ICL8211, ICL8212
V+
The disadvantage of the simple detection circuits is that
INPUT
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.
1
2
3
4
8
7
6
5
R2
INPUT
VOLTAGE
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 performance
but may be expanded into highly complex systems with
multiple voltage level detection and memory applications-
refer to specific applications section.
R1
V-
Input voltage to change to output states
(R + R )
1
2
=
x 1.15V
R
1
FIGURE 24. RANGE OF INPUT VOLTAGE GREATER THAN
+1.15 VOLTS
There are two simple methods to apply hysteresis to a circuit
for use in supply voltage level detection. These are shown in
Figure 27.
Setup Procedures For Voltage Level Detection
Case 1. Simple voltage detection no hysteresis
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.
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.
MAY BE ANY STABLE VOLTAGE
VOLTAGE REFERENCE
GREATER THAN 1.15V
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.
VREF (+VE)
V+
1
2
3
4
8
7
6
5
R2
R1
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 35µA which will increase to 7mA when the
lamp is turned on. R3 will provide hysteresis if required.
Range of input voltage less than +1.15V
Input voltage to change the output states
(R + R ) x 1.15
R V
2
1
2
REF
1
-
=
R
R
1
Nonvolatile Low Voltage Detector (Figure 29)
FIGURE 25. INPUT RESISTOR NETWORK SETUP
PROCEDURES
In this application the high trip voltage VTR2 is set to be
above the normal supply voltage range. On power up the
initial condition is A. On momentarily closing switch S1 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
if the supply voltage is reduced below VTR1 (even to zero
For supply voltage level detection applications the input
resistor network is connected across the supply terminals as
shown in Figure 26.
V+
1
2
3
4
8
7
6
5
volts) and then raised back to VNOM
.
R2
Nonvolatile Power Supply Malfunction Recorde
(Figure 30 and Figure 31)
INPUT VOLTAGE
OR
SUPPLY VOLTAGE
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.
R1
VO
It is, therefore, necessary to be able to detect and store the
fact that an out-of-operating range supply voltage condition
has occurred, even in the case where a supply voltage may
FIGURE 26. COMBINED INPUT AND SUPPLY VOLTAGES
Case 2. Use of the HYSTERESIS function
7-170
HARRIS [ HARRIS CORPORATION ]
