12

LTC1235

Choose R3 = 300k

Ω and R1 = 51kΩ. Also select R4 =

10k

Ω which is much smaller than R3.

R2 = 9.7k

Ω, Choose nearest 5% resistor 10k and recalcu-

S

APPLICATI

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.

When PFO output is high, the series combination of R3 and

R4 source current into the PFI summing junction.

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

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

maximum reset voltage threshold and the dropout voltage

10µF

100µF

ADJ

LTC1235 F07

0.1µF

TO

µP

GND

PFO

LT1086-5

R4

10k

PFI

LTC1235

R1

51k

R2

10k

R3

300k

+

+

+5V

BACKUP

10µF

10µF

ADJ

LTC1235 F08

0.1µF

TO µP

PFO

GND

LT1086-5

R4

10k

PFI

+

R1

27k

R3

2.7M

R2

8.2k

+

+5V

R5

3.3k

LTC1235

BACKUP

Figure 9. Monitoring

Regulated DC Supply with the LTC1235

Power Fail Comparator

Figure 8. Monitoring

Unregulated DC Supply with the

LTC1235 Power Fail Comparator

R3

≈ 5.88 R1

V = 1.3V 1+

R1

R2

R1

R3

H

+









V

1.3V 1

R1

R2

–

(5V – 1.3V)R1

1.3V(R3 R4)

+









Assuming R4«R3,V

5V

R1

R3

V5V

R1

R3

850mV

7.5V = 1.3V 1+

51k

R2

–

(5V – 1.3V)51k

1.3V(310k )

ΩΩ

Ω









V

1.3V 1

51k

10k

–

(5V – 1.3V)51k

1.3V(310k

7.32V









=

Ω

Ω

Ω

Ω)

V

1.3V 1

51k

10k

51k

300k

8.151V

+









=

Ω

Ω

Ω

Ω

(7.32V – 6.25V)

100mV/ms

10.7ms

=