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ADD8706 Datenblatt(PDF) 9 Page - Analog Devices |
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ADD8706 Datenblatt(HTML) 9 Page - Analog Devices |
9 / 12 page ADD8706 Rev. 0 | Page 9 of 12 APPLICATION INFORMATION THEORY The ADD8706 is designed for use in LCD gamma correction circuits. This is an ideal on-chip solution for low-end panels. It provides five gamma voltages and a VCOM output. These gamma voltages provide the reference voltages for the column driver RDACs. Due to the capacitive nature of LCD panels, it is necessary for these drivers to provide high capacitive load drive. The VCOM output is the center voltage common to all the LCD pixels. The VCOM circuit is common to all the pixels in the panel. This requires the VCOM driver to supply continuous currents up to 35 mA. INPUT/OUTPUT CHARACTERISTICS The ADD8706 has five buffers specifically designed for the needs of an LCD panel. Figure 20 shows a typical gamma correction curve for a normally white twisted nematic LCD panel. The symmetric curve comes from the need to reverse the polarity on the LC pixels to avoid “burning” in the image. Therefore, the application requires gamma voltages that come close to both supply rails. To accommodate this transfer function, the five ADD8706 buffers have been designed with three different buffer designs in one package. GRAY SCALE BITS 016 32 48 64 VDD VG2 VG1 VG3 VG4 VG5 VG6 VG7 VG8 VG9 VG10 VSS Figure 20. LCD Gamma Correction Curve The nature of LCD panels introduces a large amount of parasitic capacitance from the column drivers as well as the capacitance associated with the liquid crystals via the common plane. This makes capacitive drive capability an important factor when designing the gamma correction circuit. The outputs of the buffers and amplifier have been designed to match the performance needs of the gamma correction and VCOM circuits. All have rail-to-rail outputs, but the current drive capabilities differ. The difference in current drive and input voltage range determine the buffer and amplifier use. Buffer A has an NPN emitter-follower input stage, which provides an input range that includes the top rail, but is limited to 1.7 V away from the bottom rail. It is designed to source 15 mA of continuous current, making this buffer ideal for providing the top voltage on the RDAC string. Buffers B, C, and D use a single-supply PNP input stage with an intermediate common-mode voltage range. The output was designed to sink or source up to 15 mA of continuous current. The limited input range and equivalent sink and source current make these buffers suitable for the middle voltage ranges on the RDAC string. Buffer E also uses a single-supply PNP input stage, but the output is designed to sink only up to 15 mA of continuous current. This buffer is designed for the RDAC’s lower range. Amplifier F is designed with an input range limited to midscale applications. It is capable of delivering 35 mA of continuous current. These qualities make Amplifier F suitable for VCOM applications. IMPORTANT NOTE Because of the asymmetric nature of Buffers A and E, care must be taken to connect an input that forces the amplifiers to operate in their most productive output states. Buffer A has very limited sink capabilities, while Buffer E does not source well. Set the Buffer A input to enable the amplifier output to source current and set the Buffer E input to force a sinking output current. This means making sure the input is above the midpoint of the common-mode input range for Buffer A and below the midpoint for Buffer E. Mathematically speaking, make sure VIN > VS/2 for Buffer A and VIN < VS/2 for Buffer E. Figure 21 shows an application using the ADD8706 to generate 10 gamma outputs. Note that the five outputs are routed through another resistor network to generate the extra five output voltages, which feed into the column driver. |
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