EDN Access -- 03.17.94 Digital potentiometer controls LCD bia

-- EDN, March 17, 1994

Design Ideas:March 17, 1994

Digital potentiometer controls LCD bias

Michael Cortopassi, Dauphin Technology,
Lombard, IL

Designers of pen-based computers can easily relegate some controls that were previously mechanical, such as switches and potentiometers, to on-screen icons. For example, the circuit in Fig 1 shows one way that digital logic can control the -24V-dc LCD bias using two general-purpose I/O command lines. The DS-1669 from Dallas Semiconductor is a 64-step potentiometer available in 10-, 50-, and 100-kV ranges. The up-count (UC) and down-count (DC) pins digitally control the wiper of the potentiometer. A low-going pulse to either of these pins increases or decreases, respectively, the wiper's position on the pot relative to RL. This change in position adjusts the base current in Q₁, whose collector connects to the adjust pin on an LM337 negative voltage regulator. By changing the amount of current injected into the LM337's adjust pin, the circuitry simulates having another resistor in parallel with R₁, and the voltage output at V_OUT changes accordingly. Tapping a pen on icons that represent contrast-or other computer functions, including brightness, LCD/CRT and suspend resume-controls the I/O pins.

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