Patent ID: 7459867

Claim:
A ballast for powering at least one gas discharge lamp having a pair of lamp filaments, the ballast comprising: an inverter having an inverter output terminal and being operable to provide, at the inverter output terminal, an inverter output voltage having an operating frequency; a resonant output circuit coupled between the inverter output terminal and the lamp, and operable to provide: (i) preheating of the lamp filaments; (ii) an ignition voltage for igniting the lamp; and (iii) a magnitude-limited current for operating the lamp; a control circuit coupled to the output circuit and the inverter, wherein the control circuit is operable: (a) to monitor a voltage within the resonant output circuit; (b) in response to the monitored voltage reaching a first specified level, to control the inverter to maintain its operating frequency at a first present value for a predetermined preheating period so as to allow the resonant output circuit to provide preheating of the lamp filaments during the preheating period; (c) upon completion of the preheating period, to allow the operating frequency of the inverter to decrease from the first present value; and (d) in response to the monitored voltage reaching a second specified level, to control the inverter to maintain its operating frequency at a second present value for a predetermined ignition period so as to allow the resonant output circuit to provide an ignition voltage for igniting the lamp.