ADM706P/R/S/T, ADM708R/S/T
Power-Fail Comparator
The power-fail comparator is an independent comparator which
may be used to monitor the input power supply. The compara-
tor’s inverting input is internally connected to a 1.25 V reference
voltage. The noninverting input is available at the PFI input.
This input may 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 compara-
tor may 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 proce-
dure is implemented before the power is lost.
INPUT
POWER
1.25
V
CC
– 1.25
V
L
=
1.25
+
R1
–
R2
R3
R1
+
R2
V
MID
1.25
R2
Valid RESET Below 1 V V
CC
The ADM70x family of products are guaranteed to provide a
valid reset level with V
CC
as low as 1 V. Please refer to the Typi-
cal Performance Characteristics. As V
CC
drops below 1 V, the
internal transistor will not have sufficient drive to hold it ON so
the voltage on
RESET
will no longer be held at 0 V. A pull-
down resistor as shown in Figure 7 may be connected externally
to hold the line low if it is required.
R1
1.25V
PFO
POWER-FAIL
OUTPUT
ADM70x
RESET
R1
GND
POWER-FAIL PFI
INPUT
R2
ADM70x
Figure 5. Power-Fail Comparator
Adding Hysteresis to the Power-Fail Comparator
Figure 7.
RESET
Valid Below 1 V
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
between the
PFO
output and the PFI input as shown in Figure
6. When
PFO
is low, resistor R3 sinks current from the sum-
ming 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
may be achieved by connecting a capacitor between PFI and GND.
ADM663A
INPUT
POWER
R1
1.25V
PFI
R2
+3.3V
V
CC
PFO
TO P NMI
Typical Performance Characteristics
V
CC
RESET
ADM70x
400ms/DIV
R3
Figure 8. ADM706/ADM708
RESET
Output Voltage vs.
Supply Voltage
3.3V
PFO
0V
0V
V
L
V
IN
V
H
V
CC
Figure 6. Adding Hysteresis to the Power-Fail
Comparator
R2
+
R3
V
H
=
1.25
1
+
R1
R2
×
R3
RESET
400ms/DIV
Figure 9. RESET Output Voltage vs. Supply Voltage
–6–
REV. A
AD [ ANALOG DEVICES ]
