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Highly Efficient and Dimmable Led Back Light Driver for Portable Devices

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Highly Efficient and Dimmable Led Back Light Driver for Portable Devices HIGHLY EFFICIENT AND DIMMABLE LED BACK LIGHT DRIVER FOR PORTABLE DEVICES BY Venu Siripurapu, B.E A thesis submitted to the Graduate School in partial fulfillment of the requirements for the degree Master of Sciences, Engineering Specialization in: Electrical Engineering New Mexico State University Las Cruces, New Mexico April 2016 \HIGHLY EFFICIENT AND DIMMABLE LED BACK LIGHT DRIVER FOR PORTABLE DEVICES," a thesis prepared by Venu Siripurapu in partial fulfill- ment of the requirements for the degree, Master of Sciences has been approved and accepted by the following: Dr. Louis Reyes Dean of the Graduate School Chair of the Examining Committee Date Committee in charge: Dr. Paul M. Furth, Associate Professor, Chair. Dr. Wei Tang, Assistant Professor. Dr. Rolfe Sassenfeld, Assistant Professor. ii DEDICATION Dedicated to my mother Venkata Laxshmi, father Nageswara Rao, brother Manoj Kumar, my advisor Dr. Paul Furth, and all my friends and family mem- bers. iii ACKNOWLEDGMENTS I express my gratitude to all those people who supported me in the suc- cessful accomplishment of my research work at NMSU. My advisor Dr. Paul Furth, is a role model in the teaching industry. His way of thinking on "How to improve the educational standards" is what sets him apart. He always motivates students to perceive and analyze things through. Due to this students always reach higher levels of success. His encouragement and continuous advising led this research work to move at a faster pace. Dr. Wei Tang is one of the youngest professors in the Electrical Engineering Department. I have always been inspired by his new ideas and working style. I also had a great opportunity to work under Dr. Rolfe Sassenfeld for developing the power laboratory in Engineering Technology Department. I am happy to be a part of this success story that established an engineering technology power lab within a limited budget. Co-advisors for this research work are Ph.D. candidates Punith R Surkanti and Sri Harsh Pakala. I could not imagine my research work with out either one of their technical support and continuous status tracking. I am thankful for their great support. My parents Nageswara Rao and Venkata Lakshmi, always oversaw my in- dividual steps to ensure my journey towards successful completion of my Master's iv Degree in Electrical Engineering. All my life, my father inspired me to be a better human being. Co-workers Anurag Veerabathini, Mahendar Manda, Sai Kiran Ramidi, Rohith Gaddam created a right balance between peer support and peer competi- tion. Thank you all for your constant support in my successful journey at NMSU, in Las Cruces, New Mexico. v VITA Education 2008 - 2012 B.E. Electronics and Communication Engineering, Jawaharlal Nehru University, India 2013 - 2016 MSEE. in Electrical Engineering, New Mexico State University, USA Presentations Electro Static discharge (ESD), New Mexico State University, March, 2014. Professional Experience [1] Analog Design Engineer in RF Domain such as Oscillator, PLLs, LDOs etc., Palma Ceia SemiDesign, Dallas, Texas, from Aug-2015 - Jan-2016 [2] Hardware Engineer in Hybrid Vehicle Power Control Unit, Robert BOSCH Engineering and Business Solutions, Bangalore, India, from July-2011 - Dec-2013 [3] Associate Engineer in Power Electronics Domain, Larsen & Toubro limited- IES, Mysore, India, from June-2010 - July-2011 [4] Associate Member Technical Staff in Embedded Domain, HCL Technologies, Bangalore, India, from July-2008 - June-2010 vi ABSTRACT HIGHLY EFFICIENT AND DIMMABLE LED BACK LIGHT DRIVER FOR PORTABLE DEVICES BY Venu Siripurapu, B.E Master of Sciences, Engineering Specialization in Electrical Engineering New Mexico State University Las Cruces, New Mexico, 2016 Dr. Paul M. Furth, Chair MS Thesis defense scheduled on 04/07/2016, 9 AM Thomas & Brown Hall, Room 207. Effective power utilization is an important issue for portable devices as it defines the battery life. The display module consumes the largest portion of power and efficient power converters will reduce unwanted conversion losses and improves the battery life. A highly efficient and dimmable boost converter, which is suitable for LED back-light drivers in smart phones, is presented in this paper. A hysteretic-mode synchronous boost converter is implemented as an LED driver using the IBM 0.18 µm CMOS technology. The output of this converter is designed vii as a current source to drive 5 parallel LED strings (20 mA through each string) with two series-connected color LEDs in each string. The simple and fast response to load changes makes this system applicable for digital dimmable LED driver applications. To reduce the power dissipation and improve the transient response, the feedback loop is implemented using 1.8 V devices, whereas the power switches can switch up to 5.5 V. A peak efficiency of 92% is measured in simulation. Dimming control is implemented with 10 dimming ratios ranging from 1:1 to 512:1. viii TABLE OF CONTENTS LIST OF TABLES xii LIST OF FIGURES xiv 1 INTRODUCTION 1 1.1 Motivation . 1 1.2 Project Background . 3 1.3 Objectives and Unique Contributions . 4 1.4 Report Organization . 5 2 LITERATURE REVIEW 6 2.1 Boost Converter . 6 2.1.1 Conventional Boost Converter . 6 2.1.2 Synchronous Boost Converter . 8 2.2 Boost Control Loop . 10 2.2.1 PWM Mode Control . 10 2.2.2 PFM Mode Control . 12 2.3 LED Driver Topologies in the Literature . 13 2.4 Hysteretic Boost Converter in the Literature . 15 2.5 Current Mirrors and Current Matching . 16 2.6 Minimum or Lowest Voltage Selector . 18 ix 2.7 Hysteretic Comparator . 19 2.8 SR Latch . 20 2.9 Non-Overlapping Clock Generator . 21 2.10 MOS Gate Drivers . 23 2.11 Level Shifters . 23 2.12 Inductor Current Sense Module . 25 2.13 Zero Cross Detector . 25 2.14 Dimming . 27 3 DESIGN AND BLOCK-LEVEL SIMULATIONS 30 3.1 Design Specifications . 30 3.2 Boost Converter Design . 30 3.2.1 Inductor Value Calculation . 32 3.2.2 Capacitor Value Calculation . 34 3.3 Power FETs sizing . 37 3.4 Power MOSFET Gate Driver Sizing . 40 3.5 Hysteretic Voltage Mode Control . 42 3.6 Hysteretic Current Mode Control . 44 3.7 Minimum Voltage Selector . 44 3.8 Current Matching Circuit . 51 3.9 LED Model . 52 3.10 Inductor Full-Range Current Monitor . 54 4 CHIP-LEVEL SIMULATIONS AND LTA MEASUREMENT RE- SULTS 59 4.1 Simulation Results of Closed-Loop Design . 59 4.1.1 Virtuoso Simulation Test Bench . 59 x 4.1.2 Virtuoso Simulation Result - Full Load . 60 4.1.3 Virtuoso Simulation Results - During Digital Dimming . 63 4.2 Layout of Boost Converter, Power Switches & the control loop . 66 4.3 Measurement Results . 68 4.3.1 Parametric Failure Due To Inrush Current . 69 4.3.2 NC Pins In The Layout . 70 4.4 Measurement Results For Minimum Voltage Selector . 71 5 CONCLUSIONS 78 5.1 Summary . 78 5.2 Issues . 78 5.3 Comparison with the State-of-the-Art . 79 5.4 Future Work . 79 REFERENCES 81 APPENDICES 85 A. Test Document 86 A.1 Pin Diagram of the Boost Converter . 87 A.2 Supply Voltages and Currents . 87 A.3 Test Procedure . 88 B. External Capacitor and Inductor Selection 95 B.4 External Inductor and Capacitor Selection . 96 xi LIST OF TABLES 2.1 Truth Table of SR Latch using NOR Gates. 21 3.1 Boost Converter Specifications. 31 3.2 Inductor and Capacitor Design Values. 36 3.3 Power MOSFET Sizing. 41 3.4 Sizes of the MOSFETs used in Current Matching Circuit. 42 3.5 Hysteretic Comparator MOSFET Sizes. 46 3.6 LTA comparison with state-of-art. 51 3.7 Sizes of the MOSFETs used in Current Matching Circuit. 52 3.8 Values of Parasitic Components in LED. 54 3.9 NMOS Current Sensing Module sizes. 56 3.10 PMOS Current Sensing Module sizes. 57 4.1 Test Bench Input and Component Specifications. 61 4.2 Simulation Results Summary. 65 4.3 Load Transient Analysis. 66 4.4 Module Sizing Summary. 68 4.5 Input Specification for LTA. 76 4.6 Simulation and Measured Results of LTA. 77 5.1 Simulation Results Summary of LED Driver. 80 xii 1 Pin Description of Fabricated Chip. 92 2 Off-Chip Inductor and Capacitor. 96 xiii LIST OF FIGURES 1.1 Power Management Schemes for Smartphone Devices [1]. 2 1.2 LED Driver Application. 4 2.1 Conventional Boost Converter. 7 2.2 Synchronous Boost Converter. 9 2.3 Synchronous Boost Converter (ON State). 9 2.4 Synchronous Boost Converter (OFF State). 10 2.5 PWM Mode Control Scheme for DC-DC Converter. 11 2.6 PFM Mode Control Scheme for DC-DC Converter. 12 2.7 Energy Recycling Technology for Back light Unit [2]. 13 2.8 Double-Loop Control LED Driver [3]. 14 2.9 Current Mode Hysteretic Control [4]. 15 2.10 Double-Loop Control LED Driver [3]. 17 2.11 High Output Impedance and Low Output Voltage Current Mirror Circuit [5]. 17 2.12 Loser-Take-All Circuit [6]. 19 2.13 Comparator with Positive Feedback Decision Circuit [7]. 20 2.14 SR Latch using two NOR Gates. 21 2.15 Nonoverlapping clock generation circuit [8]. 22 2.16 Waveforms of non-overlapping clock generator [8]. 22 xiv 2.17 MOS Transistor Gate Driver with scale factor S. 24 2.18 Level Shifter. 24 2.19 Full Range Inductor Current Sensor [9]. 26 2.20 Zero Cross Detector [9]. 27 2.21 Digital Dimming [10]. 28 3.1 LED Driver Application. 31 3.2 Inductor Voltage & Current Waveforms while Operating in BCM. 33 3.3 Inductor Current at the Boundary of CCM and DCM. 35 3.4 Inductor Current at the Boundary of CCM and DCM.
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