Datasheet
ADM705/ADM706/ADM707/ADM708
Rev. G | Page 9 of 12
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 16
7V TO 15
V
INPUT
POWER
TO
MICROPROCESSOR
NMI
R3
5V
V
CC
R1
R2
1.25V
PFI
PFO
ADP3367
5V
0V
0V
V
L
V
IN
V
H
–
+
00088-010
PFO
ADM705/ADM706/
ADM707/ADM708
Figure 16. Adding Hysteresis to the Power-Fail Comparator
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. The
equations used to calculate the hysteresis are as follows:
⎟
⎠
⎞
⎜
⎝
⎛
+
=
⎟
⎠
⎞
⎜
⎝
⎛
−
−+=
⎥
⎦
⎤
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛
×
+
+=
R2
R2R1
V
R3
V
R2
R1V
R1
R3R2
R3R2
V
MID
CC
L
H
25.1
25.1
25.1
25.1
125.1
VALID RESET BELOW 1 V V
CC
The ADM705/ADM706/ADM707/ADM708 are guaranteed to
provide a valid reset level with V
CC
as low as 1 V (see the Typical
Performance Characteristics section). As V
CC
drops below 1 V,
the internal transistor does not have sufficient drive to hold the
voltage
RESET
at 0 V. A pull-down resistor can be connected
externally, as shown in , to hold the line low if
required.
Figure 17
ADM705/ADM706/
ADM707/ADM708
GND
RESET
R1
00088-011
Figure 17.
RESET
Valid Below 1 V