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| s s ----------------------------------------------------------------------------------------- INTRODUCTION With the continuing demand for smaller and more efficient systems, designers are seeking new ways to reduce power and increase integration. This paper describes how good use of OKI Semiconductor's monolithic display drivers can reduce power consumption and component count in systems that incorporate liquid crystal displays (LCDs). Information in this paper is applicable both to small LCD panels (with a dozen or less segmented digits) and to large LCD panels (such as used for laptop screens). LCD elements are capacitive in nature and dissipate virtually no power, whether selected or deselected. When an LCD element is selected, a voltage is applied across the liquid crystal that varies the element's reflectivity. Removing the voltage deselects the LCD element and returns the liquid crystal to its original state. In general, an alternating-current (AC) source is required to illuminate the individual elements in an LCD panel. A direct-current (DC) source is not suitable for driving an LCD panel because of electrolytic reactions in the LCD's liquid. Placing a potential difference across the liquid in an LCD panel causes ion migration, which gradually erodes the anode terminal and causes destructive deposition on the cathode terminal. The electrolytic reaction could be diminished by plating the electrodes with a non-reactive conductive element, such as gold, for example; however, the high cost of gold and similar non-reactive elements makes this solution impractical for all but the most esoteric applications. A far simpler way to eliminate electrolytic corrosion is to use an AC source to drive the LCD panel, eliminating unidirectional ion migration and the associated problems. AC configurations for driving LCDs fall in two main categories, which are: * Static driver configurations * Multiplexed driver configurations The next two sections describe these two configurations. The third section in this application note addresses powerrelated issues. This application note concludes with some complete circuit examples. USING STATIC DRIVERS In configurations using static drivers, a separate driver signal (SEG) provides an AC source for each element, and all elements use a single shared common (COM) terminal. LCD elements are often segments of an alphanumeric digit. In larger configurations, LCD elements are individual dots that emulate the appearance of a CRT monitor. Static driver configurations are simple to use and are generally suitable for LCDs with less than 80 segments. OKI Semiconductor supplies a range of single-chip solutions for static LCD driver configurations, some of which also include built-in RC oscillation circuits, in small-outline IC (SOIC) and plastic quad flat pack (PQFP) packages. The table below lists the main characteristics of single-chip static LCD drivers from OKI Semiconductor. LCD Drivers for Static Configurations Part Number MSM5219B MSM5221 MSM5265 Segments 48 56 80 On-Chip Oscillator X Drive Voltage (VLCD) 4.0 - 7.0 3.0 - 7.0 3.0 - 6.0 Package 60-lead PQFP 80-lead PQFP 100-lead PQFP Figure 1 below shows static driver connections to a single-digit, seven-segment alphanumeric display. Separate drivers, SEG1 - SEG8, power each segment. All segments share a common ground connection, COM. 1 OKI SEMICONDUCTOR ---------------------------------------------------------------------------------------- s s SEG7 KEY: SEG6 SEG5 Segment connections over the liquid crystal. Common base connections under the liquid crystal. COM SEG1 SEG4 SEG3 SEG8 SEG2 Figure 1. Segment Connections for Static-Drive Configurations Figure 2 below shows the AC waveforms for driving the illustrated display. When an element is deselected, the combined COM and SEG signals negate each other, as shown for the (COM - SEG1) signal illustrated below. When an element is selected, the combined COM and SEG signals constructively reinforce each other, as shown for the (COM - SEG2) signal below. VDD COM VL VDD SEG1 (Deselected) COM - SEG1 (Deselected) VL 0V VLCD VDD SEG2 (Selected) VL VLCD COM - SEG2 (Selected) 1 Frame 0V -VLCD Figure 2. Functional Waveforms for Static Drive Configurations In static drive configurations, the AC frequency used to drive each segment is identical. This AC frequency is called the frame frequency. In static drive configurations, a single element is selected or deselected in any one individual frame, as shown in Figure 2 above. Static driver configurations generally use a frame frequency in the 20-200 Hz range. Lower frame frequencies can cause visible flicker. Higher frame frequencies do not provide sufficient time for charging the capacitive LCD elements. OKI SEMICONDUCTOR 2 s s ----------------------------------------------------------------------------------------- USING MULTIPLEXED DRIVERS For configurations requiring more than about 80 drivers, it is more efficient to multiplex the COM and SEG signals than to use a static driver configuration. In multiplexed configurations, each SEG driver signal powers more than one segment, and the circuit uses more than one COM signal. The SEG and COM signals actually form a grid, with each segment driven by a unique SEG/COM node. The multiplexed drive method reduces the number of driver circuits and the number of connections between the circuit and the display cell. This reduces cost when driving many display elements. Figure 1 below illustrates this reduction in driver count by comparing static and multiplexed drive configurations for a six-digit display. The static driver configuration requires 49 connections to the LCD, whereas the multiplexed configuration requires only 21 connections. Increasing the degree of multiplexing can further reduce the number of connections; however, increased multiplexing also reduces the circuit's tolerance to voltage variation. 7A Static 6A 1A 2A 7B 6B 1B 2B 7C 6C 1C 2C 7D 6D 1D 2D 7E 6E 1E 2E 7F 6F 1F 2F Common 5A 4A 8A 3A 5B 4B 8B 3B 5C 4C 8C 3C 5D 4D 8D 3D 5E 4E 8E 3E 5F 4F 8F 3F ComA Multiplex 1/3 bias, 1/3 duty cycle ComB ComC S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 Figure 3. Static versus Multiplexed Configurations A range of multiplexed configurations are possible, distinguished by:* Bias, indicating the number of voltage levels used to power the LCD display. * Duty Cycle, indicating the number of segments driven by each individual output driver. * Frame Frequency Type, indicating whether the COM signal alternates over one frame (Type A) or two frames (Type B). For example, in a 1/2 bias, 1/2 duty-cycle configuration, each individual output driver uses two voltage levels to drive two segments. Similarly, in a 1/3 bias, 1/3 duty-cycle configuration, each driver uses three voltage levels to drive three segments (Figure 1 above is a 1/3 bias, 1/3 duty cycle configuration). Frame frequency determines the degree of flickering and vividness. For a high degree of multiplexing, the type-B configuration can make the display more vivid, but can also introduce flickering at lower clock frequencies.Figure 4 3 OKI SEMICONDUCTOR ---------------------------------------------------------------------------------------- s s and Figure 5 below illustrate the difference between type-A and type-B configurations for the LCD driver network shown in Figure 1 above. COM SEG COM - SEG 1 Frame Figure 4. A-Type Waveforms for a 1/3 Duty Cycle, 1/3 Bias Configuration COM SEG COM - SEG 1 Frame Figure 5. B-Type Waveforms for a 1/3 Duty Cycle, 1/3 Bias Configuration Type B configurations are more common, as the maximum required frequency is lower. Figure 6 through Figure 10 illustrate various multiplexed configuration, all of which use a Type B configuration to reduce frame frequency. Figure 6 on the next page illustrates how a 1/2 bias, 1/2 duty cycle, Type B configuration can drive 62 outputs. This particular example is suitable for systems using the MSM6660. Full VLCD voltage is applied across the selected segment for display and less than full VLCD voltage is applied across the deselected segment. OKI SEMICONDUCTOR 4 s s ----------------------------------------------------------------------------------------- SEG1 COMB SEG2 COMA SEG3 SEG4 Figure 6. A 1/2 Bias, 1/2 Duty Cycle, Type B Configuration Figure 7 below shows the waveforms for the 1/2 bias, 1/2 duty cycle configuration shown in Figure 6 above. VDD COM1 VLC1, 2 VLC3 VDD COM2 VLC1, 2 VLC3 VDD SEGn On Off On Off On Off On Off On Off On Off On VLC1, 2 VLC3 VLCD COM2 - SEGn 0 -VLCD Note:When 1/2 duty is selected and 1/2 bias is used, perform the following: When the code is -01, short VLC1 and VLC2 to supply the bias voltage. When the code is -02 or -03, externally short VLC1 and VLC2. Figure 7. Waveforms for 1/2 Bias, 1/2 Duty Cycle, Type B Configuration Figure 8 illustrates a 1/3 bias, 1/3 duty cycle, Type B LCD network, suitable for connection to OKI's MSM6606. 5 OKI SEMICONDUCTOR ---------------------------------------------------------------------------------------- s s SEG1 COM3 COM2 COM1 SEG2 SEG3 Figure 8. 1/3 Duty Cycle, 1/3 Bias, Type B Configuration Figure 9 below depicts waveforms for the 1/3 bias, 1/3 duty cycle, Type B network in Figure 8 above. VDD COM1 VLC1 VLC2 VLC3 VDD COM2 VLC1 VLC2 VLC3 VDD COM3 VLC1 VLC2 VLC3 VDD SEGn On Off On Off On Off On Off On Off On Off On VLC1 VLC2 VLC3 V3 V2 V1 COM2 - SEGn VSS -V1 -V2 -V3 VLCD VLCD Figure 9. 1/3 Duty Cycle, 1/3 Bias, Type B Waveforms OKI SEMICONDUCTOR 6 s s----------------------------------------------------------------------------------------- Figure 10, below, illustrates a 1/5 bias, 1/16 duty cycle, Type B configuration, together with COM and SEG waveforms. This example uses the dot-matrix configuration for display. 1 2 3 4 16 1 01 02 03 04 05 06 07 08 09 010 011 012 013 014 015 016 01 02 03 04 05 COM 01 VDD Va Vb Vc Vd Ve VDD Va Vb Vc Vd Ve VDD Va Vb Vc Vd Ve VDD Va Vb Vc Vd Ve VLCD COM 02 SEG 01 Va = VDD - 1/5 VLCD Vb = VDD - 2/5 VLCD Vc = VDD - 3/5 VLCD Vd = VDD - 4/5 VLCD Ve = VDD - 5/5 VLCD SEG 02 COM 01 - SEG 01 (Select Waveform) VLCD 4/ V 5 LCD 3/ V 5 LCD 2/ V 5 LCD 1/ V 5 LCD 0 -1/5 VLCD -2/5 VLCD -3/5 VLCD -4/5 VLCD -VLCD VLCD 4/ V 5 LCD 3/ V 5 LCD 2/ V 5 LCD 1/ V 5 LCD 0 -1/5 VLCD -2/5 VLCD -3/5 VLCD -4/5 VLCD -VLCD COM 02 - SEG 02 (Non-Select Waveform) 1 Frame Figure 10. A 1/5 bias, 1/16 duty cycle, Type B Configuration and Waveforms 7 OKI SEMICONDUCTOR ---------------------------------------------------------------------------------------- s s Larger configurations are simply extensions of the above principles. OKI Semiconductor supplies a range of singlechip and multi-chip solutions for multiplexed LCD driver configurations supporting any required number of dots or segments. The table below lists single-chip LCD drivers, indicating major differences for each part. Single-Chip LCD Drivers for Multiplexed Configurations Part Number MSM6544 MSM6606 MSM6660-01 MSM6660-02 MSM6660-03 MSM5265 MSM5260 MSC5301B-01 MSC5301B-02 Dots 42 40 62 62 62 80 80 16 COM 64 SEG 8 COM 64 SEG Duty Cycle Ratio 1/2 1/2 1/2 1/3 1/3 1/2 1/32 - 1/64 1/16 1/8 On-Chip Oscillator X X X[1] X [1] X X X Drive Voltage (VLCD) 3.0 - 6.0 4.5 - 5.5 4.0 - 6.0 4.0 - 6.0 4.0 - 6.0 3.0 - 6.0 8.0 - 18.0 4.0 - 16.0 4.0 - 16.0 Package 56-lead PQFP 64-lead PQFP 80-lead PQFP 80-lead PQFP 80-lead PQFP 100-lead PQFP 100-lead PQFP 100-lead PQFP 100-lead PQFP [1] On-chip bias resistors. For larger dot configurations, such as found in laptop displays for example, common and segment drivers are located on separate chips. The table below shows critical features for ICs that provide common drivers only. Note the use of thin quad flat packs (TQFPs) and Tape Automated Bonding (TAB) for devices with higher lead counts. Common Drivers for Multiplexed Configurations Part Number MSM5238 MSM5298A MSM6368 Drivers 32 68 80 Duty Cycle Ratio 1/32 - 1/64 1/64 - 1/256 1/256 - 1/480 Max. Dots 2,048 17,408 38,400 Drive Voltage (VLCD) 3.0 - 16.0 8.0 - 28.0 25.0 - 40.0 Package 44-lead PQFP 80-lead TQFP 100-lead TQFP The next table, opposite, shows the same critical features for ICs that provide segment drivers only. Segment Drivers for Multiplexed Configurations Part Number MSM5259 MSM5839B MSM5839C MSM5299A Drivers 40 40 40 80 Duty Cycle Ratio 1/8 - 1/16 1/32 - 1/128 1/3 - 1/64 1/64 - 1/256 Max. Dots 640 5,120 2,560 20,480 Inputs 1 1 1 4 Drive Voltage (VLCD) 3.0 - 6.0 8.0 - 18.0 4.0 - 11.0 8.0 - 28.0 Package 56-lead PQFP 56-lead PQFP 56-lead PQFP 100-lead PQFP OKI SEMICONDUCTOR 8 s s----------------------------------------------------------------------------------------- Segment Drivers for Multiplexed Configurations (Continued) Part Number MSM5299A-01 MSM5299C MSM6669 Drivers 80 80 80 Duty Cycle Ratio 1/64 - 1/256 1/64 - 1/256 1/100 - 1/256 Max. Dots 20,480 20,480 20,480 Inputs 4 4 4 Drive Voltage (VLCD) 8.0 - 28.0 8.0 - 28.0 14.0 - 28.0 Package 100-lead TQFP 100-lead PQFP 0.25mm TAB POWER SUPPLY The table below shows the relationship between the required number of driving biases and the display duty ratios. Relationship between Duty Cycle and Driving Bias Duty Ratio Static 1/2 1/3 1/4 1/7 1/8 1/11 1/12 1/14 1/16 1/24 1/32 1/64 Drive Bias - 1/2 1/3 1/3 1/4 1/4 1/4 1/4 1/5 1/5 1/5 1/5 1/5 Voltage Levels 2 3 4 4 5 5 5 5 6 6 6 6 6 Either passive or active bias generation circuitry can provide the required voltage levels, as described in the next two subsections. Passive Bias Generation A resistor ladder is the most common way to generate the required bias voltage levels. Figure 11 below shows a typical resistor ladder. VREF may be tied to VSS, or, for displays with larger voltage bias ranges, VREF may be tied to a negative power source. 9 OKI SEMICONDUCTOR ---------------------------------------------------------------------------------------- s s VDD VLC1 VLC2 VLC3 VDD VLC1 VLC2 VLC3 VREF RLCD RLCD RLCD RLCD RLCD For 1/2 bias (when 1/2 duty is selected) For 1/3 bias (when 1/3 duty is selected) VREF Figure 11. Network for Simple Passive Bias Generation The required operating margin and power consumption determines the appropriate resistor values. Because the LCD load is capacitive, the current during element charging and discharging distorts the waveform. Generally, the value of resistor R is 1 k to 10 k. Lower resistor values reduce distortion but increase power dissipation. Larger LCD panels exhibit greater capacitance, and so resistor values may be decreased proportionally as the display size increases. No capacitor is required, but a 1-F capacitor can be used if necessary. Connecting a capacitor in parallel to the resistors can reduce waveform distortion during the charge and discharge periods, but only to a limited degree. Larger capacitor values generate a voltage level shift and reduce the operating margin. Figure 12 below illustrates how to connect capacitors in a passive bias network. VDD VLC1 VLC2 VLC3 VDD VLC1 VLC2 VLC3 C RLCD C C VREF RLCD C RLCD C RLCD C VREF RLCD For 1/2 bias (when 1/2 duty is selected) For 1/3 bias (when 1/3 duty is selected) Figure 12. Network for Passive Bias Generation with Capacitor Filters In large arrays, the LCD source and common signals form a matrix configuration that complicate the path of the charge/discharge current through the load. Moreover, current varies according to the demand for the power consumption of the equipment in which the LCD is incorporated. As a result, it is not possible to generate precise equations for calculating resistor and capacitor values. Active Bias Generation In larger displays, such as graphic displays, the liquid crystal is larger and the duty cycle ratio is smaller. Stability of the liquid crystal's drive level is therefore more important for a large display than for a small display. Because graphic displays are large and contain many picture elements, the LCD driver's impedance produces distortion in the drive waveforms and degrades display quality. For this reason, the impedance of the LCD driver bias sources should be reduced with operational amplifiers. Figure 13 below shows examples of op amp configurations to provide this reduced impedance. OKI SEMICONDUCTOR 10 s s----------------------------------------------------------------------------------------- VDD VLC1 VLC2 VLC3 VDD VLC1 VLC2 VLC3 RLCD RLCD VREF RLCD RLCD RLCD VREF For 1/2 bias (when 1/2 duty is selected) For 1/3 bias (when 1/3 duty is selected) Figure 13. Network for Active Bias Generation No load current flows through the dividing resistors because of the high input impedance of the operational amplifiers. A resistor value for R of 10 k is suitable for the above circuit. APPLICATION CIRCUITS This application note concludes with two illustrative application circuits. The first, shown in Figure 14 below, uses two MSM6660 LCD drivers to provide three bias signals and 124 segment drivers for a 1/3 bias, 1/3 duty cycle, Type B LCD panel containing 372 dots. LCD Panel BL CK DATA CE SEG0 ~ SEG61 COM1 COM2 COM3 COMOUT SYNC BL CK DATA CE SEG0 ~ SEG61 COM1 COM2 COM3 COMOUT SYNC SEL VLC1 VLC2 VLC3 OSC R0 RLCD RLCD RLCD C0 SEL VLC1 VLC2 VLC3 OSC BL CK DATA CE1 CE2 Figure 14. Application Circuit Example Using MSM6660 LCD Drivers In the example on the previous page, a microcontroller may be necessary to provide an appropriate interface to the overall system. Figure 15 below depicts a more complex example that may not require an additional microcontroller. 11 OKI SEMICONDUCTOR ---------------------------------------------------------------------------------------- s s Instead of using a separate LCD driver and microcontroller, this circuit integrates LCD driver and controller functions in the MSM6222GS LCD Controller, available from OKI Semiconductor. LCD Panel COM1 ~ 16 SEG1 ~ 40 DO MSM6222GS 01 ~ 040 DI1 DO40 CP DO20 LOAD DI21 DF MSM5259GS VDD VSS V2 V3 V5 01 ~ 040 DI1 DO40 CP DO20 LOAD DI21 DF MSM5259GS VDD VSS V2 V3 V5 01 ~ 040 DI1 DO40 CP DO20 LOAD DI21 DF MSM5259GS VDD VSS V2 V3 V5 CP L DF VDD GND V1 V2 V3 V4 V5 R C 0V C R C R C R C +5V R Figure 15. Typical Application Circuit Using the MSM6222GS LCD Controller LCD controllers can increase system integration and provide a more comprehensive feature set. For small and midsize displays, a single LCD controller may provide all the required COM and SEG signals. For larger configurations, combinations of LCD controllers and drivers can provide exactly the required degree of functionality. In the above example, the LCD controller provides all required COM signals and some of the SEG signals. Additional SEG drivers augment the LCD controller with the additional SEG drivers required. OKI Semiconductor provides a family of advanced LCD controllers and drivers that augment the basic functionality of standard LCD drivers and controllers with enhanced capabilities. Additional features found in OKI's LCD controllers include key-scan logic, power saving modes, on-chip RAM, integrated oscillators, and optional on-chip bias generation. The table on the following page lists some of the members of OKI's advanced LCD controller/driver family. The following table lists the main features of devices in OKI's advanced LCD controller/driver family described in this document. OKI SEMICONDUCTOR 12 s s----------------------------------------------------------------------------------------- OKI's Advanced LCD Controller/Driver Family Part No. Controller Drivers Duty Cycle VLCD Interface Pins / Package Special Features MSM5298A -- 68 COM 1/64 - 1/256 8.0- 28.0 Serial shift 80/ PQFP register I/O 4-bit shift register I/O 100/ PQFP Bidirectional 68-bit shift register Multi-chip configuration support On-chip or external bias generation Bidirectional 4x20-bit shift register 80-bit latch Multi-chip configuration support On-chip or external bias generation 1 kbit on-chip RAM Multi-chip configuration support Blanking support On-chip RC oscillator 1 kbit on-chip RAM Multi-chip configuration support Blanking support On-chip RC oscillator Bidirectional 160-bit shift register Multi-chip configuration support On-chip or external bias generation Unidirectional 160-bit shift register 160-bit latch Power-saving mode Multi-chip configuration support On-chip or external bias generation Internal 5x6 key-scan circuit (supporting up to 30 key switches) Single LED driver output Integrated bias voltage generation On-chip RC oscillator 256 5x7 characters in on-chip ROM 80 dot arbitrators Character and arbitrator blink functions On-chip RC oscillator 256 5x7 characters in on-chip ROM 80 dot arbitrators Character and arbitrator blink functions Integrated bias voltage generation MSM5299A -- 80 SEG 1/64 - 1/256 8.0- 28.0 MSC5301B01 -- 64 SEG 16 COM 1/16 (1/5 bias) 4.0 - 16.0 Serial MCU interface Serial MCU interface 100/ PQFP MSC5301B02 -- 64 SEG 8 COM 1/8 (1/4 bias) 4.0 - 16.0 100/ PQFP MSM6568A -- 160 COM 1/200 - 1/480 14.0 - 28.0 2-bit shift Slim TAB register I/O 160 SEG 1/200 - 1/480 20.0 - 40.0 8-bit shift Slim TAB register I/O MSM6569 -- MSM6606 -- 40 SEG 2 COM 1/2 5.5 Serial MCU interface 64/ PQFP MSM666501 80 SEG 17 COM 1/9 or 1/17 3.0 - 6.0 Serial MCU interface Serial MCU interface 128/ PQFP MSM6665B 80 SEG 17 COM 1/9 or 1/17 -- PBGA For more information, see the LCD Driver Controller Data Book and Advanced LCD Controller/Driver Products from OKI Semiconductor, or contact your local OKI Semiconductor sales representative for additional assistance. 13 OKI SEMICONDUCTOR |
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