Pulse width modulation circuit and illuminating device incorporating same

Abstract

In a pulse width modulation circuit, which generates an output whose pulse width is modulated by controlling a duty ratio of an oscillation circuit adapted to generate an oscillation at a frequency determined by a resistor and a capacitor to be electrically charged via the resistor, there is provided a resistance value varying means to vary a value of the resistor. The value of the resistor is varied by shorting the both ends of the resistor with a transistor, whereby the capacitor is charged in a reduced amount of time realizing a high-speed pulse width modulation circuit.

1 - 2 . (canceled)  
   
   
       3 . An illuminating device comprising: 
 a pulse width modulation circuit in which a duty ratio of an oscillation circuit is varied by varying a value of a resistor which, in combination with a capacitor to be electrically charged via the resistor, determines a cycle of an oscillation generated by the oscillation circuit; and    a rare-gas discharge lamp, which is driven by an output signal of the pulse width modulation circuit, and which has its luminance varied depending on a lighting frequency of the pulse width modulation circuit.