ViewSonic VA550 Service Manual - Page 15

ViewSonic Corporation THEORY OF CIRCUIT OPERATION Service Manual VA550 The VA550 is a multi-frequency and multi-mode color TFT LCD display. It supports true XGA resolution of 1024x768 including SVGA, VGA and other various high resolution modes up to 1024x768 for IBM, PC compatibles, Power PC and Macintosh. The TFT LCD panel, with a 0.297mm pixel pitch, provides sharp, flicker-free images. As the previous block diagram illustrates, the VA550 uses the Analog Interface Controller (IC20) ASIC for mode detection and resolution amplifying from its preset values. The purpose of the two sets of PLL's is to provide the clock from multiple horizontal synchronous frequencies. PLLO provides ND converter's sample clock with ICS1523 (IC09) while PLL1 produces the LCD panel's dot clock with TLC2933 (IC03). Each TFT LCD display uses the 24LC21 (IC14) chip to provide DCCITM and DCC2BTM with Plug & Play. Also, included in each display is a mode detection feature to evaluate the H-Sync frequency input level to determine the correct power saving mode. Power saving will shut down certain components, including excess electrical power, to reduce power consumption. Upon receiving video signal input, the Analog Interface Controller (Digital Process and Control System) will trigger the mode detection function such that the internal controller can use the ROM's preset information to drive the Analog Interface Controller. In addition, the preset values determine; ND converter clock, LCD dot clock, line buffer input/output rate, V-Sync and H-Sync pulse width, back porch and front porch, to provide optimal performance for the TFT LCD display. The digital process and control system allows users to control OSD menu values to change display settings that include; audio volume, contrast, brightness, color, HN position, and H size. The following sections are a breakdown of the TFT LCD display control board's major functions. CONFIDENTIAL - DO NOT COPY Page 13