IMP690A, 692A, 802L, 802M, 805L
Application Information
Power Fail Hysteresis
Monitoring Capabilities of the Power-Fail Input
A noise margin can be added to the simple monitoring circuit of Although designed for power supply failure monitoring, the PFI
Figure 7 by adding positive feedback from the PFO pin. The cir- pin can be used for monitoring any voltage condition that can be
cuit of Figure 8 adds this positive “latching” effect by means of an scaled by means of a resistive divider. An example is the negative
additional resistor R3 connected between PFO and PFI which power supply monitor configured in Figure 9. In this case a good
helps in pulling PFI in the direction of PFO and eliminating an negative supply will hold the PFI pin below 1.25V and the PFO
indecision at the trip point. Resistor R3 is normally about 10 times pin will be at a logic “0”. As the negative voltage declines,
higher in resistance than R2 to keep the hysteresis band reason- the voltage at the PFI pin will rise until it exceeds 1.25V and
able and should be larger than 10kΩ to avoid excessive loading on the PFO pin will go to a logic “1”.
the PFO pin. The calculations for the correct values of resistors to
+5V
set the hysteresis thresholds are given in Figure 8. A capacitor can
be added to offer additional noise rejection by low-pass filtering.
VCC
PFI
VIN
IMP690A
IMP692A
IMP802L
IMP802M
IMP805L
R1
+5V
VCC
PFO
IMP690A
IMP692A
R1
IMP802L
IMP802M
R2
PFI
GND
IMP805L
R3
V–
R2
C1*
–
PFO
V
= VTRIP
GND
+5V
PFO
0V
To µP
*Optional
VTRIP
V–
+5V
PFO
0V
0V
5 −1.25 1.25 − V
TRIP
=
VL
VH
0V
VTRIP
VIN
R
R
2
690A_11.eps
1
1.25
V
=
TRIP
R
2
+R
2
Figure 9. Using PFI to monitor negative supply voltage
R
2
1.25
V
=
IH
R
R
2
2
3
R
3
R +R
1
V −1.25 5 −1.25 1.25
L
+
=
R
R
R
2
1
3
690A_10.eps
Figure 8. Hysteresis added to PFI pin
8
IMP [ IMP, INC ]
