IMP690A , 692A , 802L, 802M, 805L
Application Information
Power Fail Hysteresis
A noise margin can be added to the simple monitoring circuit of
Figure 7
by adding positive feedback from the PFO pin. The cir-
cuit of
Figure 8
adds this positive “latching” effect by means of an
additional resistor R3 connected between PFO and PFI which
helps in pulling PFI in the direction of PFO and eliminating an
indecision at the trip point. Resistor R3 is normally about 10 times
higher in resistance than R2 to keep the hysteresis band reason-
able and should be larger than 10kΩ to avoid excessive loading on
the
pin. The calculations for the correct values of resistors to
PFO
set the hysteresis thresholds are given in
Figure 8.
A capacitor can
be added to offer additional noise rejection by low-pass filtering.
V
IN
+5V
V
CC
R
1
IMP690A
IMP692A
IMP802L
PFI
IMP802M
IMP805L
PFO
GND
To
µP
*Optional
+5V
PFO
0V
0V
V
TRIP
=
1.25
R
2
R
+
R
2
2
1.25
R
2
R
3
R
+
R R
1
2
3
V
L
V
TRIP
V
IN
V
H
+5V
PFO
0V
V
TRIP
V–
5
−
1.25 1.25
−
V
TRIP
=
R
1
R
2
0V
R
1
Monitoring Capabilities of the Power-Fail Input
Although designed for power supply failure monitoring, the PFI
pin can be used for monitoring any voltage condition that can be
scaled by means of a resistive divider. An example is the negative
power supply monitor configured in
Figure 9.
In this case a good
negative supply will hold the PFI pin below 1.25V and the PFO
pin will be at a logic “0”. As the negative voltage declines,
the voltage at the PFI pin will rise until it exceeds 1.25V and
the PFO pin will go to a logic “1”.
+5V
V
CC
IMP690A
IMP692A
IMP802L
PFO
PFI
IMP802M
IMP805L
GND
R
2
V–
R
3
R
2
C1*
V– = V
TRIP
690A_11.eps
Figure 9. Using PFI to monitor negative supply voltage
V
IH
=
V
L
−
1.25 5
−
1.25 1.25
+
=
R
1
R
3
R
2
690A_10.eps
Figure 8. Hysteresis added to PFI pin
8
IMP [ IMP, INC ]
