Datasheet

Data Sheet ADM8690/ADM8691/ADM8695
Rev. C | Page 15 of 24
APPLICATIONS INFORMATION
INCREASING THE DRIVE CURRENT
If the continuous output current requirements at V
OUT
exceed
100 mA, or if a lower V
CC
− V
OUT
voltage differential is desired,
an external PNP pass transistor can be connected in parallel
with the internal transistor. The BATT ON output (ADM8691/
ADM8695) can directly drive the base of the external transistor.
PNP TRANSISTOR
0.1µF 0.1µF
BATTERY
V
BATT
V
CC
V
OUT
BATT
ON
5V INPUT
POWER
00093-024
ADM8691/
ADM8695
Figure 24. Increasing the Drive Current
USING A RECHARGEABLE BATTERY FOR BACKUP
If a capacitor or a rechargeable battery is used for backup, the
charging resistor should be connected to V
OUT
to eliminate the
discharge path that would exist during power-down if the
resistor were connected to V
CC
.
R
0.1µF
0.1µF
V
BATT
V
CC
V
OUT
5V INPUT
POWER
RECHARGEABLE
BATTERY
V
OUT
–
V
BATT
R
I =
00093-025
Figure 25. Rechargeable Battery
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 26. When
PFO
is low, Resistor R3 sinks current from the
summing junction at the PFI pin. When
PFO
is high, the series
combination of R3 and R4 sources current into the PFI summing
junction. This results in differing trip levels for the comparator.
5V
0V
PF
O
0V V
L
V
H
V
IN
V
H
= 1.3V
V
L
= 1.3V
ASSUMING R4 < < R3 THEN
HYSTERESIS V
H
– V
L
= 5V
1+
R1
R2
R1
R3
)(
+
1+
R1
R2
R1 (5V – 1.3V)
R3 (1.3V (R3 + R4))
)(
–
1.3V
PFI
PFO
TO
MICROPROCESSOR
NMI
5V
V
CC
7V TO 15V
INPUT
POWER
7805
R1
R2
R3
R4
00093-026
R1
R2
)(
Figure 26. 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
(see Figure 27). The PFI input samples the battery voltage and
generates an active low
PFO
signal when the battery voltage
drops below a selected threshold. It may be necessary to apply
a test load to determine the loaded battery voltage. This is done
under processor control using
CE
OUT
. Because
CE
OUT
is forced
high during the battery backup mode, the test load is not applied
to the battery while it is in use, even if the microprocessor is not
powered.
PFI
BATTERY
10MΩ
10MΩ
R1
R2
5V INPUT
POWER
20kΩ
OPTIONAL
TEST LOAD
LOW BATTERY
SIGNAL TO
MICROPROCESSOR
I/O PIN
V
BATT
V
CC
CE
OUT
CE
IN
PFO
00093-027
FROM
MICROPROCESSOR
I/O PIN APPLIES
TEST LOAD
TO BATTERY
Figure 27. Monitoring the Battery Status