7-170

ICL8211, ICL8212

FIGURE 24. RANGE OF INPUT VOLTAGE GREATER THAN

+1.15 VOLTS

Setup Procedures For Voltage Level Detection

Case 1. Simple voltage detection no hysteresis

Unless an input voltage of approximately 1.15V is to be

detected, resistor networks will be used to divide or multiply

the unknown voltage to be sensed. Figure 25 shows

procedures on how to set up resistor networks to detect

INPUT VOLTAGES of any magnitude and polarity.

FIGURE 25. INPUT RESISTOR NETWORK SETUP

PROCEDURES

For supply voltage level detection applications the input

resistor network is connected across the supply terminals as

shown in Figure 26.

FIGURE 26. COMBINED INPUT AND SUPPLY VOLTAGES

Case 2. Use of the HYSTERESIS function

The disadvantage of the simple detection circuits is that

there is a small but finite input range where the outputs are

neither totally ‘ON’ nor totally ‘OFF’. The principle behind

hysteresis is to provide positive feedback to the input trip

point such that there is a voltage difference between the

input voltage necessary to turn the outputs ON and OFF.

The advantage of hysteresis is especially apparent in

electrically noisy environments where simple but positive

voltage detection is required. Hysteresis circuitry, however,

is not limited to applications requiring better noise perfor-

mance but may be expanded into highly complex systems

with multiple voltage level detection and memory applica-

tions-refer to specific applications section.

There are two simple methods to apply hysteresis to a circuit

for use in supply voltage level detection. These are shown in

Figure 27.

The circuit of Figure 27A requires that the full current flowing

in the resistor network be sourced by the HYSTERESIS out-

put, whereas for circuit Figure 27B the current to be sourced

by the HYSTERESIS output will be a function of the ratio of

the two trip points and their values. For low values of hyster-

esis, circuit Figure 27B is to be preferred due to the offset

voltage of the hysteresis output transistor.

A third way to obtain hysteresis (ICL8211 only) is to connect

a resistor between the OUTPUT and the THRESHOLD

terminals thereby reducing the total external resistance

between

the

THRESHOLD

and

GROUND

when

the

OUTPUT is switched on.

Practical Applications

Low Voltage Battery Indicator (Figure 28)

This application is particularly suitable for portable or remote

operated equipment which requires an indication of a depleted

or discharged battery. The quiescent current taken by the sys-

tem will be typically 35

µA which will increase to 7mA when the

Nonvolatile Low Voltage Detector (Figure 29)

above the normal supply voltage range. On power up the

operating point changes to B and will remain at B until the

supply voltage drops below VTR1, at which time the output

will revert to condition A. Note that state A is always retained

Nonvolatile Power Supply Malfunction Recorde

(Figure 30 and Figure 31)

In many systems a transient or an extended abnormal (or

absence of a) supply voltage will cause a system failure.

This failure may take the form of information lost in a volatile

semiconductor memory stack, a loss of time in a timer or

even possible irreversible damage to components if a supply

voltage exceeds a certain value.

It is, therefore, necessary to be able to detect and store the

fact that an out-of-operating range supply voltage condition

1

2

3

4

8

7

6

5

V+

INPUT

V-

INPUT

VOLTAGE

Input voltage to change to output states

=

x 1.15V

1

2

3

4

8

7

6

5

V+

MAY BE ANY STABLE VOLTAGE

VOLTAGE REFERENCE

GREATER THAN 1.15V

Range of input voltage less than +1.15V

Input voltage to change the output states

=

-

1

2

3

4

8

7

6

5

V+

INPUT VOLTAGE

OR

SUPPLY VOLTAGE