Data Sheet

ADM705/ADM706/ADM707/ADM708

Rev. H | Page 9 of 12

Adding Hysteresis to the Power Fail Comparator

For increased noise immunity, hysteresis can be added to the

power fail comparator. Because the comparator circuit is

noninverting, hysteresis can be added by connecting a resistor

between the PFO output and the PFI input as shown in Figure 16.

7V TO 15V

INPUT

POWER

TO

MICROPROCESSOR

NMI

R3

5V

R1

R2

1.25V

PFI

PFO

ADP3367

5V

0V

–

+

PFO

ADM705/ADM706/

ADM707/ADM708

RESET

TO

MICROPROCESSOR

RESET

Figure 16. Adding Hysteresis to the Power Fail Comparator

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 immunity can be

achieved by connecting a capacitor between PFI and GND. The

equations calculate the hysteresis are as follows:





























R1

R3

R2

R3

R2

V

H

1

25

.

1





















R3

V

R2

R1

V

CC

L

25

.

1

25

.

1

25

.

1

















R2

R2

R1

V

MID

25

.

1

The ADM705/ADM706/ADM707/ADM708 are guaranteed to

the internal transistor does not have sufficient drive to hold the

voltage RESET at 0 V. A pull-down resistor can connect externally,

as shown in Figure 17, to hold the line low if required.

ADM705/ADM706/

ADM707/ADM708

GND

RESET

R1

Figure 17. RESET Valid Below 1 V