Topics on light-emitting-diode driver research

In this dissertation, light-emitting-diode (LED) drivers are investigated for efficiency issues related to driving Red-Green-Blue (RGB) pixels and multiple LED strings in parallel. A high-efficiency digitally controlled RGB LED driver was designed for driving a 3x3 RGB LED display panel. A multiplexer was used to sense the voltage drop across the current controllers. This driver maintained a minimum drive voltage across the RGB LED pixels required to keep it in regulation leading to a reduction of unwanted power losses in the RGB LED pixels by selecting the minimum drop across the current-controllers as the reference voltage of the digital controller. Additionally, analog dimming was implemented to dim each individual LED in a 3x3 RGB LED display panel. Efficiencies of 85.6 %, 93.3 %, and 91.1 % were experimentally obtained for red, green, and blue LEDs, respectively at the rated output current. For comparison, efficiencies of 38.3% for the red LED, 66.2 % for the green LED, and 64.5 % for the blue LED at the nominal current of 20mA using a 5 V supply were achieved in conventional LED drivers. For parallel connected LED strings, a current controller is required for each string to maintain the desired current with the drive voltage provided by a switch-mode power converter (SMPC). A novel analog/digital LED driver controller was designed for driving a two-string LED load with three white LEDs in each string for backlight application in liquid-crystal-displays (LCDs). In this implementation, an analog controller was used to control the output voltage of the SMPC while a digital controller was used to achieve the minimum drive voltage across the output LED strings, leading to an efficiency of 89 % at the rated output current. A phase shifted pulse width modulation (PSPWM) dimming was implemented to reduce load stresses, improve electromagnetic-interferences (EMI) and increase system efficiency. A mathematical model, based on multirate simulation technique, for merging the analog and digital controllers was proposed. Finally, a LED driver chip was designed and fabricated using a 0.5 micron CMOS process to reduce size of the overall LED driver system. The integrated circuit consisted of a 16-channel analog multiplexers, five current controllers, and buffer circuits. The LED driver chip function was experimentally verified using a SMPC to drive a five-string LED load in parallel with a single green LED in each string for display panel applications by maintaining a minimum drive voltage across the LED strings, thus leading to an efficiency of 75 % at the rated output current.