IMP708 데이터 시트보기 (PDF) - IMP, Inc

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IMP708

Low-Power µP Supervisor Circuits

IMP, Inc 

IMP705/6/7/8, 813L

Application Information

Monitoring Voltages Other Than VCC

The IMP705-708 can monitor voltages other than VCC using the

Power Fail circuitry. If a resistive divider is connected from the

voltage to be monitored to the Power Fail input, PFI, the PFO

(output) will go LOW if the divider voltage goes below its 1.25V

reference. Should hysteresis be desired, connect a resistor (equal

to approximately 10 times the sum of the two resistors in the

divider) between the PFI and PFO pins. A capacitor between PFI

and GND will reduce circuit sensitivity to input high-frequency

noise. If it is desired to assert a RESET in addition to the PFO flag,

this may be achieved by connecting the PFO output to MR.

Monitoring a Negative Voltage

The Power-Fail circuitry can also monitor a negative supply rail.

When the negative rail is OK, PFO will be LOW, and when the

negative rail is failing (not negative enough), PFO goes HIGH

(the opposite of when positive voltages are monitored). To trigger

a reset, these outputs need to be inverted: adding the resistors

and transistor as shown achieves this. The RESET output will

then have the same sense as for positive voltages: good = HIGH,

bad = LOW. It should be noted that this circuit’s accuracy depends

on the VCC line, the PFI threshold tolerance, and the resistors.

VCC = 5V

VCC

+5V

+

VCC

VIN = 12V

1M9

MR

IMP70x

PFO

MR

IMP70x

RP

PFO

PFI

RESET

130k9

GND

To Processor

12V Threshold

~10.87V

705_07.eps

PFI

RESET

RN

—

Negative Input

Voltage

GND

5 − 1.25 = 1.25 − VTRIP

RP

RN

705_09.eps

Figure 4. Monitoring Voltages Other Than VCC

Figure 5. Monitoring a Negative Voltage

© 1999 IMP, Inc.

408-432-9100/www.impweb.com

7

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