Lighting circuit for vehicular discharge lamp

In a lighting circuit for a vehicular discharge lamp having a DC voltage booster circuit for boosting an input voltage from a DC voltage input terminal to provide an output voltage to be applied to the discharge lamp, and a control circuit for varying the output voltage of the DC voltage booster circuit to execute lighting control including constant power control of the discharge lamp, the improvement comprising:

• a voltage drop detector for detecting a DC input voltage applied to the DC voltage input terminal,
• an output voltage detector for detecting the output voltage of the DC voltage booster circuit and outputting a signal corresponding to a difference between the detected output voltage and a reference value;
• and
• an output current detector for detecting an output current of the DC voltage booster circuit and outputting a signal corresponding to a difference between the detected output current and a reference value,
• whereby in response to a signal received from the voltage drop detector in accordance with a drop of the DC input voltage below a predetermined normal level, the control circuit performs voltage boosting control to set power supplied to the discharge lamp lower than rated power to prevent excess current consumption by said discharge lamp;
• and
• whereby the control circuit produces control signals corresponding to the signals from the output voltage detector and the output current detector and supplies the control signals to the DC voltage booster circuit to control the output voltage thereof, and when the DC input voltage falls to lower than a predetermined value, the voltage drop detector varies the reference value of the output current detector in accordance with the drop of the DC input voltage to thereby effect voltage boosting control to set the power supplied to the discharge lamp lower than the rated power.