ADM705/ADM706/ADM707/ADM708

Rev. G | 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 simply 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

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 used to calculate the hysteresis are as follows:

⎟

⎠

⎞

⎜

⎝

⎛

+

=

⎟

⎠

⎞

⎜

⎝

⎛

−

−

+

=

⎥⎦

⎤

⎢⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

×

+

+

=

R2

R2

R1

V

R3

V

R2

R1

V

R1

R3

R2

R3

R2

V

MID

CC

L

H

25

.

1

25

.

1

25

.

1

25

.

1

1

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 be connected

externally, as shown in

, to hold the line low if

required.

Figure 17

ADM705/ADM706/

ADM707/ADM708

GND

RESET

R1

Figure 17. RESET Valid Below 1 V