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Frequently Asked Questions

When direct driving a liquid crystal display, each individual segment has an individual lead which comes to the edge of the display to a unique pin. Maximum contrast and viewing angle can be attained by direct driving a display. As the number of display segments increases, direct drive may become impractical due to the number of drive circuits and external interconnections required. Both can be reduced by means of time-multiplexing the display. A full discussion of direct driving an LCD is presented in our App Note Section.

direct_drv

        Direct_drive_eq_circuit

In this example, a single backplane (in red) is shown. This backplane is common to every segment, and the segments are all routed out to a unique pin on the display contact ledge. An individual segment is activated by applying the driving squarewave between the backplane and the individual segment trace. This part will require 14 contacts for the segments and 1 contact for the backplane.

The equivalent electrical circuit, in this case trivial, is shown in the drawing above.

When multiplexing an LCD, appropriate segments are connected together to form groups which are sequentially addressed by means of multiple backplane (sometimes called common plane) electrodes. These groups are organized in a matrix of rows and columns. Typical multiplex drivers generate amplitude varying, time synchronized waveforms, and apply them to the row and column lines of the matrix at a high rate. A full discussion of multiplexing is presented in our App Note Section.

LCD MUX x4

LCD Mux x4 eq_circuit

In this example, a 4:1 multiplex, the four different colors represent the four backplanes. The equivalent circuit above is more meaningful in this case. As can be seen, BP1 (in red) controls the "A" and "B" segments of the two digits. The "A", "E", and "F" segments all are routed to the edge of the part using a single trace. The "A" segment is therefore turned on by applying the squarewave between BP1 and the first pint on the lower left of the bottom row.

By using this multiplexing scheme, the LCD will require only four contacts for the segments, plus an additional four contacts for the backplanes. We have therefore reduced the pin count significantly.
LCD Mux x7 This is an example of a pinout for a dot matrix display. Here, every row must be brought to the left edge, and every column must be brought to the top of the part for connection to the PC board. This is usually accomplished by a heat seal or elastomeric connector. See "What is a Heat Seal? Elastomer?" for an explanation of these connection methods.

The gaps between the pixels can be as small as about .001", which makes the precision of the heat seal connector, and the application of the heat seal to the display, extremely critical. Our page "Products Related to LCD's" contains a listing of companies supplying heat seal connectors. They also have the necessary equipment to attach the seals to the LCD.
 


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