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 twisted nematic 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.

The first thing that needs to be determined in a particular application is the relationship between display RMS "OFF" voltage, and display driver circuit supply voltage. Since different drive schemes (Mux rates)   are utilized, this is best determined from the manufacturer's data sheet. Also required are the temperature coefficient and threshold voltage of the liquid crystal fluid. (Help in selecting the correct LCD fluid for your application is available from our application engineering department at (440)786-8700).

The fluid temperature coefficient must be divided by the ratio of RMS "OFF" voltage to driver circuit supply voltage, to calculate the ultimate temperature coefficient of the driver circuit supply voltage. In this way, the display RMS "OFF" voltage will track its optimum value over the temperature range.

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 R1provides a means by which the display operating voltage can be set.

Note that range limiting resistor R1' and R1" may be added to increase the resolution of R1 and to determine suitable end points. Operational amplifier U2 may be any type capable of supplying sufficient output current to power the display driver circuit (s). An LM307 is a good choice for small systems since its output can swing close to the supply rail, minimizing the magnitude of the positive supply required. R1 must be fed from a regulated supply to maintain a stable output voltage at U2.