EBL having a feedback circuit and a method for ensuring low temperature lamp operation at low dimming levels

A dimmable ballast lamp, including:

• a lamp;
• a ballast circuit for converting AC input power to AC output power and for delivering said AC output power to said lamp to produce a lamp voltage and a lamp current;
• a dimming dimming interface circuit for generating a dimming control signal having a voltage level indicative of a selected dimming level of said lamp;
• a dimming circuit responsive to said dimming control signal for controlling operation of said ballast circuit in such a manner as to adjust said lamp current to a level proporational to the voltage level of said dimming control signal, to thereby drive said lamp to said selected dimming level;
• a feedback control circuit for detecting said lamp voltage and for producing a feedback control signal in response to said lamp voltage exceeding a perscribed threshold voltage level, said feedback control signal having a voltage exceeds said perscribed threshold voltage level;
• and,
• wherein said dimming circuit is responsive to said dimming control signal being raised by addition thereto of said feedback control signal for controlling operation of said ballast circuit in such a manner as to increase said lamp current, to thereby increase the dimming level of said lamp.

A method for operating a dimmable ballast lamp in such a manner as to protect against shutoff of the lamp, comprising the steps of:

• generating a dim signal representing a desired brightness:
• detecting a shutoff condition of the lamp;
• producing a control signal in response to detection of said shutoff condition;
• and,
• using said control signal to increase the level of the dim signal such that the desired brightness of the lamp is increased to a higher level and the shutoff condition is avoided.