Energy efficient grey scale driver for electroluminescent displays

Abstract

A circuit and method are provided of driving a display panel requiring gray scale control wherein the voltage applied to a row of pixels is equal to the sum of voltages of opposite sign with respect to ground applied respectively to the row electrode and column electrodes whose intersection with the row defines the pixels. The pixels have a capacitance that may be voltage dependent such that energy is stored in the pixels when a voltage is applied across them. The driving circuit incorporates a resonant circuit that is able to efficiently recover capacitive energy stored on the row of pixels and transfer it to another row of pixels as the rows are addressed by the sequential application of a voltage on each row. The resonant circuit comprises a step down transformer, a capacitor across the primary winding, either the rows or columns of the display panel connected across the secondary winding and an input voltage and FET switches to drive the resonant circuit synchronous with the timing pulses governing the addressing of the display. The value of the capacitor connected across the transformer primary winding is chosen commensurate with the turns ratio on the transformer and the anticipated range of panel capacitance values to effectively limit variations in the resonance frequency with respect to the frequency of the timing pulses. Limiting the resonance frequency variation in this way maintains high energy recovery efficiency irrespective of random variations in panel capacitance occurring as a result of changes in the displayed image. The improvement of the present invention is an additional secondary winding on the transformer that is connected to a rectifier and DC storage capacitor that is connected in series with the rows or columns of the panel. The additional circuit facilitates clamping of the driver voltage to a constant level irrespective of variations in the load due to the fluctuations in load impedance.

Dependencies are inferred from the claim text.