A Fourth Generation of LCD Backlight Technology - analog.com

1 Application Note 65AN65-1an65faNovember 1995A Fourth Generation of LCD Backlight TechnologyComponent and Measurement Improvements Refine PerformanceJim WilliamsL, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by Patents including Generation portable computers and instruments utilize backlit LCDs (liquid crystal displays). These displays have also appeared in applications ranging from medical equipment to automobiles, gas pumps and retail terminals.

2 Cold cathode fluorescent lamps (CCFLs) provide the high-est available efficiency for backlighting the display. These lamps require high voltage AC to operate, mandating an efficient high voltage DC/AC converter. In addition to good efficiency, the converter should deliver the lamp drive in sine wave form. This is desirable to minimize RF emissions. Such emissions can cause interference with other devices, as well as degrading overall operating efficiency. The sine wave excitation also provides optimal current-to-light conversion in the lamp. The circuit should permit lamp control from zero to full brightness with no hysteresis or pop-on, and must also regulate lamp intensity vs power supply small size and battery-powered operation associated with LCD equipped apparatus mandate low component count and high efficiency for these circuits.

3 Size constraints place severe limitations on circuit architecture and long battery life is usually a priority. Laptop and handheld por-table computers offer an excellent example. The CCFL and its power supply are responsible for almost 50% of the battery drain. Additionally, these components, including PC board and all hardware, usually must fit within the LCD enclosure with a height restriction of practical, efficient LCD Backlight design is a classic study of compromise in a transduced electronic system. Every aspect of the design is interrelated, and the physical embodiment is an integral part of the electrical circuit.

4 The choice and location of the lamp, wires, display housing and other items have a major effect on electrical characteristics. The greatest care in every detail is required to achieve a practical high efficiency LCD Backlight . Getting the lamp to light is just the beginning!First Generation backlights were crude, with poor per-formance in almost all areas. LTC (Linear TechnologyCorporation) has introduced feedback stabilization and optimized lamp driving configurations in three successive generations of Technology . The effort has culminated in dedicated ICs for Backlight Fourth publication reviews our recent work in com-ponents and measurement techniques applicable to LCD backlighting.

5 Theoretical considerations are presented with practical suggestions, remedies and circuits. As always, we welcome reader comments, questions and requests for LED backlights have mostly replaced CCFLs, this Application Note shows proper design and layout for high voltage invertersApplication Note 65AN65-2an65faTABLE OF CONTENTSPREFACE .. 1 INTRODUCTION ..4 PERSPECTIVES ON DISPLAY EFFICIENCY .. 5 Cold Cathode Fluorescent Lamps (CCFLs) ..5 CCFL Load Characteristics ..7 Display and Layout Losses ..8 Considerations for Multilamp Designs ..31 CCFL Power Supply Circuits ..32 Low Power CCFL Power Supplies.

6 37 High Power CCFL Power Supply ..39 Floating Lamp Circuits ..40IC-Based Floating Drive Power Floating Lamp Criteria for CCFL Circuits ..46 Summary of Circuits ..49 General Optimization and Measurement Considerations ..52 Electrical Efficiency Optimization and Measurement ..53 Electrical Efficiency Measurement ..55 Feedback Loop Stability Issues ..55 REFERENCES ..59 APPENDIX A .. 60 HOT CATHODE FLUORESCENT LAMPS .. 60 APPENDIX B .. 60 MECHANICAL DESIGN CONSIDERATIONS FOR LIQUID CRYSTAL DISPLAYS .. 60 Introduction ..60 Flatness and Rigidity of the Bezel ..61 Avoiding Heat Buildup in the Display.

7 61 Placement of the Display Components ..62 Protecting the Face of the Display ..62 APPENDIX C .. 63 ACHIEVING MEANINGFUL ELECTRICAL 63 Current Probe Circuitry ..64 Current Calibrator ..67 Voltage Probes for Grounded Lamp Circuits ..70 Voltage Probes for Floating Lamp Circuits ..72 Differential Probe Calibrator ..76 RMS Correlation of Electrical Efficiency Measurements ..84 Application Note 65AN65-3an65faAPPENDIX D .. 87 PHOTOMETRIC MEASUREMENTS .. 87 APPENDIX E .. 92 OPEN LAMP/OVERLOAD PROTECTION .. 92 Overload Protection ..93 APPENDIX F .. 94 INTENSITY CONTROL AND SHUTDOWN METHODS.

8 94 About Potentiometers ..96 Precision PWM Generator ..98 APPENDIX G .. 99 LAYOUT, COMPONENT AND EMISSIONS CONSIDERATIONS .. 99 Circuit Segmenting ..99 High Voltage Layout ..99 Discrete Component Selection ..106 Basic Operation of Converter ..107 Requisite Transistor Characteristics ..108 Additional Discrete Components Considerations ..110 Emissions ..110 APPENDIX H .. 110LT 1172 OPERATION FROM HIGH VOLTAGE INPUTS .. 110 APPENDIX I .. 111 ADDITIONAL CIRCUITS .. 111 Desktop Computer CCFL Power Supply ..111 Dual Transformer CCFL Power Supply ..112 HeNe Laser Power Supply ..113 APPENDIX J.

9 114 LCD CONTRAST CIRCUITS .. 114 Dual Output LCD Bias Voltage Generator ..115LT118X Series Contrast Supplies ..116 APPENDIX K .. 119 WHO WAS ROYER AND WHAT DID HE DESIGN? .. 119 APPENDIX L .. 120A LOT OF CUT OFF EARS AND NO VAN GOGHS .. 120 Not-So-Great Backlight Circuits ..120 Not-So-Great Primary Side Sensing Ideas ..122 Application Note 65AN65-4an65faNote 1: Previous publications are annotated in References 1, 18 and scribing marks the Fourth LTC publication in asmany years concerning LCD The extraor-dinary user response to previous efforts has resulted in a continuing LCD Backlight development effort by ourcompany.

10 This level of interest, along with significant performance advances since the last publication, justifies further discussion of LCD of attractive solutions for LCD illumination has necessitated the longest sustained LTC application engineering effort to date. A single circuit in a 1991 publication (Measurement and Control Circuit Collection, LTC Application Note 45, June 1991) has resulted in four years of continuous investigation, summarized in three successive, dedicated impetus for all this bustle has been an overwhelming and continuously ascending reader response.