Patent ID: 8362701

Claim:
An electronic ballast for operating at least one lamp, comprising: an inverter operative to convert a DC voltage to provide an AC output voltage at an inverter output to drive at least one lamp: and an end-of-life (EOL) detection circuit directly connected with the inverter output to sense an absolute DC lamp voltage of a lamp and operative to generate an end-of-life signal based on a maximum of the absolute DC lamp voltage, where the end-of-life detection circuit comprises: a plurality of absolute DC lamp voltage sensing circuits operatively coupled with the inverter output to sense absolute DC lamp voltages of individual lamps, the individual absolute DC lamp voltage sensing circuits comprising: a resistor coupled between a corresponding line of the inverter output and an intermediate node, and a capacitance coupled between the intermediate node and a sensing node; a maximum lamp DC voltage circuit operatively coupled with the absolute DC lamp voltage sensing circuits to determine a maximum one of the absolute DC lamp voltages and to generate the end-of-life signal at a maximum lamp DC voltage output based on the maximum absolute DC lamp voltage, the maximum lamp DC voltage circuit comprising: a plurality of first diodes coupled between corresponding intermediate nodes and the maximum lamp DC voltage output, a plurality of second diodes coupled between the corresponding intermediate nodes and a circuit ground, a positive sensing capacitance and a positive sense resistor coupled in parallel between the maximum lamp DC voltage output and the sense node, and a negative sensing capacitance and a negative sense resistor coupled in parallel between the sense node and the circuit ground; and a comparator operative to compare the end-of-life signal from the maximum lamp DC voltage output with a threshold and to generate a comparator output signal having a first state when the end-of-life signal is less than the threshold and a second state indicating at least one lamp has reached an end-of-life condition when the end-of-life signal is greater than the threshold; where the inverter is operative to convert the DC voltage to provide the AC output voltage at the inverter output to drive a plurality of lamps: and where the EOL detection circuit is directly connected with the inverter output to sense absolute DC lamp voltages of individual lamps and operative to generate the end-of-life signal based on a maximum one of the absolute DC lamp voltages.