Basic Over/Undervoltage

Detection Circuits

Figures 3, 4, and 5 show the three basic voltage detec-

tion circuits.

The simplest circuit, depicted in Figure 3, does not

have any hysteresis. The comparator trip-point formulas

can easily be derived by observing that the comparator

nal resistors form a voltage divider that attenuates the

1.3V when the input voltage is at the desired compara-

tor trip point. Since the bias current of the comparator

is only a fraction of a nanoamp, the current in the volt-

age divider can be less than one microamp without los-

ing accuracy due to bias currents. The ICL7665A has a

2% threshold accuracy at +25°C, and a typical temper-

ature coefficient of 100ppm/°C including comparator

offset drift, eliminating the need for external poten-

tiometers in most applications.

Figure 4 adds another resistor to each voltage detector.

This third resistor supplies current from the HYST out-

old. As the formulas show, this hysteresis resistor

affects only the lower trip point. Hysteresis (defined as

the difference between the upper and lower trip points)

keeps noise or small variations in the input signal from

repeatedly switching the output when the input signal

remains near the trip point for a long period of time.

The third basic circuit, Figure 5, is suitable only when the

voltage to be detected is also the power-supply voltage for

the ICL7665. This circuit has the advantage that all of the

current flowing through the input divider resistors flows

through the hysteresis resistor. This allows the use of

higher-value resistors, without hysteresis output leakage

having an appreciable effect on the trip point.

Resistor-Value Calculations

Figure 3

1) Choose a value for R11. This value determines the

amount of current flowing though the input divider,

range of 10k

Ω to 10MΩ.

2) Calculate R21 based on R11 and the desired trip

point:

R21 = R11

1.3V

Microprocessor Voltage Monitor with

Dual Over/Undervoltage Detection

_______________________________________________________________________________________

7

Figure 3. Simple Threshold Detector

Figure 4. Threshold Detector with Hysteresis

ICL7665

OUT1

OUT2

SET2

SET1

R21

R11

R22

R12

V+

OUT1

OUT2

ICL7665

OUT1

OUT2

SET2

SET1

R21

R11

R22

R12

V+

HYST1

HYST2

R31

R32

V+

OUT1

0V

V+

OUT2

0V