US Patent 7755304 Three-way dimming ballast circuit

A three-way dimming ballast circuit for driving a ballast power switching circuit powering a gas discharge lamp such as a CFL. The circuit includes a driver circuit for driving high and low side switches of the ballast power switching circuit; a control circuit for driving the driver circuit including an oscillator circuit for providing an oscillating signal to control the frequency of operation of the ballast power switching circuit, the ballast power switching circuit outputting lamp-powering pulsed signals; and a dimming control circuit having an input, the dimming control circuit receiving an AC lamp current feedback signal at the input, the dimming control circuit further receiving a DC input voltage reference at the input, whereby the DC input voltage reference determines a desired dimming level of the lamp and the AC lamp current feedback signal maintains the lamp brightness at the desired dimming level. The DC input voltage reference is provided by one or both of a setting circuit comprising forward-biased diodes and/or an override circuit comprising transistors that are switched on to apply a maximum DC voltage.