1.  Technical Field
The present invention relates to a lighting control circuit for vehicle lighting equipment and, more particularly, a lighting control circuit for a vehicle lighting equipment constructed to control the lighting of a semiconductor light source that is formed of a semiconductor light emitting device. 
2.  Related Art
In the prior art, as the vehicle lighting equipment, the equipment using a semiconductor light emitting device such as LED (Light Emitting Diode), or the like as a light source is known.  Also, the vehicle lighting equipment of this type is equipped with a lighting control circuit that controls the lighting of the LED. 
In northern Europe, North America, and others, particularly in the district where an amount of solar radiation is small even in the daytime of winter, the driver is bound to turn on the headlamp of his or her car in the daytime.  Therefore, in the vehicle such as the car, or the like sold in these countries, the configuration for turning on the headlamp as the DRL (Daytime Running Light) is employed, as the lamp control system that is used to turn on the headlamp in a dimmed lighting mode even in the daytime (see JP-A-10-86746 (page 2 to page 5, FIG.  1). 
In turning on the headlamp of the vehicle as the DRL, there are two ways, i.e., one is the method of using the dedicated lamp as such headlamp and the other is the method of applying the existing lamp to such headlamp.  For example, when the bulb having a filament made of halogen, or the like is used as the dedicated lamp, such bulb can be turned on in a dimmed lighting mode by causing the bulb to emit a light at the designated brightness.  While, when the LED is used as the dedicated lamp, such LED can be turned on in a dimmed lighting mode by supplying a predetermined power or current to the LED. 
In contrast, when the existing lamp is also applied to such headlamp, e.g., when the low-beam headlamp or the high-beam headlamp is also applied to such headlamp, these lamps can be turned on in a dimmed lighting mode using a PWM (Pulse WidthModulation) signal, for example, from the viewpoint of energy saving a quantity of light is too much if these lamps are turned on in a full lighting mode.  The PWM signal is defined as such a signal that has a frequency in a range of several hundreds Hz to several tens kHz, for example, and turns on/off the power (voltage/current) request at a particular duty ratio.  When a duty ratio of the PWM signal is set to 50%, half of the power required in the full lighting mode is put into the lamp and thus the brightness emitted from the lamp gives a quantity of light correspondingly.  In case the lamp is turned on in a dimmed lighting mode using the PWM signal, either the bulb using the halogen filament or the LED can be employed as the lamp.