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The paper proposes an electronic ballast in which the resonant frequency of the circuit is continuously tracked by the phase-locked loop (PLL). This electronic ballast, which employs PLL control, has a high tolerance for the variations that exist in the equivalent resistance of a lamp. At present, the PLL control is used to drive cold-cathode fluorescent lamps (CCFLs), in which variations in equivalent resistance between ignition and the steady state are small, so that phase differences at the resonant frequency are almost fixed; thus the resonant frequency can be tracked continuously by the operating frequency. However, if variations in equivalent resistance are large, the phase difference at the resonant frequency may be not fixed, and thus the resonant frequency may be not continuously tracked by the operating frequency. The paper proposes the electronic ballast for fluorescent lamps with large variations in equivalent resistance between ignition and the steady state. According to the phase characteristics of various resonant tanks and an optimum design of the resonant tank, the circuit's resonant frequency is continuously tracked by the PLL. Since the current regulator circuit limits the value of the lamp current as the operating frequency increases from the resonant frequency, the lamp current is accurately controlled, regardless of variations in the load. The paper discusses the implementation of the proposed ballast with PLL control, which offers high tolerance for the variations of the equivalent resistance in the lamp.