Patent ID: 8384297

Claim:
An electronic ballast for driving a gas discharge lamp, the ballast comprising: an inverter circuit for converting a DC bus voltage to a high-frequency output voltage having an operating frequency and an operating duty cycle; a resonant tank circuit characterized by a resonant frequency and operable to couple the high-frequency output voltage to the lamp to generate a lamp current through the lamp and a lamp voltage across the lamp; and a control circuit coupled to the inverter circuit for controlling the operating frequency and the operating duty cycle of the high-frequency output voltage, so as to adjust the intensity of the lamp to a target intensity, the control circuit operable to control the duty cycle of the high-frequency output voltage to adjust the magnitude of the lamp current through the lamp, so as to control the intensity of the lamp to the target intensity, the control circuit operable to control the operating frequency of the high-frequency output voltage to a low-end frequency when the target intensity of the lamp is at a low-end intensity; wherein the control circuit is operable to determine an approximation of the resonant frequency of the resonant tank circuit, and to control the operating frequency of the high-frequency output voltage in response to the approximation of the resonant frequency and the target intensity of the lamp, the low-end frequency being an offset frequency away from the approximation of the resonant frequency.