To conserve pins, many LCD Drivers triplex their drive signals—a technique that enables AC waveforms on trhree common lines and three segment lines to activate any standard character of a seven-segment display. Large LCDs of 1" or more exhibit a large capacitance between the common and segment electrodes (several nonofarads), which presents a problem for standard LCD drivers.
These drivers' high output impedance (50KΩ, for example) causes difficulty in driving capacitance, and the consequent AC-Waveform distortioncan produce ghosting and shadow segments in the display. The drive circuit in Figure 1
solves this problem by introducing a buffer amplifier for each of the three common lines. Each amplifier may be programmed independently for a quiescent current of 10, 100 or 1000µA. In this application, the bias network applies a voltage that sets the three quiescent currents to 100µA.
The display driver and triple op amp operate between 5V and ground, and the COM signals range from 5V to ~ 1V. To assure that these signals remain within the amplifiers' common-mode range. We attenuate the signalsby one half and operate the buffers at a gain of two. The circuit drives eight 1" displays and is suitable for ambient temperature variations of 15°F or less. At the highest expected temperature, you should adjust R1
so that no "off" segments are visible.
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