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.

R1

R2

+

R1

R3

When PFO output is high, the series combination of R3

and R4 source current into the PFI summing junction.

R1

R2

±

(5V ± 1.3V)R1

1.3V(R3+R4)

Assuming R4«R3,V

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

200mV. With these assumptions in mind, we can reason-

maximum reset voltage threshold and the dropout voltage

APPLICATIONS INFORMATION

V

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

R2

±

(5V ± 1.3V)51k

1.3V(310k)

R2 = 9.7k, Choose nearest 5% resistor 10k and recalculate

51k

10k

±

(5V ± 1.3V)51k

1.3V(310k)

= 7.32V

51k

10k

+

51k

300k

= 8.151V

(7.32V – 6.25V)

100mV/ms

10.7ms

=

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

ADJ

1235 F08

0.1μF

TO μP

GND

PFO

LT1086-5

R4

10k

PFI

LTC1235

R1

51k

R2

10k

R3

300k

+

+

+5V

BACKUP

10μF

10μF

ADJ

1235 F09

0.1μF

TO μP

PFO

GND

LT1086-5

R4

10k

PFI

+

R1

27k

R3

2.7M

R2

8.2k

+

+5V

R5

3.3k

LTC1235

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