ICM7232 Intersil Corporation, ICM7232 Datasheet - Page 10

• Manufacturer Part Number: ICM7232
• Description: Numeric/alphanumeric Triplexed Lcd Display Drivers
• Manufacturer: Intersil Corporation

Figure 8 shows the curve of contrast versus applied RMS volt-
age for a liquid crystal material tailored for V
typical value for
that the RMS OFF voltage V
“threshold” voltage where contrast begins to increase. This
places the RMS ON voltage at 2.1V, which provides about
85% contrast when viewed straight on.
All members of the ICM7231 and ICM7232 family use an internal
resistor string of three equal value resistors to generate the volt-
ages used to drive the display. One end of the string is connected
on the chip to V
pin 2 (V
(V
used. Remember that V
three times the threshold voltage of the liquid crystal material
used. Also it is very important that pin 2 never be driven below
V
Temperature Effects and Temperature Compensation
The performance of the LCD material is affected by tempera-
ture in two ways. The response time of the display to changes
of applied RMS voltage gets longer as the display tempera-
ture drops. At very low temperatures (-20
may take several seconds to change a new character after the
new information appears at the outputs. However, for most
applications above 0
able multiplexed LCD materials, and for low-temperature
applications, high-speed liquid crystal materials are available.
At high temperature, the effect to consider deals with plastic
materials used to make the polarizer.
Some polarizers become soft at high temperatures and per-
manently lose their polarizing ability, thereby seriously
degrading display contrast. Some displays also use sealing
materials unsuitable for high temperature use. Thus, when
SS
DISP
. This can cause device latchup and destruction of the chip.
100
90
80
70
60
50
40
30
20
10
FIGURE 8. CONTRAST vs APPLIED RMS VOLTAGE
0
) to be optimized for the particular liquid crystal material
DISP
0
T
A
1.1V
) on each chip. This allows the display voltage input
= 25
V
OFF
DD
RMS
o
C
1
/
=
3
and the other end (user input) is available at
= -30
1
multiplexed displays in calculators. Note
0-
o
APPLIED VOLTAGE (V
C this will not be a problem with avail-
o
PEAK
= -10
PEAK
= V
o
2
= +10
DD
/3
0+
- V
o
V
ON
RMS
1V is just below the
DISP
= 2.1V
o
C) some displays
)
3
PEAK
= 0
and should be
ICM7231, ICM7232
= 3.1V, a
4
9-28
specifying displays the following must be kept in mind: liquid
crystal material, polarizer, and seal materials.
A more important effect of temperature is the variation of
threshold voltage. For typical liquid crystal materials suitable for
multiplexing, the peak voltage has a temperature coefficient of -7
to -14mV/
old voltage goes down. Assuming a fixed value for V
threshold voltage drops below V
be visible. Figure 9 shows the temperature dependence of peak
voltage for the same liquid crystal material of Figure 8.
FIGURE 9. TEMPERATURE DEPENDENCE OF LC THRESHOLD
For applications where the display temperature does not
vary widely, V
make the RMS OFF voltage, V
old voltage at the highest temperature expected. This will
prevent OFF segments turning ON at high temperature (this
at the cost of reduced contrast for ON segments at low
temperatures).
For applications where the display temperature may vary to
wider extremes, the display voltage V
may require temperature compensation to maintain sufficient
contrast without OFF segments becoming visible.
Display Voltage and Temperature Compensation
These circuits allow control of the display peak voltage by
bringing the bottom of the voltage divider resistor string out at
pin 2. The simplest means for generating a display voltage
suitable to a particular display is to connect a potentiometer
from pin 2 to V
a maximum value of 200k
adjustment to suit most displays. This method for generating
display voltage should be used only in applications where the
temperature of the chip and display won’t vary more than
perature coefficient, which will tend to increase the display
peak voltage with an increase in temperature. The display
voltage also depends on the power supply voltage, leading to
tighter tolerances for wider temperature ranges.
5
o
C ( 9
6
5
4
3
2
1
0
-10
o
o
F), as the resistors on the chip have a positive tem-
C. This means that as temperature rises, the thresh-
PEAK
SS
10% CONTRAST (OFF)
PEAK VOLTAGE FOR
0
as shown in Figure 10. A potentiometer with
AMBIENT TEMPERATURE (
may be set at a fixed voltage chosen to
10
PEAK VOLTAGE FOR
90% CONTRAST (ON)
should give sufficient range of
PEAK
PEAK
20
/3 OFF segments begin to
/3, just below the thresh-
DISP
30
o
(and thus V
C)
40
P
, when the
50
PEAK
)