ADM690–ADM695
+AP P LICATIO N INFO RMATIO N
Incr easing the D r ive Cur r ent
When PFO is low, resistor R3 sinks current from the summing
junction at the PFI pin. When PFO is high, the series combina-
tion of R3 and R4 source current into the PFI summing junc-
tion. T his results in differing trip levels for the comparator.
If the continuous output current requirements at VOUT exceed
100 mA or if a lower VCC–VOUT voltage differential is desired,
an external PNP pass transistor may be connected in parallel
with the internal transistor. T he BAT T ON output (ADM691/
ADM693/ADM695) can directly drive the base of the external
transistor.
+5V
7805
+7V TO +15V
INPUT
V
POWER
CC
R
4
R
1
2
1.3V
PNP TRANSISTOR
PFO
+5V
TO
µP NMI
INPUT
POWER
0.1µF
0.1µF
PFI
R
ADM69x
BATT
ON
V
OUT
V
CC
R
3
V
BATT
ADM691
ADM693
ADM695
BATTERY
5V
Figure 17. Increasing the Drive Current
PFO
Using a Rechar geable Batter y for Back Up
If a capacitor or a rechargeable battery is used for back up then
the charging resistor should be connected to VOUT since this
eliminates the discharge path that would exist during power
0V
V
V
H
0V
L
V
IN
down if the resistor is connected to VCC
.
R
1
R
1
V
H
= 1.3V
= 1.3V
(
1+ ––– + –––
)
R
R
VOUT – VBATT
3
2
I =
R
+5V
INPUT
POWER
R
(5V – 1.3V)
R
1
1
V
L
(
1+ ––– – –––––––––––––
)
0.1µF
R
1.3V (R R )
2
3 +
4
R
0.1µF
ASSUMING R < <
R THEN
3
4
R
1
HYSTERESIS V – V = 5V
(
–––
)
H
L
R
2
V
V
OUT
CC
V
BATT
Figure 19. Adding Hysteresis to the Power Fail Com parator
RECHARGEABLE
BATTERY
ADM69x
Monitor ing the Status of the Batter y
T he power fail comparator can be used to monitor the status of
the backup battery instead of the power supply if desired. T his
is shown in Figure 20. T he 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-
age. T his can be done under processor control using CEOUT .
Since CEOUT is forced high during the battery backup mode, the
test load will not be applied to the battery while it is in use, even
if the microprocessor is not powered.
Figure 18. Rechargeable Battery
Adding H yster esis to the P ower Fail Com par ator
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.
–10–
REV. A
ADI [ ADI ]
