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ADM8692 Datasheet(PDF) 14 Page - Analog Devices

Part No. ADM8692
Description  Microprocessor Supervisory Circuits
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ADM8692 Datasheet(HTML) 14 Page - Analog Devices

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ADM8690/ADM8691/ADM8692/ADM8693/ADM8695
Rev. B | Page 14 of 20
APPLICATION INFORMATION
INCREASING THE DRIVE CURRENT
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 can be connected in parallel
with the internal transistor. The BATT ON output (ADM8691/
ADM8693/ADM8695) can directly drive the base of the
external transistor.
PNP TRANSISTOR
0.1µF
0.1µF
BATTERY
VBATT
VCC
VOUT
BATT
ON
5V INPUT
POWER
ADM8691/
ADM8693/
ADM8695
Figure 24. Increasing the Drive Current
USING A RECHARGEABLE BATTERY FOR BACKUP
If a capacitor or a rechargeable battery is used for backup then
the charging resistor should be connected to VOUT because this
eliminates the discharge path that would exist during power-
down if the resistor is connected to VCC.
ADM869x
R
0.1µF
0.1µF
VBATT
VCC
VOUT
5V INPUT
POWER
RECHARGEABLE
BATTERY
VOUT – VBATT
R
I =
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
PFO
0V
VL
VH
VIN
VH = 1.3V
VL = 1.3V
ASSUMING R4 < < R3 THEN
HYSTERESIS VH – VL = 5V
1+
R1
R2
R1
R3 )
(
+
1+
R1
R2
R1 (5V – 1.3V)
R3 (1.3V (R3 + R4)) )
(
ADM869x
1.3V
PFI
PFO
TO
MICROPROCESSOR
NMI
5V
VCC
7V TO 15V
INPUT
POWER
7805
R1
R2
R3
R4
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. This
is shown in Figure 27. The PFI input samples the battery voltage
and generates an active low PFO signal when the battery voltage
drops below a chosen threshold. It can be necessary to apply a
test load to determine the loaded battery voltage. This is done
under processor control using CEOUT. Because CEOUT 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.
ADM869x
PFI
BATTERY
10M
10M
R1
R2
5V INPUT
POWER
20k
OPTIONAL
TEST LOAD
LOW BATTERY
SIGNAL TO
MICROPROCESSOR
I/O PIN
VBATT
VCC
CEOUT
CEIN
PFO
FROM
MICROPROCESSOR
I/O PIN APPLIES
TEST LOAD
TO BATTERY
Figure 27. Monitoring the Battery Status


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