Patent ID: 7642731

Claim:
An inverter for driving a lamp, comprising: an inner pulse width modulation (PWM) generator that generates an inner pulse width modulation (PWM) signal; a monitoring unit that monitors a duty ratio of an external pulse width modulation (PWM) signal, to selectively output the external PWM signal or the inner PWM signal in accordance with the monitored duty ratio; and an AC drive signal generator that generates an AC drive signal in response to the external PWM signal or the inner PWM signal and supplies the AC drive signal to the lamp, wherein the monitoring unit comprises: a duty ratio detector that detects the duty ratio of the external PWM signal in the form of a DC voltage; and an output controller that bypasses the external PWM signal or the inner PWM signal in accordance with the detected duty ratio, wherein the duty ratio detector comprises: an integrator that integrates the external PWM signal; and a DC converter that converts the integrated signal from the integrator into a DC voltage, and wherein the output controller comprises: a first switching device that supplies a control signal in response to the DC voltage from the duty ratio detector; a second switching device that bypasses the external PWM signal to ground in response to the control signal; and a third switching device that bypasses the inner PWM signal to around in response to the DC voltage from the duty ratio detector.