Datasheet
ADM706P/ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/ADM708T
Rev. C | Page 11 of 16
POWER-FAIL COMPARATOR
6435-011
INPUT
POWER
TO MICROPROCESSOR NMI
R3
3.3
V
The power-fail comparator is an independent comparator that
can be used to monitor the input power supply. The inverting
input of the comparator is internally connected to a 1.25 V
reference voltage. The noninverting input is available at the PFI
input. This input is 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 is used to monitor the preregulator input simply
by choosing an appropriate resistive divider network. The
PFO
output is used to interrupt the processor so that a shutdown
procedure is implemented before the power is lost.
INPUT
POWE
R
R1
R2
POWER-FAIL
INPUT
1.25V
PFI
PFO
POWER-FAIL
OUTPUT
06435-010
ADM706P/ADM706R/
ADM706S/ADM706T/
ADM708R/ADM708S/
ADM708T
Figure 17. Power-Fail Comparator
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 is added simply by connecting a resistor
between the
PFO
output and the PFI input as shown in .
When
Figure 18
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 is achieved
by connecting a capacitor between PFI and GND.
0
V
CC
R1
R2
1.25V
PFI
PFO
ADM663A
–
+
3.3V
0V
0V
V
L
V
IN
V
H
PFO
ADM706P/ADM706R/
ADM706S/ADM706T/
ADM708R/ADM708S/
ADM708T
Figure 18. Adding Hysteresis to the Power-Fail Comparator
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
+
+= R1
R3R2
R3R2
V
H
125.1
⎟
⎠
⎞
⎜
⎝
⎛
−
−+=
R3
V
R2
R1V
CC
L
25.1
25.1
25.1
⎟
⎠
⎞
⎜
⎝
⎛
+
=
R2
R2R1
V
MID
25.1
VALID RESET BELOW 1 V V
CC
The ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/
ADM708T are guaranteed to provide a valid reset level with V
CC
as low as 1 V. Refer to the Typical Performance Characteristics
section. As V
CC
drops below 1 V, the internal 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 , can
be connected externally to hold the line low if it is required.
Figure 19
GND
RESET
R1
06435-012
ADM706R/ADM706S/
ADM706T/ADM708R/
ADM708S/ADM708T
Figure 19.
RESET
Valid Below 1 V