sp690tep-l Exar Corporation, sp690tep-l Datasheet - Page 15

• Manufacturer Part Number: sp690tep-l
• Description: Sp690s -low Power Microprocessor Supervisory With Battery Switch-over
• Manufacturer: Exar Corporation

Figure 22A. Adding Additional Hysteresis to the Power-Fail Comparator.
Figure 22B. Shifting the Additional Hysteresis above V
Connecting an ordinary signal diode in series
with R3, as in Figure 22B, causes the lower trip
point (V
hysteresis (V
window occurs above V
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.
Figure 23. Using the Power-Fail Comparator to Monitor an Additional Power Supply
SP690T/S/R JAN 30-06
A.)
L
) to coincide with the trip point without
R1
R2
V
IN
PFO
TRIP
V
V
V
WHERE V
0V
TRIP
H
L
= R1
0V
=
+
*C1
SP690T/S/R, 802/T/S/R, 804T/S/R, 805T/S/R Low Power Microprocessor Supervisory
(
= V
), so the entire hysteresis
V
R2
R1
PFT
TO P
[ (
PFT
V
+ V
V-
PFT
V
PFT
PFO
R3
PFH
PFH
V
V
WHERE V
NOTE: V
(
R1
)
1 + 1 + 1
TRIP
L
= 1.25V
V
V
R1 + R2
( )
= 10mV
= R2
L
PFI
PFO
R1
CC
TRIP
R2
R2
= R2
V
(
TRIP
[ (
SP690T/S/R
SP802T/S/R
SP804T/S/R
SP805T/S/R
R1
1 + 1 + 1
V
TRIP
)
R3
PFT
[
V
GND
. This method pro-
PFT
V
(
PFH
)
V
R2
IS NEGATIVE
CC
V
H
R1
1 + 1
- V
PFT
= 1.25V
= 10mV
R3
V
+ V
R3
CC
L
PFI
R2
]
)
PFH
)
)
- V
SP690T/S/R
SP802T/S/R
SP804T/S/R
SP805T/S/R
(
R3
R1
CC
1 + 1
V
GND
V
IN
V
]
CC
TRIP
R2
)
PFO
0V
- V
R1
CC
]
*OPTIONAL
V-
PFT
15
R1
R2
V
IN
PFO
V
V
The current through R1 and R2 should be at least
1 A to ensure that the 25nA (max over extended
temperature range) PFI input current does not
shift the trip point. R3 should be larger than
10k
Capacitor C1 adds additional noise rejection.
TRIP
H
V
=
CC
(
= V
V
PFT
B.)
PFT
+ V
PFH
so it does not load down the PFO pin.
)
(
R1 + R2
(
V
R1 + R2
TRIP
R2
PFI
R2
R1
R2
)
3.0V OR 3.3V
SP690T/S/R
SP802T/S/R
SP804T/S/R
SP805T/S/R
V
V
V
WHERE V
)
IN
TRIP
L
PFO
GND
= R1
V
V
0V
CC
= V
H
[
+
0V
*C1
PFT
(
V
V
V
PFO
PFT
PFT
PFH
D
TO P
= DIOD FORWARD VOLTAGE DROP
(
+ V
= 1.25V
R1 + R2
= 10mV
PFH
R3
R2
)
V
V
(
IN
TRIP
R1
1 + 1 + 1
PFI
)
PFO
R2
SP690T/S/R
SP802T/S/R
SP804T/S/R
SP805T/S/R
© 2006 Sipex Corporation
GND
R3
V
V
CC
)
H
- (V
CC -
R3
V
D
)
]
V
IN