Datasheet

LTC1235
12
1235fa
Power Fail Warning
The LTC1235 generates a Power Failure Output (PFO) for
early warning of failure in the microprocessor’s power sup-
ply. This is accomplished by comparing the Power Failure
Input (PFI) with an internal 1.3V reference. PFO goes low
when the voltage at PFI pin is less than 1.3V. Typically
PFI is driven by an external voltage divider (R1 and R2 in
Figures 8 and 9) which senses either an unregulated DC
input or a regulated 5V output. The voltage divider ratio can
be chosen such that the voltage at PFI pin falls below 1.3V
several milliseconds before the +5V supply falls below the
maximum reset voltage threshold 4.75V. PFO is normally
used to interrupt the microprocessor to execute shutdown
procedure between PFO and RESET or RESET.
The power fail comparator, C3, does not have hysteresis.
Hysteresis can be added however, by connecting a resistor
between the PFO output and the noninverting PFI input
pin as shown in Figures 8 and 9. The upper and lower trip
points in the comparator are established as follows:
When PFO output is low, R3 sinks current from the sum-
ming junction at the PFI pin.
V
H
=1.3V 1+
R1
R
2
+
R1
R3






When PFO output is high, the series combination of R3
and R4 source current into the PFI summing junction.
V
L
=1.3V 1+
R1
R
2
±
(5V ±1.3V)R1
1.3V(R3+R4)






Assuming R4«R3,V
HYST
= 5V
R1
R3
Example 1: The circuit in Figure 8 demonstrates the use
of the power fail comparator to monitor the unregulated
power supply input. Assuming the rate of decay of the
supply input V
IN
is 100mV/ms and the total time to execute
a shut-down procedure is 8ms. Also the noise of V
IN
is
200mV. With these assumptions in mind, we can reason-
ably set V
L
= 7.5V which 1.25V greater than the sum of
maximum reset voltage threshold and the dropout voltage
of LT1086-5 (4.75V + 1.5V) and V
HYST
= 850mV.
APPLICATIONS INFORMATION
V
HYST
= 5V
R1
R3
= 850mV
R3 ≈ 5.88 R1
Choose R3 = 300k and R1 = 51k. Also select R4 = 10k
which is much smaller than R3.
7.5V =1.3V 1+
51k
R
2
±
(5V ±1.3V)51k
1.3V(310k)






R2 = 9.7k, Choose nearest 5% resistor 10k and recalculate
V
L
,
V
L
=1.3V 1+
51k
10
k
±
(5V ±1.3V)51k
1.3V(310k)






= 7.32V
V
H
=1.3V 1+
51k
10
k
+
51k
300k






= 8.151V
(7.32V – 6.25V)
100mV/ms
10.7ms=
V
HYST
= 8.151V – 7.32V = 831mV
Figure 8. Monitoring Unregulated DC Supply with the
LTC1235 Power Fail Comparator
Figure 9. Monitoring Regulated DC Supply with the
LTC1235 Power Fail Comparator
10μF
100μF
V
IN
V
OUT
ADJ
1235 F08
V
CC
0.1μF
TO μP
GND
PFO
LT1086-5
V
IN
≥ 7.5V
R4
10k
PFI
LTC1235
R1
51k
R2
10k
R3
300k
+ +
+5V
BACKUP
10μF
10μF
V
IN
V
OUT
ADJ
1235 F09
V
CC
0.1μF
TO μP
PFO
GND
LT1086-5
V
IN
≥ 6.5V
R4
10k
PFI
+
R1
27k
R3
2.7M
R2
8.2k
+
+5V
R5
3.3k
LTC1235
BACKUP