adm708san Analog Devices, Inc., adm708san Datasheet - Page 10

• Manufacturer Part Number: adm708san
• Description: 3 V, Voltage Monitoring Microprocessor Supervisory Circuit
• Manufacturer: Analog Devices, Inc.

ADM706P/ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/ADM708T
The watchdog monitor can be deactivated by floating the
watchdog input (WDI). The WDO output can now be used as a
low line output because it only goes low when V
the reset threshold.
RESET
POWER-FAIL COMPARATOR
The power-fail comparator is an independent comparator that
can be used to monitor the input power supply. The
comparator’s inverting input is internally connected to a 1.25 V
reference voltage. The noninverting input is available at the PFI
input. This input can be used to monitor the input power
supply via a resistive divider network. When the voltage on the
PFI input drops below 1.25 V, the comparator output ( PFO )
goes low indicating a power failure. For early warning of power
failure, the comparator can be used to monitor the preregulator
input simply by choosing an appropriate resistive divider
network. The PFO output can be used to interrupt the
processor so that a shutdown procedure is implemented before
the power is lost.
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 18. When PFO is low, Resistor R3 sinks current from the
summing junction at the PFI pin. When PFO is high, Resistor R3
sources current into the PFI summing junction. This results in
differing trip levels for the comparator. Further noise immunity
can be achieved by connecting a capacitor between PFI and GND.
WDO
POWER
WDI
t
INPUT
WP
R1
R2
POWER-FAIL
Figure 17. Power-Fail Comparator
Figure 16. Watchdog Timing
t
WD
INPUT
PFI
ADM706P/ADM706R/
ADM708R/ADM708S/
ADM706S/ADM706T/
RESET EXTERNALLY
TRIGGERED BY MR
1.25V
ADM708T
t
WD
PFO
CC
t
RS
falls below
POWER-FAIL
OUTPUT
t
WD
Rev. B | Page 10 of 16
POWER
VALID RESET BELOW 1 V V
The ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/
ADM708T family of products are guaranteed to provide a valid
reset level with V
Characteristics section. As V
transistor does not have sufficient drive to hold it on so the
voltage on RESET is no longer held at 0 V. A pull-down resistor
as shown in Figure 19 can be connected externally to hold the
line low if it is required.
INPUT
V
V
V
R1
R2
H
L
MID
Figure 18. Adding Hysteresis to the Power-Fail Comparator
3.3V
PFO
=
=
0V
. 1
. 1
=
ADM663A
0V V
25
25
. 1
25
⎡
⎢
⎢
⎣
ADM706R/ADM706S/
ADM706T/ADM708R/
+
ADM708S/ADM708T
L
1
CC
R1
⎛
⎜
⎝
+
V
as low as 1 V. Refer to the Typical Performance
Figure 19. RESET Valid Below 1 V
R1
IN
⎛
⎜
⎜
⎝
⎛
⎜
⎝
PFI
V
3.3V
ADM706P/ADM706R/
ADM706S/ADM706T/
ADM708R/ADM708S/
R2
R2
. 1
R2
H
+
R2
1.25V
25
R2
+
×
ADM708T
R3
−
R3
⎞
⎟
⎠
V
V
CC
CC
⎞
⎟
⎟
⎠
CC
+
–
GND
drops below 1 V, the internal
R1
R3
RESET
R3
−
⎤
⎥
⎥
⎦
CC
. 1
PFO
25
⎞
⎟
⎠
TO MICROPROCESSOR NMI
R1