ADM705–ADM708

REV. B

–5–

Power-Fail Comparator

The power-fail comparator is an independent comparator that

may be used to monitor the input power supply. The comparator’s

inverting input is internally connected to a 1.25 V reference

voltage. The noninverting input is available at the PFI input.

This input may be used to monitor the input power supply via

a resistive divider network. When the voltage on the PFI input

drops below 1.25 V, the comparator output (

PFO) goes low,

indicating a power failure. For early warning of power failure,

the comparator may be used to monitor the preregulator input

simply by choosing an appropriate resistive divider network.

The

PFO output can be used to interrupt the processor so that

a shutdown procedure is implemented before the power is lost.

INPUT

POWER

R1

R2

POWER-FAIL

INPUT

1.25V

PFI

PFO

POWER-FAIL

OUTPUT

ADM70x

Figure 5. Power-Fail Comparator

Adding Hysteresis to the Power-Fail Comparator

For increased noise immunity, hysteresis may be added to the

power-fail comparator. Since the comparator circuit is non-

inverting, hysteresis can be added simply by connecting a

resistor between the

PFO output and the PFI input as shown in

Figure 6. When

PFO is low, resistor R3 sinks current from the

summing junction at the PFI pin. When

PFO is high, resistor

R3 sources current into the PFI summing junction. This results

in differing trip levels for the comparator. Further noise immu-

nity may be achieved by connecting a capacitor between PFI

and GND.

7V TO 15V

INPUT POWER

TO

P NMI

R3

5V

R1

R2

1.25V

PFI

PFO

ADM70x

ADM663

5V

PFO

0V

0V

Figure 6. Adding Hysteresis to the Power-Fail Comparator

V

RR

RR

R

VR

R

V

RE

V

RR

R

H

L

CC

MID

=

+

×









=+









=

+









125 1

23

23

1

125

1

125

2

125

125

12

2

.]

.

.

–

– .

.

Valid

RESET Below 1 V V

CC

The ADM70x family of products is guaranteed to provide a

internal transistor will not have sufficient drive to hold it ON so

the voltage on

RESET will no longer be held at 0 V. A pull-down

resistor as shown in Figure 7 may be connected externally to

hold the line low if it is required.

ADM70x

GND

RESET

R1

Figure 7.

RESET Valid Below 1 V