LCD Drivers
 






There are two typesof drivers for LCD's: direct or multiplex. DIRECT DRIVE, which is also referred to as STATIC indicates that each segment of a display has an independent connection to the driver. Such displays have the highest contrast ratio over the widest operating temperatures. Static driven displays require drive frequencies between 30Hz and 60Hz. Frequencies below 30Hz will cause the display to flicker, whilst 60Hz will result in excessive current draw. Moreover, if a static-driven display is overdriven, OFF segments can become energized resulting in CROSS TALK.
 
A MULTIPLEX DRIVE (also called MUX or Duty) can simplify the connection requirements of a display with a
large number of segments, e.g. 16 x 2 dot matrix displays. Multiplexed displays always have more than one
back plane. Multiplexing makes it possible to sequentially address a segment on each of the commons. The
number of segments addressed by one common is the multiplex ratio. A 1/4 Mux indicates that the LCD has 4
commons. Multiplexed segments are grouped into a matrix. Mux drives produce amplitude-varying,
time-synchronized waveforms for both rows and columns in order to address a particular segment.


 
         


Since multiplexed LCD's have voltage applied to the off segments, it is important to minimize voltages that
could produce cross talk. Voltage bias levels are used to divide the voltage and reduce cross talk. A driver with
a ¼ bias is divided into four levels, Vdd, ¾ Vdd, 2/4 Vdd and ¼ Vdd. Contrast is relative to the RMS voltage on
the back plane minus the segment waveform at any given matrix location. Waveforms and their resultant RMS
voltage can either be above saturation Von or below the threshold voltage Voff.

Contrast is relative to the RMS voltage on the back plane minus the segment waveform at any given matrix
location. Waveforms and their resultant RMS voltage can either be above saturation Von or below the
threshold voltage Voff. In any case, the voltages can be adjusted to improve the contrast of the display. By
varying the external resistor dividers, it is possible to fine-tune the voltages. However, it is more common to
employ a 10k S - 20k S potentiometer to adjust the drive voltage. As the number of commons/back-planes
increases, the Von/Voff ratio approaches unity (1:1) because of the correlation of properties of the driver
waveforms. As the multiplex ratio is increased, the following conditions will result:

  1. Reduced viewing cone
  2. Reduced contrast
  3. Reduced operating temperature range

The minimum number of contacts on a LCD is determined by the formula C=(S/N)+ N, in which S equals the number of segments;
N equals the multiplex rate; and C equals the number of contacts.



ENERGY CONSUMPTION

LCD's require little energy to operate. Typically, 5:A to 25:A at 5 volts per square inch for a Twisted Nematic
Display. However, there is a temperature coefficient, which is important for multiplexed displays. Moreover,
adding a backlight or heater will necessitate additional energy.
LCD's require a pure AC drive voltage. Exposure to a DC component will reduce the life of the display
significantly and must be limited to 50mVdc. When that happens, ELECTROPHORESIS occurs: Traveling
through the LC layer, conductive particles migrate from one glass to the other, and attach themselves to the
ITO pattern. A conductive spike is created thus causing a dead short.


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