TB1098 Low-Power Techniques for LCD Applications Author: Brian Claveria Microchip Technology Inc. INTRODUCTION Low power is often a requirement in LCD applications. The low-power features of PIC® microcontrollers and the ability to drive an LCD directly can help in meeting this requirement. While the LCD Driver module makes driving LCDs very easy, there are important factors to take into account in configuring the module so that application can be optimized for low power. Two specific factors that affect power consumption with respect to the LCD Driver module are the resistor ladder, which generates the bias voltages for the LCD waveforms, and the clock source configuration. By intelligently selecting a resistor ladder size and clock source best suited for the application, an LCD application using PIC microcontrollers can be optimized for low power. Resistor Ladder Sizing Voltage is provided to the LCD Driver module in one of three configurations (Static, 1/2, and 1/3 biasing). In 1/2 and 1/3 biasing, a resistor ladder is used to provide the bias levels for the LCD Driver. In Static mode, VDD must be provided and no resistor ladder is needed. The goal is to maximize the resistor ladder values, as the resistor ladder will draw current at all times. Resistor value is constrained by two factors: the refresh rate of the LCD and the size of the LCD. The LCD module is essentially an analog multiplexer that connects the LCD bias voltages to various segment and common pins that connect to LCD pixels. Each pixel/segment of the LCD display can be modeled as a capacitor. Including both the internal common resistance and switch circuit multiplexing resistance, the circuit can be simplified to a Thévenin equivalent circuit shown below. VTH is equal to either 2/3 VDD or 1/3 VDD for cases where the Thévenin resistance in non-zero. FIGURE 2: SIMPLIFIED LCD CIRCUIT RTH = (2R*R)/(2R+R) RTOTAL = RTH + RSW + RCOM RTH = 2R2/3R RTH = 2R/3 RSW = 4.7K VTH RCOM = 0.4K Note: RSW and RCOM are estimates. Using the simplified circuit, the step response of the voltage across each pixel can be described by the following equation: EQUATION 1: V PIXEL = V TH ( 1 – e VPIXEL/VTH = 1 - e-t/RC ln (.02) = -t/RC ® t = ~ 4 RC LCD PIC Microcontroller VLCD3 VLCD2 VLCD1 VSS VPIXEL VTH 0.98 VTH SEGn R R ) EQUATION 2: LCD CIRCUIT R – t ⁄ ( R T OT A L C P I X EL ) Manipulation of this equation (see Equation 2) dictates that it will take approximately 4 time constants (RC) for the pixel voltage to reach 98% of VTH. Ideally, the resistor ladder should be sized such that pixel voltage reaches at least 98% of VTH. 2% = e-t/RC VDD CPIXEL 98% = 1 -e-t/RC Shown below is an LCD circuit. FIGURE 1: + - CPIXEL (n x m) 0 t = 4 RC t COMm VLCD0 © 2007 Microchip Technology Inc. DS91098A-page 1 TB1098 This analysis says that the resistor ladder can be made larger if: 1. 2. More time is allowed for the pixel to charge (slower refresh rate), or The capacitance of each pixel is smaller (smaller size display). Refresh rate should be as slow as possible in order to maximize resistor ladder size. These settings are configured in <LP3:LP0> bits of the LCDPS register. Keep in mind however, that human visual perception can detect a frequency of 30 Hz or less. The capacitance of each LCD pixel depends on its size. A larger display will typically have more capacitance than a smaller display. Therefore, a larger display will require smaller resistors. While it is possible to calculate the maximum resistor value analytically (see LCD Tips n’ Tricks (DS41261) at www.microchip.com/lcd), these calculations should be used as a guide rather than a solution. Ultimately, it is a reiterative bench testing exercise to determine whether or not resistor size is optimal. If the resistor value is too large, some pixels will be darker than others and the contrast will not be constant. Shown below are two screenshots of LCD segment waveforms, with a 10K resistor ladder (Figure 3) and 220K resistor ladder (Figure 4). FIGURE 3: 10K RESISTOR LADDER WAVEFORM In the next figure, Figure 4, the resistor value selected for the resistor ladder was 220 kΩ. The output waveforms show a much more obvious RC charge and discharge that does not meet the 4 time constant requirement. Nonetheless, the LCD display looked exactly the same as the LCD display with a 10 kΩ resistor ladder. The lessons from this are that there is not an analytic solution to calculating the ideal resistor values for the voltage divider. Bench testing by stepping through various resistor values and visual inspecting the LCD output is a better method. In general, the larger the LCD display, the smaller the resistors must be. In addition, the higher the refresh rate the smaller the resistors. Consequently, a lower refresh rate will allow for larger resistor values. These are factors to be taken to account when selecting an optimum resistor size. LCD CLOCK SOURCE SELECTION LCD clock source selection depends upon application. Because the LCD glass can be modeled as a capacitive load, a higher frequency drive waveform will require more current. Therefore, a lower frequency clock source will consume less current. Selecting the proper clock source can affect power consumption in an application. This is because specific low-power LCD modes can only be utilized by certain clock sources. Therefore, each clock source has a number of advantages and disadvantages. Below, are some pros and cons of each available clock source: 1. FOSC Pros: - No External Circuitry - Dual Purpose (also used for instruction execution) FIGURE 4: Cons: 220K RESISTOR LADDER WAVEFORM 2. - Cannot drive LCD in Sleep Mode - Typically draws more current than the LFINTOSC LFINTOSC Pros: - The segment output waveforms from the PIC microcontroller in Figure 3 are sized based on the PIC microcontroller 10 kΩ data sheet design guidance. The LCD had good contrast. DS91098A-page 2 No External Circuitry Low frequency (31 kHz) Can drive the LCD is Sleep Mode Dual Purpose (also used for instruction execution) Cons: - May consume more current than an external crystal © 2007 Microchip Technology Inc. TB1098 3. External Crystal Pros: - Can drive the LCD in Sleep Mode - Dual Purpose (can be used for accurate time keeping) Cons: - Requires external circuitry CONCLUSION Two techniques that can significantly affect power consumption are resistor ladder sizing and clock source selection. A deterministic guide for selecting resistor ladder size has been demonstrated as well as a practical approach to finding an optimal resistor for an LCD application. Finally, the pros and cons of specific clock source selections have been shown. By following these techniques and taking these factors into account, low power can be achieved for specific LCD applications. REFERENCES LCD PIC® MCU Tips ‘n Tricks, DS41261 AN658, “LCD Fundamentals Microcontroller”, DS00658 using PIC16C92X TB084, “Contrast Control Circuits for the PIC16F91X”, DS91084 DS41250, “PIC16F946/917/916/914/913 Data Sheet”, AN1070, “Driving Liquid Crystal Displays with the PIC16F913/914/916/917/946”, DS01070 © 2007 Microchip Technology Inc. DS91098A-page 3 TB1098 NOTES: DS91098A-page 4 © 2007 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. 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Analog-for-the-Digital Age, Application Maestro, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, PICkit, PICDEM, PICDEM.net, PICLAB, PICtail, PowerCal, PowerInfo, PowerMate, PowerTool, REAL ICE, rfLAB, rfPICDEM, Select Mode, Smart Serial, SmartTel, Total Endurance, UNI/O, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2007, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. 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