Power Supply Fundamentals

Abstract

Liquid Crystal Displays require dedicated power supply circuits to support their specific requirements. Many different display technologies coexist in the market and compete for their market share. While the passive matrix Twisted Nematic (TN) or Super Twisted Nematic (STN) LCDs require a fairly simple bias supply circuit, the active matrix Liquid Crystal (LC) Displays require a more sophisticated bias supply. The active matrix usually requires several power rails with power up and down sequencing. In this chapter, the power supply requirements and implementation for the standard passive matrix LC Displays are discussed. The passive matrix LC Display usually requires a single supply rail. The active matrix LC Display uses Thin Film Transistors (TFT) as an active element turning each pixel on and off. To control the TFTs two additional supply rails are required. The active matrix display also requires a common voltage rail to establish an alternate current (AC) signal across the pixel. This is implemented by using a dynamic or static common voltage drive architecture. The LCD power supply not only provides several voltage rails but also generates the voltages for the gamma reference, and compensates voltage shifts due to gate signal pulses and parasitic gate line capacitance and resistance. This chapter discusses the LCD requirements and how they are supported and influenced by the LCD bias supply and control circuits.