Date of Award

10-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Electrical Engineering

Advisor

Poole, Kelvin

Committee Member

Harrell , William

Committee Member

Singh , Rajendra

Abstract

An integrated circuit (IC) powering up to nine series connected high brightness light emitting diodes (HBLED's) was designed. The LED's are split into three groups by color (RGB), and the average current through each group is independently varied using a separate microprocessor in conjunction with the IC controller. Integrated comparators compare the voltage across each LED group with internally generated references to check for open or short circuit diodes. In the event of an open circuit failure, the IC controller switches in a SCR to bypass the open circuit device thus maintaining light output at reduced intensity.
Silvaco CAD tools and SPICE simulations were used to design the LDD high voltage MOSFET's. A MOSFET width of 15,000μm was sufficient to meet the specification. Simulations showed a VDSON of 100mV at 0.5A is possible.
Mentor Graphics CAD tools in conjunction with a standard TSMC 0.35μm process was used to design the op-amp and comparators used in the fault detection circuit and the PMOS current mirror used in the MOSFET gate control circuit. The gate control circuit was found to have an average delay of 4μs.
By varying the intensity of the three primary colored LED's, a spectrum of colors, including white light, can be produced. Independent control of the average current through each LED group is accomplished by connecting a MOSFET in parallel with each group. The MOSFET provides an alternate current path and, by applying a Pulse Width Modulated (PWM) signal to the gate of the MOSFET, the average current through each iii LED group can be varied from 25mA (lowest duty cycle) to 475mA (highest duty cycle). The 3.3v digital signals supplied by the microprocessor are used to switch each MOSFET transistor ON or OFF using three separate level shift circuits on the IC controller.
A hardware version of the intensity control circuit was constructed out of discrete components to demonstrate PWM LED intensity control functionality. The duty cycle of the hardware circuit's LED current was measured to have an average percent error of 2% and a maximum percent error of 12%.

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