SP791DS/08

SP791 Low Power Microprocessor Supervisory with Battery Switch-Over

© Copyright 2000 Sipex Corporation

12

POWER-FAIL INPUT

The Power-Fail Input (PFI) has a guaranteed

input leakage of +/-25nA max over temperature.

The typical comparator delay is 15

ground if not used.

POWER-FAIL OUTPUT

The Power-Fail Output (PFO) goes low when

PFI falls below 1.25V. It sinks 3.2mA with a

saturation voltage of 0.1V. With PFI above

necting PFI through a voltage divider to an un-

regulated supply allows PFO to generate an NMI

as the unregulated power begins to fall (see

Figure 9b).

LOWLINE OUTPUT

typical threshold voltage 150mV above the re-

set threshold and has 15mV of hysteresis.

LOWLINE typically sinks 3.2mA at 0.1V. For

normal operation (Vcc above the LOWLINE

cess to the unregulated supply is unavailable,

use LOWLINE to provide a nonmaskable in-

terrupt (NMI) to the

µP as shown in Figure 9a.

POWER-FAIL COMPARATOR

The power-fail comparator is an uncommitted

comparator that has no effect on the other func-

tions of the SP791. Common uses include moni-

toring supplies other than 5V (see the Typical

Operating Circuit and the Monitoring a Nega-

tive Voltage section) and early power-fail de-

tection when the unregulated power is easily ac-

cessible as shown in Figure 9b.

Figure 9. a) If the unregulated supply is inaccessible, LOWLINE generates the NMI for the

µP.

b) Use PFO to generate the

µP NMI if the unregulated supply is accessible.

FROM

REGULATED

SUPPLY

0.1

µF

3

2

1

15

10

11

0.1

µF

3.0V

4

POWER TO

CMOS RAM

µP

RESET

RESET

NMI

LOWLINE

I/O LINE

WDI

GND

Vcc

Corporation

UNREGULATED

SUPPLY

a.)

b.)

µP POWER

3

2

1

15

6

11

0.1

µF

3.0V

4

POWER TO

CMOS RAM

µP

RESET

RESET

NMI

PFO

I/O LINE

WDI

GND

Vcc

Corporation

µP POWER

PFI

7

0.1

µF

VOLTAGE

REGULATOR