ADM705/ADM706/ADM707/ADM708
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
7V TO 15V
INPUT
POWER
R1
1.25V
PFI
R2
ADP3367
5V
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
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 Figure 17, to hold the line low if
required.
ADM705/ADM706/
ADM707/ADM708
V
CC
–
+
PFO
TO
MICROPROCESSOR
NMI
RESET
00088-011
ADM705/ADM706/
ADM707/ADM708
R3
GND
R1
Figure 17. RESET Valid Below 1 V
5V
PFO
0V V
L
V
IN
V
H
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
+
R3
⎞ ⎤
⎡
V
H
=
1.25
⎢
1
+ ⎛
⎜
⎟
R1
⎥
⎣ ⎝
R2
×
R3
⎠ ⎦
⎛
1.25
V
CC
−
1.25
⎞
−
V
L
=
1.25
+
R1
⎜
⎟
R3
⎝
R2
⎠
R1
+
R2
⎞
V
MID
=
1.25
⎛
⎜
⎟
⎝
R2
⎠
Rev. G | Page 9 of 12
00088-010
0V
AD [ ANALOG DEVICES ]
