This research investigated a method to optimize the start-up performance of a dimmable light-emitting diode (LED) driver based on half-bridge Class D series-parallel LCC topology. This LED driver is designed for outdoor or high bay applications with 347-480 V-ac input voltage. The total maximum output power is 100 W with a wide tunable output current range (from 50 to 800 mA). The wide load range makes it difficult to achieve a startup time of less than 1 s under each load condition as well as limit the output overshoot level to below 150% by implementing only one set of KI values in the feedback loop. Moreover, in this design, a fluorescent control IC is used due to the cost efficiency of repurposing existing fluorescent ballast manufacturing systems. The start-up sequence of the fluorescent control IC introduces an additional challenge to the start-up design. In order to comply with Energy Star, which requires the output current to reach 75% of its nominal value and also limit the overshoot level, a self-adjusted-duty-cycle control algorithm is designed. This duty-cycle signal generated by measuring the load condition through a secondary side microcontroller is used to determine the switching frequency of the LCC tank. With this method, the start-up performance is greatly improved and meets all requirements.