Patent ID: 7755304

Claim:
A dimming ballast control circuit for driving a ballast power switching circuit powering a gas discharge lamp comprising: first and second AC line inputs; a driver circuit powered from said AC line inputs for driving high and low side switches of the ballast power switching circuit; a control circuit powered from said AC line inputs 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 a dimming input, the dimming control circuit receiving an AC lamp current feedback signal at the dimming input, the dimming control circuit further receiving a DC input voltage reference at the dimming 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 dimming control circuit further comprising a resistive divider stage coupled to said dimming input to provide said DC input voltage reference, said resistive divider stage comprising: a first diode having its anode connected to said first AC line input and its cathode connected to one end of a first resistor, and a second diode with its anode connected to said second AC line input and its cathode connected to one end of a second resistor, other ends of said first and second resistors being connected at a common node to one end of a third resistor, the other end of the third resistor being connected to a common point of said driver and control circuits; and said common node being connected to said dimming input for supplying said DC input voltage reference, whereby when the first AC line input is connected to AC power, the series connection of the first diode, first resistor and third resistor provides a first DC voltage level at said common node and to said dimming input for powering said gas discharge lamp at a first light level; and when the second AC line input is connected to AC power, the series connection of the second diode, second resistor and third resistor provides a second DC voltage level at said common node and to said dimming input for powering said gas discharge lamp at a second light level.