Low-Power Techniques for LCD Applications

TB1098 Low-Power Techniques for LCD Applications Each pixel/segment of the LCD display can be Author: Brian Claveria modeled as a capacitor. Including both the internal Microchip Technology Inc. common resistance and switch circuit multiplexing resistance, the circuit can be simplified to a Thévenin TH is equal to eitherequivalent circuit shown below. V INTRODUCTION 2/3 VDD or 1/3 VDD for cases where the Thévenin resistance in non-zero.Low power is often a requirement in LCD applications. ® microcontrollers andThe low-power features of PIC the ability to drive an LCD directly can help in meeting FIGURE 2: SIMPLIFIED LCD CIRCUIT this requirement. While the LCD Driver module makes TH = (2R*R)/(2R+R)R RTOTAL = RTH + RSW + RCOMdriving LCDs very easy, there are important factors to 2 RTH = 2R /3R take into account in configuring the module so that TH = 2R/3R application can be optimized for low power. RSW = 4.7K + VTHTwo specific factors that affect power consumption with - RCOM = 0.4K CPIXELrespect to the LCD Driver module are the resistor lad- der, which generates the bias voltages for the LCD waveforms, and the clock source configuration. By intelligently selecting a resistor ladder size and clock Note: RSW and RCOM are estimates. source best suited for the application, an LCD applica- tion using PIC microcontrollers can be optimized for Using the simplified circuit, the step response of the low power. voltage across each pixel can be described by the following equation: Resistor Ladder Sizing Voltage is provided to the LCD Driver module in one of EQUATION 1: three configurations (Static, 1/2, and 1/3 biasing). In -tR? ()TOTALCPIXEL VPIXEL = VTH ()1 - e 1/2 and 1/3 biasing, a resistor ladder is used to provide DDthe bias levels for the LCD Driver. In Static mode, V must be provided and no resistor ladder is needed. The Manipulation of this equation (see Equation 2) dictates goal is to maximize the resistor ladder values, as the that it will take approximately 4 time constants (RC) for resistor ladder will draw current at all times. . Ideally, the resis-the pixel voltage to reach 98% of V TH tor ladder should be sized such that pixel voltage Resistor value is constrained by two factors: the refresh .reaches at least 98% of V TH rate of the LCD and the size of the LCD. The LCD mod- ule is essentially an analog multiplexer that connects EQUATION 2: the LCD bias voltages to various segment and common -t/RC pins that connect to LCD pixels. VPIXEL/VTH = 1 - e VPIXEL -t/RC 98% = 1 -eShown below is an LCD circuit. VTH -t/RC 2% = e FIGURE 1: LCD CIRCUIT ln (.02) = -t/RC 0.98 VTH VDD t = ~ 4 RC ® MicrocontrollerLCD PIC VLCD3