IMP802 IMP Inc, IMP802 Datasheet - Page 8

• Manufacturer Part Number: IMP802
• Description: P POWER SUPPLY SUPERVISOR WITH BATTERY BACKUP SWITCH
• Manufacturer: IMP Inc

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 10k to avoid excessive loading on
the PFO pin. The calculations for the correct values of resistors to
set the hysteresis thresholds are given in Figure 8. A capacitor can
be added to offer additional noise rejection by low-pass filtering.
8
Application Information
PFO
+5V
0V
0V
V
Figure 8. Hysteresis added to PFI pin
IN
V
V
V
R
R
IH
TRIP
L
1
2
R
1 25
1
.
R
1
R
R
1 25
2
C1*
1 25
R
5 1 25
2
.
R
.
R
2
R
2
To P
R
3
R
3
.
2
3
R
+5V
3
1 25
R
.
2
V
L
V
PFI
PFO
CC
V
V
TRIP
IN
IMP802M
GND
IMP690A
IMP692A
IMP802L
IMP805L
V
H
*Optional
690A_10.eps
IMP690A
IMP690A
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”.
Figure 9. Using PFI to monitor negative supply voltage
, 692A
, 692A
PFO
+5V
0V
+5V
5 1 25
R
R
R
V–
1
2
1
.
, 802L, 802M, 805L
, 802L, 802M, 805L
V
PFI
1 25
CC
.
IMP802M
IMP690A
IMP692A
IMP802L
IMP805L
V
R
V–
TRIP
GND
V
2
TRIP
PFO
V – = V
0V
690A_11.eps
TRIP