Power-Fail Comparator

PFI is the noninverting input to an uncommitted com-

The power-fail comparator is intended to monitor the

preregulated input of the power supply, providing an

early power-fail warning so software can conduct an

orderly shutdown. It can also be used to monitor sup-

plies other than 5V. Set the power-fail threshold with a

resistor-divider, as shown in Figure 10.

Power-Fail Input

PFI is the input to the power-fail comparator. The typical

unused, connect this input to ground.

Power-Fail Output

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

divider to a preregulated supply allows PFO to gener-

ate an NMI as the preregulated power begins to fall

(Figure 11b). If the preregulated supply is inaccessible,

use LOW LINE to generate the NMI (Figure 11a). The

LOW LINE threshold is typically 52mV above the reset

threshold (see the Low-Line Comparator section).

Full-Featured µP Supervisory Circuit with

±1.5% Reset Accuracy

12

______________________________________________________________________________________

MAX807

GND

PFI

PFO

R1

R2

0V

PFO

WHERE

+

R1

1

)

(

R2

1

–

R1

+

R1

1

)

(

R2

1

–

R1

MAX807

GND

PFI

PFO

R1

R2

PFO

)

(

R2

R1

+ R2

MR

b)

a)

)

(

R2

R1

+ R2

MAX807

OUT

FROM

REGULATED

SUPPLY

POWER TO

CMOS RAM

BATT

RESET

LOW LINE

WDI

GND

RESET

NMI

I/O LINE

µP

µP POWER

0.1

µF

0.1

µF

2.8V

a)

b)

MAX807

OUT

PFI

POWER TO

CMOS RAM

BATT

RESET

PFO

WDI

GND

VOLTAGE

REGULATOR

RESET

NMI

I/O LINE

µP

µP POWER

0.1

µF

0.1

µF

2.8V

Figure 11. a) If the preregulated supply is inaccessible, LOW

LINE generates the NMI for the µP. b) Use PFO to generate the

µP NMI if the preregulated supply is accessible.