LTC692CN8 データシートの表示（PDF） - Linear Technology

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LTC692CN8

Microprocessor Supervisory Circuits

Linear Technology 

LTC692/LTC693

APPLICATIONS INFORMATION

VIN ≥ 7.5V

+

10μF

R1

51k

R2

10k

LT1086-5

VIN VOUT +

ADJ

100μF

R3

300k

5V

0.1μF

R4

10k

TO μP

VCC

LTC692

LTC693

PFO

PFI GND

692_3 • F08

Figure 8. Monitoring Unregulated DC Supply with the

LTC692/LT693 Power-Fail Comparator

VIN ≥ 6.5V

LT1086-5

+

VIN VOUT +

10μF ADJ

10μF

5V

R1 R4

27k 10k

R3

2.7M

0.1μF

R2

8.2k

TO μP

R5

3.3k

VCC

LTC692

LTC693

PFO

PFI GND

692_3 • F09

Figure 9. Monitoring Regulated DC Supply with the

LTC692/LT693 Power-Fail Comparator

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 shutdown procedure is 8ms. Also, the noise of VIN is

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

ably set VL = 7.25V which is 1.25V greater than the sum of

maximum reset voltage threshold and the dropout voltage

of LT®1086-5 (4.5V + 1.5V) and VHYSTERESIS = 850mV.

VHYSTERESIS

=

5V

R1

R3

=

850mV

R3 ≈ 5.88 R1

Choose R3 = 300k and R1 = 51k. Also select R4 = 10k

which is much smaller than R3.

7.25V

=

1.

3V

⎛

⎝⎜

1+

51k

R2

–

(5V – 1.3V)51k⎞

1.3V(310k) ⎠⎟

R2 = 10.1k, Choose nearest 5% resistor 10k and recal-

culate VL,

VL

=

1.3V

⎛

⎝⎜

1+

51k

10 k

–

(5V – 1.3V)51k⎞

1.3V(310 k) ⎟⎠

= 7.32V

VH

=

1.3V

⎛

⎝⎜

1+

51k

10k

+

51k ⎞

300k ⎠⎟

=

8.151V

(7.32V – 6.25V) = 10.7ms

100mV/ms

VHYSTERESIS = 8.151V – 7.32V = 831mV

The 10.7ms allows enough time to execute shutdown pro-

cedure for microprocessor and 831mV of hysteresis would

prevent PFO from going low due to the noise of VIN.

0692fa

13

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