Patent ID: 8436548

Claim:
A ballast circuit to drive a light source, the ballast circuit comprising: a rectifier circuit configured to receive an AC input voltage from a dimmer circuit and to provide a rectified output voltage; a power conversion circuit configured to provide a regulated output to the light source in response to the rectified output voltage and a dimmer reference level signal representative of a dimmer setting of the dimmer circuit; and a conduction angle detection circuit, the conduction angle detection circuit coupled to an output of the rectifier to receive the rectified voltage, and coupled to an input of the power conversion circuit to provide the dimmer reference level signal to the power conversion circuit, the conduction angle detection circuit comprising: a comparator having a first input and a second input and configured to provide a pulse-width modulated output in response to comparison of signals at the first input with signals at the second input, the pulse width modulated output having a pulse width representative of the dimmer setting of the dimmer circuit; a limiting circuit coupled to the comparator and configured to receive the rectified voltage and provide a voltage-limited output in response to the rectified voltage to the first input of the comparator; a threshold supply circuit configured to provide a threshold voltage to the second input of the comparator; and a filter coupled to the comparator, the filter being configured to convert the pulse-width modulated output of the comparator to the dimmer reference level signal.