Patent ID: 7378804

Claim:
A ballast for a discharge lamp comprising: a DC power supply which receives an input voltage and provides a regulated DC output voltage, said DC power supply including a switching element which is driven to turn on and off in a controlled manner to provide said DC output voltage; an inverter circuit which converts said DC output voltage into a high frequency voltage, said inverter circuit including at least one pair of two switching elements which are connected in series across said DC output voltage and are controlled to alternately turn on and off to provide said high frequency voltage; a load circuit which receives said high frequency voltage to apply it to the discharge lamp for driving said discharge lamp; and an integrated control circuit which controls said switching elements of said DC power supply as well as said inverter circuit; wherein said DC power supply is a step-up chopper circuit for boosting the input voltage to said DC output voltage; wherein said load circuit including an inductor and a capacitor which are cooperative to form a resonant circuit through which said high frequency voltage is applied to said discharge lamp; wherein said integrated control circuit comprises: a first control unit which generates a first control signal for control of said switching element of said DC power supply, a second control unit which generates a second control signal for control of the switching elements of said inverter circuit, a driver circuit which provides drive signals for driving each one of the switching elements of said DC power supply and said inverter circuit in accordance with said first control signal and said second control signal, a starter circuit providing a controlled power for energizing said first control unit, said second control unit, and said driver circuit, a first timer which starts counting time upon energization of said first control unit, said second control unit, and said driver circuit so as to define a preheating time period within which filaments of the discharge lamp are heated, and to define a subsequent lamp-start time period within which a starting voltage is applied to the discharge lamp, and a selector which, in response to the output from said first timer, varies at least one of a duty ratio of and a switching frequency of the switching elements of said inverter circuit for providing a preheating current to said filaments within said preheating time period and for providing said starting voltage within said lamp-start time period.