LCD's require an AC drive voltage with minimal DC component. Prolonged DC operation may cause electrochemical reactions inside the displays which will cause significantly reduced life. The initial indications of display degradation because of excessive DC current is an electro-plating of liquid crystal components onto the surface electrodes. These will appear as a mirror-like "burn-in" of the electrode pattern that was energized with DC. Elevated temperature will accelerate this effect.
Because an LCD is made up of several dielectric layers, the Equivalent Circuit shown below is a series of capacitors and a shunted resistor.
There are also series resistances to consider, including the resistivity of the indium oxide electrode paths and the crossover resistance. When voltages are specified for LCD operation, they refer to RMS (Root-Mean-Square) voltage, measured across the glass. For a static drive LCD, this is from an energized pin to the common plane.
Drive frequencies for direct drive displays are typically between 60Hz and 150Hz. Depending on the display size and design, displays can be operated at higher frequencies, but this will result in increased power consumption. Operation below 60Hz may result in display flicker. Electroplating may also occur at low frequencies when the speed of response of the liquid crystal is faster than the drive frequency. The net voltage that the liquid crystal would see would then have a DC component with respect to the speed of the fluid.
LCD's can be overdriven by a combination of voltage and frequency, which will result in cross talk or "ghosting" Ghosting is the appearance or partial activation of an "off" segment. At a specified viewing direction, the liquid crystal is formulated to begin turning on at a specific voltage, which is designated as the voltage threshold. By convention, this is the 10% on-state, or the condition where the contrast is 10% of the full-on contrast. Ghosting occurs when the effective RMS voltage across the display exceeds the voltage threshold of the liquid crystal.
On large displays with specific geometric designs, the backplane impedance could become a significant issue, where the voltage and current are out of phase. High drive voltage and/or frequency can contribute to produce voltages across segments that are at or above the voltage threshold. This will cause ghosting. This ghosting will always occur first at the off-axis condition called out as the optimum viewing angle. Since the current is directly proportional to the frequency, there is a voltage-frequency product which must not be exceeded. These values are very dependent on the design and layout of any given part, so proper display design and choice of driving conditions are important. It is also very important that all unused segments be connected to the backplane, and not allowed to float.
An actual drive circuit will be a symmetrical square wave with less than 50mV DC offset. The low DC offset drive circuit is best designed using a CMOS "exclusive OR" gate. Refer to the bottom part of the figure below:
Please note that normal TTL devices are not suitable for drivers, as they typically have enough DC offset in their output signal to destroy an LCD. Today's commercially available display drivers are designed to produce the optimum signal output.
The top part of the above figure shows the output waveforms of our "exclusive OR" circuit. Plot A is the 40 Hz, 50% duty cycle square wave input to the XOR gate, or "clock" signal. Plot B is the control signal waveform which will turn a segment "on". Plot C is the output of the XOR gate. Plot D shows the RMS voltage as seen by the LCD segment.
Note that when the control input (B) is at logic 0, the resultant output signal to the segment is in phase with the clock output, resulting in a 0V RMS voltage at the segment. It will therefore remain in the "off" state. When the control input to the XOR goes high, the output shifts 180o out of phase to the backplane, resulting in an RMS value at the segment equal to the supply voltage. The segment will turn "on".
The drawing below shows a typical segment plane and common plane layout for a seven segment digit. From the drawing, it can be seen that the common plane electrode is made as large as possible to keep the resistance low and thereby eliminate "ghosting".
