LTC1235CN(Rev0) Ver la hoja de datos (PDF) - Linear Technology
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LTC1235CN Datasheet PDF : 16 Pages
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LTC1235
APPLICATI S I FOR ATIO
Power Fail Warning
The LTC1235 generates a Power Failure Output (PFO) for
early warning of failure in the microprocessor's power
supply. 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 microproces-
sor to execute shut-down procedure between PFO and
RESET or RESET.
The power fail comparator, C3, does not have hysteresis.
Hysteresis can be added however, by connecting a resis-
tor 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
summing junction at the PFI pin.
VH
=
1.3V
1+
R1
R2
+
R1
R3
When PFO output is high, the series combination of R3 and
R4 source current into the PFI summing junction.
VL
=
1.3V
1 +
R1
R2
–
(5V – 1.3V)R1
1.3V(R3 + R4)
Assuming
R4« R3, VHYSTERESIS
=
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 the rate of decay of the
supply input VIN is 100mV/ms and the total time to execute
a shut-down procedure is 8ms. Also the noise of VIN is
200mV. With these assumptions in mind, we can reason-
ably set VL = 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 VHYSTERESIS = 850mV.
VHYSTERESIS
=
5V
R1
R3
=
850mV
R3 ≈ 5.88 R1
Choose R3 = 300kΩ and R1 = 51kΩ. Also select R4 =
10kΩ which is much smaller than R3.
7.5V
=
1. 3V
1+
51kΩ
R2
–
(5V – 1.3V)51kΩ
1.3V(310kΩ)
R2 = 9.7kΩ, Choose nearest 5% resistor 10k and recalcu-
late VL,
VL
=
1.3V
1 +
51kΩ
10 kΩ
–
(5V – 1.3V)51kΩ
1.3V(310kΩ)
=
7.32V
VH
=
1.3V
1 +
51kΩ
10kΩ
+
51kΩ
300kΩ
=
8.151V
(7.32V – 6.25V) = 10.7ms
100mV/ms
VHYSTERESIS = 8.151V – 7.32V = 831mV
VIN ≥ 7.5V
+
10µF
R1
51k
LT1086-5
VIN VOUT
ADJ
+
100µF
R3
300k
+5V
0.1µF
R4
10k
R2
10k
VCC
LTC1235
PFO BACKUP
PFI GND
TO µP
LTC1235 F07
Figure 8. Monitoring Unregulated DC Supply with the
LTC1235 Power Fail Comparator
VIN ≥ 6.5V LT1086-5 10µF
+
VIN VOUT
+
10µF ADJ
+5V
R1
R4
27k 10k
R3
2.7M
R2
8.2k
R5
3.3k
0.1µF
VCC
LTC1235
PFO BACKUP
PFI GND
TO µP
LTC1235 F08
Figure 9. Monitoring Regulated DC Supply with the LTC1235
Power Fail Comparator
12
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