AN1048D datasheet(3/21 Pages) STMICROELECTRONICS

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AN1048

LCD requirements

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LCD requirements

With a low Root Mean Square (RMS i.e.:

) voltage applied to it, an LCD is

practically transparent. The LCD segment is inactive(OFF) if the RMS voltage is below the

LCD threshold voltage and is active(ON) if the LCD RMS voltage is above the threshold

voltage. The LCD threshold voltage depends on the quality of the liquid used in the LCD and

the temperature. The optical contrast is defined by the difference in transparency of a LCD

segment ON (dark) and a LCD segment OFF (transparent). The optical contrast depends on

the difference between the RMS voltage on an ON segment (VON) and the RMS voltage on

an OFF segment (VOFF). The higher the difference between VON(rms) and VOFF(rms), the

higher the optical contrast. The optical contrast also depends on the level of VON versus the

LCD threshold voltage. If VON is below or close to the threshold voltage, the LCD is

completely or almost transparent. If VOFF is close or above the threshold voltage, the LCD is

completely dark.

In this document, contrast is defined as D = VON(rms) / VOFF(rms).

The applied LCD voltage must alternate to give a zero DC value in order to ensure a long

LCD life time.

The higher the multiplexing rates, the lower the contrast. The signal period has also to be

short enough to avoid visible flickering on the display.

The LCD voltage for each segment is equal to the difference between the S and COM

voltages (see Figure 1).

Figure 1.

Equivalent Electrical Schematic of an LCD Segment

Note:

The DC Value should never be more than 100mV (refer to the LCD manufacturer’s

datasheet). Otherwise the life time can be shortened. The frequency range is 30 - 200Hz

typically. If it is less, it flickers; if it is more, the power consumption increases.

Mean Signal

()

COM

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