Driver circuit with a semiconductor light source and method for operating a driver circuit

Abstract

A driver circuit for a semiconductor light source and to a method for controlling the brightness thereof are provided. When a pulse width modulated semiconductor light source reaches a duty cycle threshold value, the voltage is increased or decreased through a variable voltage source in order to achieve a higher dynamic range.

What is claimed is: 
     
         1 . A driver circuit with a semiconductor light source, comprising:
 at least one pulse width modulation driven semiconductor light source;   at least one sensor measuring a luminous intensity of light emitted by the semiconductor light source;   a variable voltage source powering the semiconductor light; and   a controller connected to the at least one sensor and the variable voltage source, the controller controlling the luminous intensity of the light emitted by the semiconductor light source by adjusting a duty cycle of the pulse width modulation, and the controller controlling the variable voltage source such that, when the duty cycle falls below or exceeds a duty cycle threshold value, a voltage applied to the semiconductor light source is adjusted by the controller through the variable voltage source.   
     
     
         2 . The driver circuit as claimed in  claim 1 , wherein the variable voltage source comprises a variable resistor controlled by the controller. 
     
     
         3 . The driver circuit as claimed in  claim 1 , wherein the variable voltage source comprises a field effect transistor controlled by the controller. 
     
     
         4 . The driver circuit as claimed in  claim 1 , wherein the variable voltage source comprises an unregulated control element, through which the voltage can be varied. 
     
     
         5 . The driver circuit as claimed in  claim 1 , wherein the controller comprises a memory that stores a table of values representing a ratio of applied voltage and brightness. 
     
     
         6 . The driver circuit as claimed in  claim 1 , wherein the variable voltage source is adjustable in at least 5 increments. 
     
     
         7 . The driver circuit as claimed in  claim 1 , wherein the variable voltage source is adjustable in at least 10 increments. 
     
     
         8 . The driver circuit as claimed in  claim 1 , wherein the variable voltage source is adjustable in at least 100 increments. 
     
     
         9 . A method for operating a driver circuit for a semiconductor light source, comprising:
 driving at least one semiconductor light source through pulse width modulation;   sensing a luminous intensity of light emitted by the semiconductor light source;   adjusting a duty cycle of the pulse width modulation based on the sensed luminous intensity;   powering the semiconductor light source by a variable voltage source;   controlling the variable voltage source, when the duty cycle falls below or exceeds a duty cycle threshold value, to increase or decrease voltage applied to the semiconductor light source.   
     
     
         10 . The method as claimed in  claim 9 , wherein the voltage applied to the semiconductor light source is reduced in case of a duty cycle of less than 30%. 
     
     
         11 . The method as claimed in  claim 9 , wherein the voltage applied to the semiconductor light source is reduced in case of a duty cycle of less than 20%. 
     
     
         12 . The method as claimed in  claim 9 , wherein the voltage applied to the semiconductor light source is increased in case of a duty cycle of more than 70%. 
     
     
         13 . The method as claimed in  claim 9 , wherein the voltage applied to the semiconductor light source is increased in case of a duty cycle of more than 80%. 
     
     
         14 . The method as claimed in  claim 9 , wherein the step of driving at least one semiconductor light source through pulse width modulation comprises driving a plurality of semiconductor light sources, each of the plurality of semiconductor light sources having a different color of light. 
     
     
         15 . The method as claimed in  claim 14 , further comprising adjusting the duty cycle of the pulse width modulation based on the sensed luminous intensity separately for each color.