AN3114 datasheet(7/43 Pages) STMICROELECTRONICS

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AN3114

LCD solution

Doc ID 16829 Rev 3

7/43

The orientation layer usually consists of a polymer (e.g. polyimide) which has been

unidirectionally rubbed using, for instance, a soft tissue. As a result, the liquid crystal

molecules are fixed with their alignment more or less parallel to the plates, in the direction of

rubbing. The crystal alignment directions at the surface of the two plates are perpendicular

so that the molecules between the two plates undergo a homogeneous twist deformation in

alignment to form a helix.

If no electric field is applied, the birefringent liquid crystal molecules keep their helical

structure and rotate linearly polarized light waves passing through the plates. The

transmitted light wave is then allowed through a crossed exit polarizer. As a result, the

modulator has a bright appearance. On the other hand, if an AC voltage of a few volts is

applied, the resulting electric field forces the liquid crystal molecules to align themselves

along the field direction and the twist deformation (the helix) is unwound. In this case, the

polarization of the incident light is not rotated by the crystal molecules and the crossed exit

polarizer blocks the light wave. As a result, the modulator appears dark.

The inverse switching behavior can be obtained with parallel polarizers. It must also be

noted that gray scale modulation is easily achieved by varying the voltage between the

crystal molecule reorientation threshold (reorientation is resisted by the elastic properties of

liquid crystals) and the saturation field.

LCDs are sensitive to root mean square voltage levels. With a low root mean square voltage

applied to it, an LCD is practically transparent (the LCD segment is then inactive or off). To

turn an LCD segment on, causing the LCD segment to turn dark (from light gray to opaque

black), an LCD RMS voltage greater than the LCD threshold voltage Vth is applied to the

LCD. The LCD RMS voltage is the RMS voltage across the capacitor C in Figure 2, which is

equal to the potential difference between the SEG and COM values.

The LCD threshold voltage Vth 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 an LCD

segment that is on (dark) and an LCD segment that is off (transparent). The optical contrast

depends on the difference between the RMS voltage on an on LCD segment (Von) and the

RMS voltage on an off LCD 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 Vth. If Von is lower or close to the threshold voltage Vth,

the LCD is completely or almost transparent. If Voff is close or higher than the threshold

voltage Vth, the LCD is completely black.

The discrimination ratio (D) specifies the contrast levels that the LCD panel can achieve. It is

defined as follows:

To prevent the electrolytic process (DC voltage applied on LCD or the temperature effect on

the performance of the LCD panel...) from occurring and consequently ensure a longer LCD

lifetime, the applied LCD voltage must also alternate to obtain a zero DC value.

Note:

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

datasheet), otherwise, the LCD lifetime may be shortened. The typical frequency ranges

from 30 to 100 Hz. If a lower frequency is used, the LCD flickers. If a larger frequency is

used, the power consumption increases.

ON RMS

() VOFF RMS

()

⁄

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