ADM8696/ADM8697
–9–
APPLICATIONS INFORMATION
Increasing the Drive Current (ADM8696)
If the continuous output current requirements at V
OUT
exceeds
100 mA or if a lower V
CC
–V
OUT
voltage differential is desired,
an external PNP pass transistor may be connected in parallel
with the internal transistor. The BATT ON output (ADM8696)
can directly drive the base of the external transistor.
BATTERY
+5V
INPUT
POWER
0.1µF
PNP
TRANSISTOR
0.1µF
V
OUT
V
CC
BATT
ON
V
BATT
ADM8696
Figure 14. Increasing the Drive Current
Using a Rechargeable Battery for Backup (ADM8696)
If a capacitor or a rechargeable battery is used for backup, the
charging resistor should be connected to V
OUT
since this elimi-
nates the discharge path that would exist during power-down if
the resistor is connected to V
CC
.
V
OUT
V
CC
RECHARGABLE
BATTERY
+5V
INPUT
POWER
0.1µF
0.1µF
V
BATT
ADM8696
R
R
V
OUT
– V
BATT
I =
Figure 15. Rechargeable Battery
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 by connecting a resistor be-
tween the PFO output and the PFI input as shown in Fig-
ure 16. 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 source current into the PFI summing
junction. This results in differing trip levels for the comparator.
Alternate Watchdog Input Drive Circuits
The watchdog feature can be enabled and disabled under pro-
gram control by driving WDI with a three-state buffer (Figure
17a). When three-stated, the WDI input will float, thereby dis-
abling the watchdog timer.
This circuit is not entirely foolproof and it is possible a software
fault could erroneously three-state the buffer. This would pre-
vent the ADM869x from detecting that the microprocessor is no
longer operating correctly. In most cases, a better method is to
ADM869x
R2
1.3V
R1
PFO
7805
R4
R3
+7V TO +15V
INPUT
POWER
+5V
PFI
V
CC
TO
µP NMI
V
H
= 1.3V (1+ ––– + ––– )
V
L
= 1.3V (1+ ––– – ––––––––––––– )
ASSUMING R
4
< < R
3
THEN
HYSTERESIS V
H
– V
L
= 5V (––– )
R
1
R
2
R
1
R
3
R
1
R
2
R
1
R
2
R
1
(5V – 1.3V)
1.3V (R
3 +
R
4
)
Figure 16. Adding Hysteresis to the Power Fail Comparator
extend the watchdog period rather than disabling the watchdog.
This may be done under program control using the circuit
shown in Figure 17b. When the control input is high, the OSC
SEL pin is low and the watchdog timeout is set by the external
capacitor. A 0.01 µF capacitor sets a watchdog timeout delay of
100 s. When the control input is low, the OSC SEL pin is
driven high, selecting the internal oscillator. The 100 ms or the
1.6 s period is chosen, depending on which diode in Fig-
ure 17b is used. With D1 inserted, the internal timeout is set at
100 ms while with D2 inserted the timeout is set at 1.6 s.
WDI
ADM869x
WATCHDOG
STROBE
CONTROL
INPUT
Figure 17a. Programming the Watchdog Input
OSC IN
OSC SEL
ADM869x
CONTROL
INPUT*
D1 D2
*LOW = INTERNAL TIMEOUT
HIGH = EXTERNAL TIMEOUT
Figure 17b. Programming the Watchdog Input