States in

Battery-Backup Mode

backup mode.

Logic high. The open-circuit output

–

W

—

D

—

P

—

O

–

16

Logic low*

–

R

—

E

—

S

—

E

—

T

–

15

Logic high. The open-circuit output

–

W

—

D

—

O

–

14

High impedance

–

C

—

E

–

IN

13

Logic high. The open-circuit output

–

C

—

E

–

OUT

12

WDI is ignored, and goes high

impedance.

WDI

11

Logic low*

–

L

—

O

—

W

—

L

—

I

—

N

—

E

–

10

–

–

M

—

R

–

is ignored.

–

M

—

R

–

9

SWT is ignored.

SWT

8

The power-fail comparator remains

active in the battery-backup mode for

PFI

7

The power-fail comparator remains

active in the battery-backup mode for

voltage,

–

P

—

F

—

O

–

is forced low.

–

P

—

F

—

O

–

6

Logic high. The open-circuit output is

BATT ON

5

GND—0V reference for all signals.

GND

4

Battery-switchover comparator

3

an internal PMOS switch.

2

Supply current is 1µA maximum.

VBATT

1

STATUS

NAME

PIN

MAX791

FROM

REGULATED

SUPPLY

POWER TO

CMOS RAM

VBATT

RESET

LOWLINE

WDI

GND

RESET

NMI

I/O LINE

µP

µP POWER

2

0.1

µF

0.1

µF

3.0V

1

3

4

a)

b)

15

10

11

MAX791

PFI

POWER TO

CMOS RAM

VBATT

RESET

PFO

WDI

GND

VOLTAGE

REGULATOR

RESET

NMI

I/O LINE

µP

µP POWER

2

0.1

µF

0.1

µF

3.0V

1

3

7

4

15

6

11

Figure 9. a) If the unregulated supply is inaccessible,

LOWLINE generates the NMI for the µP. b) Use PFO to gener-

ate the µP NMI if the unregulated supply is inaccessible.