ADM690–ADM695

REV. A

–10–

+APPLICATION INFORMATION

Increasing the Drive Current

an external PNP pass transistor may be connected in parallel

with the internal transistor. The BATT ON output (ADM691/

ADM693/ADM695) can directly drive the base of the external

transistor.

V

OUT

V

CC

BATTERY

+5V

INPUT

POWER

0.1

µF

0.1

µF

BATT

ON

V

BATT

PNP TRANSISTOR

ADM691

ADM693

ADM695

Figure 17. Increasing the Drive Current

Using a Rechargeable Battery for Back Up

If a capacitor or a rechargeable battery is used for back up then

eliminates the discharge path that would exist during power

V

OUT

V

CC

RECHARGEABLE

BATTERY

+5V

INPUT

POWER

0.1

µF

0.1

µF

V

BATT

ADM69x

R

I =

R

Figure 18. Rechargeable Battery

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 nonin-

verting, hysteresis can be added simply by connecting a resistor be-

tween the PFO output and the PFI input as shown in Figure 19.

junction at the PFI pin. When PFO is high, the series combina-

tion. This results in differing trip levels for the comparator.

ADM69x

R

2

1.3V

PFO

R

1

7805

R

4

R

3

+7V TO +15V

INPUT

POWER

+5V

PFI

V

CC

TO

µP NMI

5V

0V

0V

V

L

V

H

V

IN

PFO

V

H

= 1.3V

V

L

= 1.3V

ASSUMING R

4

< <

R

3

THEN

HYSTERESIS V

H

– V

L

= 5V

R

1

R

2

R

1

R

3

R

1

R

2

R

1

R

2

R

1

(5V – 1.3V)

1.3V (R

3 +

R

4

)

Figure 19. Adding Hysteresis to the Power Fail Comparator

Monitoring the Status of the Battery

The power fail comparator can be used to monitor the status of

the backup battery instead of the power supply if desired. This

is shown in Figure 20. The PFI input samples the battery volt-

age and generates an active low PFO signal when the battery

voltage drops below a chosen threshold. It may be necessary to

apply a test load in order to determine the loaded battery volt-

test load will not be applied to the battery while it is in use, even

if the microprocessor is not powered.