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AN2687 Datasheet(PDF) 5 Page - STMicroelectronics
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AN2687 Datasheet(HTML) 5 Page - STMicroelectronics
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An LCD panel is composed of many layers. A liquid crystal is filled between two of them
(glass plates), which are separated by thin spacers coated with transparent electrodes that
contain orientation layers. 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 (Vrms=
) 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 segment to turn dark (from
light gray to opaque black), an LCD RMS voltage greater than the LCD threshold voltage 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 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 segment (V
) and the RMS
voltage on an off segment (V
). The higher the difference between V
(rms), the higher the optical contrast. The optical contrast also depends on the level of
versus the LCD threshold voltage. If V
is below or close to the threshold voltage, the
LCD is completely or almost transparent. If V
is close or above the threshold voltage, the
LCD is completely black.
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