LCD Contrast Adjustment and Temperature Compensation
Contrast, or more appropriately put, the contrast ratio of a liquid crystal
display is the ratio of the light area of a display to the dark area. When
an LCD is used in a product, an allowance must be made to adjust the LCD
bias voltage. This adjustment will control the contrast between the LCD
segment being on and off, and this voltage needs to be optimized for the
best appearance. This adjustment needs to be made in production for most LCD
designs, and it may also need to be adjusted dynamically during your
products usage since temperature extremes may effect the LCDs contrast. This
section covers information on LCD contrast adjustment required to make the
display viewable, and also how to compensate for the correct contrast over
the full temperature range of your product.
LCD Contrast Adjustment
The LCD bias voltage that effects the LCD contrast, also effects the bias
angle of the display. So at what angle you view the
display will influence the setting of the LCD bias voltage. Adjusting the
contrast voltage, V , will effect the bias angle to some extent, but not the
viewing angle. A 12:00 display can be optimized for a 6:00 viewing position
by adjusting the contrast voltage. A 12:00 display set for 6:00 viewing
position will not have as great a contrast as a 6:00 display set for 6:00
viewing
position, and vice versa. Designers often want a display to be optimized for
straight-on viewing. Either a 12:00 or a 6:00 module can be used; and the
contrast voltage can be adjusted slightly to optimize the display for that
viewing position.
Once the viewing position has been established for a
design, the contrast setting can be determined. This is normally done during
product development on the prototype unit. A potentiometer is connected
between the appropriate power supply rails (Vdd and Vss for single supply,
and Vee and Vdd for higher voltage LCD modules). The wiper of the pot is
connected to Vo (LCD bias voltage input, see below). The LCD is then
positioned at the nominal viewing position, and the pot is adjusted to
obtain the desired LCD appearance. Depending on the optimum contrast
requirement, and the ability to use more than one vendors LCD module, you
can either leave the potentiometer in the design, or measure the voltage on
the Vo pin and select a pair of resistors to produce this voltage in the
production units.
LCD Contrast Adjustment with Temperature Compensation
Due to the contrast versus voltage versus temperature characteristics of
liquid crystal fluids, and the sensitive nature of display drive voltage
during multiplex operation, it may become necessary to compensate the LCD
drive voltage for applications where the display is subjected to wide
temperature excursions. For a typical Liquid Crystal fluid, with a negative
temperature coefficient, an under voltage condition with diminished display
contrast will result at low temperatures, and a "ghosting" or overdrive
condition will occur at high temperatures, if no compensation techniques are
employed. So if your application requires good contrast over a large
temperature range, you may need to consider temperature compensation for
your contrast adjustment circuitry.
When using an LCD module, one solution
is to order an automatic temperature compensation option. However, if you
are using one of our modules without this option, or if you are using one of
our glass products, you need to make your own temperature compensation
circuitry. Listed below are two possible temperature compensation circuits
for your reference.
Example 1
The first thing that needs to be
determined in a particular application is the typical LCD bias voltage (Vo)
and the temperature coefficient of the LCD module. The figure below shows the basic temperature regulation circuit. The heart
of the circuit is U1, a National Semiconductor LM335 monolithic temperature
sensor, which should be placed in close proximity to the LCD. The LC335 has
a basic output voltage coefficient of 110mV/C. Resistor R2 supplies
operating current to U1, 1mA nominal. Difference amplifier U2 inverts and
scales this coefficient consistent with LCD driver requirements.
Potentiometer R1 provides a means by which the display operating voltage can
be set.
Example 2
This example is more simplistic and is
directed toward LCD module applications. As with the first example, you need
to determine the typical LCD bias voltage (Vo) and the temperature
coefficient of the LCD module.
Based on the temperature coefficient requirements, a thermistor (Rth + R3)
need to be selected to provide the correct offset for the temperature
coefficient generated by the LCD module. Resistor R1 will provide the
adjustment to select the correct initial voltage (or 2 fixed voltage divider
resistors) and transistor Q1 will provide the current buffer to allow the
Thermistor to operate with minimum loading. R2 is just used as a minimum
bias resistor for open circuit conditions. Vee must be a regulated voltage
source.
 |
