Patent ID: 7525256

Claim:
A single-chip IC controller for an electronic ballast for driving high intensity discharge (HID) lamp: said single chip IC controller being connected for controlling both a buck stage and a switching stage in said electronic ballast; the buck stage having a buck MOSFET controlling current to a buck inductor, receiving a DC input voltage and outputting a DC voltage with controlled current; and the switching stage converting said DC output voltage to a switched AC voltage which is capable of driving said HID lamp; wherein said controller controls said buck MOSFET in response to buck inductor current; and wherein said buck stage is controlled to operate in either: a critical-conduction mode wherein said buck inductor current does not exceed a predetermined level, and in response thereto, said buck MOSFET is turned on according to a predetermined on-time, and following said predetermined on-time said buck inductor current discharges to zero into said switching stage, and in response thereto, said predetermined on-time is restarted, or a continuous-conduction mode wherein said buck inductor current exceeds a predetermined level, in response to which said buck MOSFET is turned off prior to the end of said predetermined on-time, whereupon said inductor current discharges into said switching stage, and following a predetermined off-time, said buck MOSFET is restarted independently of whether said inductor current has reached zero or is above zero, and to transition smoothly between said modes.