Maximum-efficiency tracking control applied in the backlight module is proposed in this paper. The backlight system incorporates a phase-locked loop as a feedback mechanism to promote both the system efficiency and stability. To overcome the drawbacks of the conventional electromagnetic transformer and miniaturize the backlight module, the piezoelectric transformer is adopted for driving the cold cathode fluorescent lamp. In addition, the PLL is utilized to track the optimal operating frequency of PT for achieving maximum output efficiency.