Patent ID: 8400074

Claim:
A discharge lamp ballast comprising: a DC-AC power converter having a plurality of switching elements; a startup circuit functional to generate a high voltage to ignite a discharge lamp, the startup circuit coupled between output terminals of the power converter; a lamp current detection circuit functional to detect an amplitude of a lamp output current; a lamp voltage detection circuit functional to detect an amplitude of a lamp output voltage; and a control circuit effective to control the switching elements and to generate an operating frequency in accordance with a startup operation to generate a high voltage output from the startup circuit, a filament heating operation wherein the operating frequency is controlled so as to adjust the amplitude of the lamp output current detected by the lamp current detection circuit to a target current amplitude, said target current amplitude determined based on the amplitude of the lamp output voltage detected by the lamp voltage detection circuit, and a steady-state operation wherein the operating frequency is decreased to maintain a stable light output.