adm693aq Analog Devices, Inc., adm693aq Datasheet - Page 10

• Manufacturer Part Number: adm693aq
• Description: Microprocessor Supervisory Circuits
• Manufacturer: Analog Devices, Inc.

ADM690–ADM695
+APPLICATION INFORMATION
Increasing the Drive Current
If the continuous output current requirements at V
100 mA or if a lower V
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.
Using a Rechargeable Battery for Back Up
If a capacitor or a rechargeable battery is used for back up then
the charging resistor should be connected to V
eliminates the discharge path that would exist during power
down if the resistor is connected to V
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.
POWER
POWER
INPUT
INPUT
RECHARGEABLE
+5V
+5V
BATTERY
BATTERY
Figure 17. Increasing the Drive Current
0.1 F
Figure 18. Rechargeable Battery
0.1 F
CC
–V
V
V
OUT
BATT
BATT
V
V
CC
CC
PNP TRANSISTOR
voltage differential is desired,
I =
ADM691
ADM693
ADM695
ADM69x
BATT
V
R
OUT
ON
CC
– V
R
.
BATT
V
V
OUT
OUT
OUT
0.1 F
0.1 F
since this
OUT
exceed
–10–
When PFO is low, resistor R
junction at the PFI pin. When PFO is high, the series combina-
tion of R
tion. This results in differing trip levels for the 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-
age. This can be done under processor control using CE
Since CE
test load will not be applied to the battery while it is in use, even
if the microprocessor is not powered.
Figure 19. Adding Hysteresis to the Power Fail Comparator
+7V TO +15V
POWER
INPUT
3
OUT
and R
is forced high during the battery backup mode, the
PFO
R
R
4
5V
0V
V
V
ASSUMING R
HYSTERESIS V
1
2
H
L
source current into the PFI summing junc-
= 1.3V
= 1.3V
0V
7805
(
(
1+ ––– – –––––––––––––
1+ ––– + –––
4
R
R
R
R
< <
H
+5V
1
2
1
2
3
PFI
V
– V
R
L
sinks current from the summing
ADM69x
3
L
R
R
R
1.3V (R
THEN
= 5V
1
3
1
1.3V
V
(5V – 1.3V)
IN
)
(
3 +
–––
V
R
R
V
CC
1
2
R
H
R
4
)
3
)
)
PFO
R
P NMI
4
OUT.
TO
REV. A