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AN2687 Datasheet(PDF) 5 Page - STMicroelectronics |
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AN2687 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 22 page AN2687 LCD principle Doc ID 14296 Rev 1 5/22 1 LCD principle Figure 1. LCD principle 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 (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 black. Mean Signal 2 |
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