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Abstract

A column driver for a matrix addressed display such as a liquid crystal display (LCD) is disclosed. The column driver accepts 10-bit inputs, provides 1024 distinct output levels, and is compatible with digital gamma correction by way of, for example, an EPROM lookup table. A complete column driver includes a group of chips which are serially interconnected and receive all the external signals applied in parallel. Only one chip is made active at any instant of time, in order to save power. In each IC, selective polarity inversion of the 10-bit data is carried out before feeding it to a switched capacitor digital to analog converter (CAPDAC) which operates in either a high or low voltage range. All CAPDACs are loaded simultaneously and present a precise output for approximately 95% of each linetime. Each CAPDAC output is connected to a buffer opamp via two CMOS switches which cause the CAPDAC to simultaneously yet transparently change range and allow the opamp buffer to continue driving. The buffer accepts a high impedance, low parasitic input voltage from the CAPDAC array and outputs a low input signal for driving columns with 150-400 pF capacitance. The buffer has an autocalibration circuit that comprises a nested, mini-operational amplifier (opamp) coupled between the output and input of the buffer. The mini-opamp amplifies the balance error produced by the buffer during a calibration interval to provide an output signal which changes the balance point of the buffer amplifier.

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First Claim