ADM8691 Analog Devices, ADM8691 Datasheet - Page 15

• Manufacturer Part Number: ADM8691
• Manufacturer: Analog Devices

Specifications of ADM8691

• Product Description: µP Supv w/ Backup Batt Switch, Adj Reset Period, Adj Watchdog Period, 4.65V Trip point, 100 mA Iout
• Supply Current: 200µA
• Us Price 1000-4999: n/a
• Display In Ist: Yes
• Active For Param Search: Yes

Data Sheet
APPLICATIONS 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 can be connected in parallel
with the internal transistor. The BATT ON output (ADM8691/
ADM8695) can directly drive the base of the external transistor.
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
discharge path that would exist during power-down if the
resistor were connected to V
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 INPUT
5V INPUT
POWER
POWER
RECHARGEABLE
BATTERY
0.1µF
BATTERY
Figure 24. Increasing the Drive Current
0.1µF
Figure 25. Rechargeable Battery
CC
V
− V
BATT
V
V
CC
V
PNP TRANSISTOR
BATT
CC
I =
CC
OUT
.
ADM8691/
ADM8695
V
OUT
BATT
voltage differential is desired,
ON
– V
R
R
BATT
V
V
OUT
OUT
OUT
to eliminate the
OUT
0.1µF
0.1µF
exceed
Rev. C | Page 15 of 24
PFO
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
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.
BATTERY
7V TO 15V
5V
0V
TEST LOAD
POWER
OPTIONAL
0V
INPUT
20kΩ
Figure 26. Adding Hysteresis to the Power-Fail Comparator
R1
R2
V
L
R1
R2
Figure 27. Monitoring the Battery Status
V
7805
IN
ADM8690/ADM8691/ADM8695
10MΩ
10MΩ
V
H
5V
PFI
V
PFI
CE
BATT
OUT
1.3V
5V INPUT
POWER
V
V
ASSUMING R4 < < R3 THEN
HYSTERESIS V
V
H
L
V
CC
= 1.3V
= 1.3V
CC
OUT
R3
. Because CE
(
(
1+
1+
PFO
R1
R2
CE
R1
R2
PFO
H
–
IN
+
– V
R3 (1.3V (R3 + R4))
R1
R3
R1 (5V – 1.3V)
L
LOW BATTERY
SIGNAL TO
MICROPROCESSOR
I/O PIN
FROM
MICROPROCESSOR
I/O PIN APPLIES
TEST LOAD
TO BATTERY
)
= 5V
R4
OUT
(
TO
MICROPROCESSOR
NMI
R1
R2
is forced
)
)