| 摘要 |
PURPOSE:To simplify a circuit constitution, and constitute a device at low cost. CONSTITUTION:A constant lamp current is supplied to a discharge lamp La till a lamp voltage of the discharge lamp La reaches a specified value after starting, and after the lamp voltage reaches the specified value, a constant lamp power is supplied to the discharge lamp La. An error amplifier circuit 8 is provided to generate a differential output between a lamp current detection circuit 3 and a reference value Vref2, and the output of the error amplifier circuit 8 is given from an OR circuit 9 to a PWM control circuit 6 till the lamp voltage of the discharge lamp La reaches the specified value after starting. In a period till the lamp voltage reaches the specified value after starting, the lamp current is controlled to be constant by the PWM control circuit 6 in accordance with the output of the error amplifier circuit 8. Need of detecting a correct lamp voltage by a lamp voltage detection circuit 2 when the lamp voltage is low, and perform correct calculation for an output of almost zero by a lamp power calculation circuit 4 is thus eliminated, constitution of the lamp voltage detection circuit 2 and the lamp power calculation circuit 4 can be simplified, thereby a device can be constituted at a low cost. |
