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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.
     The LCD bias voltage that effects the LCD contrast, also effects the bias angle of the display (see LCD Viewing Angles ) . 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.
    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 (-TC option, if available) when you buy Pacific Display Devices LCD modules. 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:
    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.
       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.

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