VIN
VIN
B.)
A.)
R1
R2
R1
R2
VCC
VCC
PFI
PFI
+
+
SP690T/S/R
SP802T/S/R
SP804T/S/R
SP805T/S/R
SP690T/S/R
SP802T/S/R
SP804T/S/R
SP805T/S/R
R3
R3
*C1
*C1
PFO
PFO
GND
GND
TOµP
TOµP
*OPTIONAL
PFO
0V
PFO
0V
VIN
VIN
0V
VTRIP
0V
VL
VTRIP
VH
VH
R1 + R2
R1 + R2
VTRIP = VPFT
VTRIP = VPFT
(
)(
)
+ 1
(
)
R2
R2
1
+ 1
VH
=
VPFT + VPFH
-
(VCC - VD
R3
)
R1
1 + 1 + 1
(
)
(
R1 R2 R3
)
]
VPFT + VPFH
VL = R1
[
(
)(R1
)
R2 R3
]
1
+
1
+
1
-
VCC
R3
VPFT
VL = R1
[
(
)
R1 R2 R3
WHERE VPFT = 1.25V
PFH = 10mV
VD = DIOD FORWARD VOLTAGE DROP
V
WHERE VPFT = 1.25V
V
PFH = 10mV
Figure 22A. Adding Additional Hysteresis to the Power-Fail Comparator.
Figure 22B. Shifting the Additional Hysteresis above VPFT
ThecurrentthroughR1andR2shouldbeatleast
1µAtoensurethatthe25nA(maxoverextended
temperature range) PFI input current does not
shift the trip point. R3 should be larger than
10kΩ so it does not load down the PFO pin.
Capacitor C1 adds additional noise rejection.
Connecting an ordinary signal diode in series
with R3, as in Figure 22B, causes the lower trip
point(VL)tocoincidewiththetrippointwithout
hysteresis (VTRIP), so the entire hysteresis
window occurs above VTRIP. This method pro-
vides additional noise margin without compro-
mising the accuracy of the power-fail threshold
when the monitored voltage is falling. It is
useful for accurately detecting when a voltage
falls past a threshold.
VIN
3.0V OR 3.3V
VCC
VCC
R1
R2
R1
R2
SP690T/S/R
SP802T/S/R
SP690T/S/R
SP802T/S/R
PFI
PFO
PFI
PFO
SP804T/S/R
SP805T/S/R
SP804T/S/R
SP805T/S/R
GND
GND
V-
VCC
VCC
PFO
PFO
VIN
V-
VTRIP
VL
VTRIP
0V
VH
-
VCC
R1 + R2
1
+
1
R2
VTRIP = VPFT
VTRIP = R2
VPFT + VPFH
(
)
[
(
R2
]
)(R1
)
R1
-
VCC
R3
1
+
1
VPFT
VL = R2
R1 + R2
[
(
]
)
R1 R2
VPFT VPFH
=
VH
+
(
)(
)
R2
WHERE VPFT = 1.25V
VPFH = 10mV
NOTE: VTRIP IS NEGATIVE
Figure 23. Using the Power-Fail Comparator to Monitor an Additional Power Supply
SP690T/S/R DS/10 SP690T/S/R, 802/T/S/R, 804T/S/R, 805T/S/R Low Power Microprocessor Supervisory
© Copyright 2000 Sipex Corporation
15
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