Abstract:
Provided is an LED drive circuit capable of controlling a current caused to flow through an LED at a proper value even when a power supply voltage is higher than a sum of a forward voltage of the LED to be driven and a voltage of a current sense element. In a configuration where an inductor and a rectifying element are connected in series to each other, at each one terminal thereof and the LED and a current sense element, which are connected in series to each other, are a smoothing capacity are respectively connected to the other terminals of the inductor and the rectifying device, in parallel with each other, the LED and these elements are separated from a power supply or a ground by a switching element.

Description:
[0001]     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2005-147453 filed May 20, 2005, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a light emitting diode drive circuit having a DC-DC converter circuit for driving a light emitting diode (hereinafter referred to as “LED”) at a constant current.  
         [0004]     2. Description of the Related Art  
         [0005]     As a common DC-DC converter circuit for driving an LED at a constant current, a booster circuit shown in  FIG. 2  is widely known.  
         [0006]     A smoothing capacity  104  is connected between a rectifying device  103  and a ground (GND). In addition, a switching element  105  is connected between a connecting point at which an inductor  102  and the rectifying device  103  are connected to each other, and the ground. An LED  108  and a current sense element  107  are connected in series to each other between a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, and the ground. Further, an output of the current sense element  107  is connected to a control circuit  106 , and an output of the control circuit  106  is connected to the switching element  105 .  
         [0007]     The control circuit  106  monitors a voltage of the current sense element  107 , and controls short-circuiting and open-circuiting of the switching element  105 , thereby controlling a current caused to flow through the LED  108  at a proper value to cause the LED to emit light properly. In other words, in order to cause a proper current to flow through the LED  108 , a voltage of the smoothing capacity  104  is controlled so that the voltage becomes a sum of a forward voltage when a proper current is caused to flow through the LED  108 , and a voltage generated when a proper current is caused to flow through the current sense element  107 .  
         [0008]     However, in driving an LED in the boost DC-DC converter circuit shown in  FIG. 2 , there is a problem in that, when a power supply voltage is increased to be higher than a forward voltage of the LED to be driven, a current flowing through the LED cannot be controlled.  
         [0009]     In other words, provided that a voltage generated at the time when a current is caused to flow through the rectifying device  103 , is set to  0  V, when a voltage of a power supply  101  exceeds a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , a current caused to flow through the LED  108  and the current sense element  107  each are increased to be larger than a proper value. As a result, the LED emits light excessively, and at worst, the LED may break down.  
       SUMMARY OF THE INVENTION  
       [0010]     In order to solve the above-mentioned problem with the conventional art, the present invention therefore has an object to provide a technique for causing a proper current to flow through an LED even when a power supply voltage is increased to be a high voltage.  
         [0011]     In order to solve the above-mentioned problem, the present invention provides a structure in which an inductor and a rectifying device are connected in series to each other, and an LED and a current sense element, which are connected in series to each other at each one terminal thereof, and a smoothing capacity are respectively connected to the other terminal of the inductor and the rectifying device, in parallel with each other.  
         [0012]     According to the present invention, it is possible to cause a proper current to flow through an LED even when a power supply voltage is a high voltage in driving the LED in a DC-DC converter circuit. Further, when a switching element is turned off, a power supply voltage is not applied to the LED and the current sense element, thereby making it possible to reduce current consumption without providing another switching element.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     In the accompanying drawings:  
         [0014]      FIG. 1  is a diagram showing an LED drive circuit according to a first embodiment of the present invention;  
         [0015]      FIG. 2  is a diagram showing a conventional LED drive circuit;  
         [0016]      FIG. 3  is a diagram showing an LED drive circuit according to a second embodiment of the present invention;  
         [0017]      FIG. 4  is a diagram showing an LED drive circuit according to a third embodiment of the present invention;  
         [0018]      FIG. 5  is a diagram showing an LED drive circuit according to a fourth embodiment of the present invention;  
         [0019]      FIG. 6  is a diagram showing an LED drive circuit according to a fifth embodiment of the present invention;  
         [0020]      FIG. 7  is a diagram showing an LED drive circuit according to a sixth embodiment of the present invention;  
         [0021]      FIG. 8  is a diagram showing an LED drive circuit according to a seventh embodiment of the present invention;  
         [0022]      FIG. 9  is a diagram showing an LED drive circuit according to an eighth embodiment of the present invention;  
         [0023]      FIG. 10  is a diagram showing an LED drive circuit according to a ninth embodiment of the present invention;  
         [0024]      FIG. 11  is a diagram showing an LED drive circuit according to a tenth embodiment of the present invention;  
         [0025]      FIG. 12  is a diagram showing an LED drive circuit according to an eleventh embodiment of the present invention; and  
         [0026]      FIG. 13  is a diagram showing an LED drive circuit according to a twelfth embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0027]      FIG. 1  is a circuit diagram according to a first embodiment of the present invention. An inductor  102  and a rectifying device  103  are connected in series with a power supply  101 , and a smoothing capacity  104  is connected in parallel with the inductor  102  and the rectifying device  103 . An LED  108  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, one terminal of a current sense element  107  is connected to the other terminal of the LED  108 , and the other terminal of the current sense element  107  is connected to the power supply  101 . A control circuit  106  is connected to both terminals of the current sense element  107 , a switching element  105  is connected between a connecting point at which the inductor  102  and the rectifying device  103  are connected to each other, and a ground, and an output of the control circuit  106  is connected to the switching element  105 .  
         [0028]     A feature of the first embodiment resides in that the smoothing capacity  104 , and the LED  108  and the current sense element  107  which are connected in series to each other, are connected in parallel with each other at both terminals each of the inductor  102  and the rectifying device  103  which are connected in series to each other. With such a configuration, when the switching element  105  is short-circuited, the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited, the electric power of the inductor  102  emits directly to the LED  108 , the current sense element  107 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of a voltage of the power supply  101 .  
         [0029]     Accordingly, assuming that a voltage generated in the rectifying device  103  is set to 0 V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Second Embodiment  
       [0030]      FIG. 3  is a circuit diagram according to a second embodiment of the present invention. The second embodiment is different from the first embodiment in that the LED  108  and the current sense device  107  are exchanged.  
         [0031]     The circuit according to the second embodiment has a configuration in which the power supply  101  is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the rectifying device  103 , the other terminal of the rectifying device  103  is connected to one terminal of the smoothing capacity  104 , and the other terminal of the smoothing capacity  104  is connected to the power supply  101 . A connecting point at which the inductor  102  and the rectifying device  103  are connected to each other, is connected to one terminal of the switching element  105 , and the other terminal of the switching element  105  is connected to the ground. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, one terminal of the current sense element  107  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, the other terminal of the current sense element  107  is connected to one terminal of the LED  108 , and the other terminal of the LED  108  is connected to the power supply  101 , thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0032]     A feature of a configuration of  FIG. 3  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0033]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Third Embodiment  
       [0034]      FIG. 4  is a circuit diagram according to a third embodiment of the present invention. The third embodiment is different from the first embodiment in that the current sense element  107  is placed between the LEDs  108 .  
         [0035]     The circuit according to the third embodiment has a configuration in which the power supply  101  is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the rectifying device  103 , the other terminal of the rectifying device  103  is connected to one terminal of the smoothing capacity  104 , and the other terminal of the smoothing capacity  104  is connected to the power supply  101 . A connecting point at which the inductor  102  and the rectifying device  103  are connected to each other, is connected to one terminal of the switching element  105 , and the other terminal of the switching element  105  is connected to the ground. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, is connected to one terminal of one of the LEDs  108 , the other terminal of one of the LEDs  108  is connected to one terminal of the current sense  107 , the other terminal of the current sense  107  is connected to one terminal of the other of the LEDs  108 , and the other terminal of the other of the LEDs  108  is connected to the power supply  101 , thereby making it possible to drive the LEDs  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0036]     A feature of a configuration of  FIG. 4  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0037]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Fourth Embodiment  
       [0038]      FIG. 5  is a circuit diagram according to a fourth embodiment of the present invention. The fourth embodiment is different from the first embodiment in a position in which the rectifying device  103  is inserted. That is, the rectifying device  103  is inserted between a connecting point at which the inductor  102  and the power supply  101  are connected to each other, and a connecting point at which the smoothing capacity  104  and the current sense element  107  are connected to each other.  
         [0039]     The circuit according to the fourth embodiment has a configuration in which the power supply  101  is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the smoothing capacity  104 , the other terminal of the smoothing capacity  104  is connected to one terminal of the rectifying device  103 , and the other terminal of the rectifying device  103  is connected to the power supply  101 . A connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the switching element  105 , and the other terminal of the switching element  105  is connected to the ground. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the LED  108 , the other terminal of the LED  108  is connected to one terminal of the current sense element  107 , and the other terminal of the current sense element  107  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0040]     A feature of a configuration of  FIG. 5  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0041]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Fifth Embodiment  
       [0042]      FIG. 6  is a circuit diagram according to a fifth embodiment of the present invention. The fifth embodiment is different from the second embodiment in a position in which the rectifying device  103  is inserted. That is, the rectifying device  103  is inserted between a connecting point at which the inductor  102  and the power supply  101  are connected to each other, and a connecting point at which the smoothing capacity  104  and the LED  108  are connected to each other.  
         [0043]     The circuit according to the fifth embodiment has a configuration in which the power supply  101  is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the smoothing capacity  104 , the other terminal of the smoothing capacity  104  is connected to one terminal of the rectifying device  103 , and the other terminal of the rectifying device  103  is connected to the power supply  101 . A connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the switching element  105 , and the other terminal of the switching element  105  is connected to the ground. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to one terminal of the LED  108 , and the other terminal of the LED  108  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0044]     A feature of a configuration of  FIG. 6  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0045]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current is caused to flow through the LED  108  and a voltage generated due to the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Sixth Embodiment  
       [0046]      FIG. 7  is a circuit diagram according to a sixth embodiment of the present invention. The sixth embodiment is different from the third embodiment in a position in which the rectifying device  103  is inserted. That is, the rectifying device  103  is inserted between a connecting point at which the inductor  102  and the power supply  101  are connected to each other, and a connecting point at which the smoothing capacity  104  and the LED  108  are connected to each other.  
         [0047]     The circuit according to the sixth embodiment has a configuration in which the power supply  101  is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the smoothing capacity  104 , the other terminal of the smoothing capacity  104  is connected to one terminal of the rectifying device  103 , and the other terminal of the rectifying device  103  is connected to the power supply  101 . A connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the switching element  105 , and the other terminal of the switching element  105  is connected to the ground. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of one of the LEDs  108 , the other terminal of one of LEDs  108  is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to one terminal of the other of the LEDs  108 , and the other terminal of the other of the LEDs  108  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LEDs  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0048]     A feature of a configuration of  FIG. 7  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0049]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current is caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Seventh Embodiment  
       [0050]      FIG. 8  is a circuit diagram according to a seventh embodiment of the present invention. The seventh embodiment is different from the fifth embodiment in a position in which the switching element  105  is inserted. That is, the switching element  105  is inserted between the power supply  101  and a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other.  
         [0051]     The circuit according to the seventh embodiment has a configuration in which the ground is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the rectifying device  103 , the other terminal of the rectifying device  103  is connected to one terminal of the smoothing capacity  104 , and the other terminal of the smoothing capacity  104  is connected to the ground. The power supply  101  is connected to one terminal of the switching element  105 , the other terminal of the switching element  105  is connected to a connecting point at which the inductor  102  and the rectifying device  103  are connected to each other. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to one terminal of the LED  108 , the other terminal of the LED  108  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0052]     A feature of a configuration of  FIG. 8  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0053]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Eighth Embodiment  
       [0054]      FIG. 9  is a circuit diagram according to an eighth embodiment of the present invention. The eighth embodiment is different from the fourth embodiment in a position in which the switching element  105  is inserted. That is, the switching element  105  is inserted between the power supply  101  and a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other.  
         [0055]     The circuit according to the eighth embodiment has a configuration in which the ground is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the rectifying device  103 , the other terminal of the rectifying device  103  is connected to one terminal of the smoothing capacity  104 , and the other terminal of the smoothing capacity  104  is connected to the ground. The power supply  101  is connected to one terminal of the switching element  105 , the other terminal of the switching element  105  is connected to a connecting point at which the inductor  102  and the rectifying device  103  are connected to each other. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point between the inductor  102  and the smoothing capacity  104  is connected to one terminal of the LED  108 , the other terminal of the LED  108  is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0056]     A feature of a configuration of  FIG. 9  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0057]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Ninth Embodiment  
       [0058]      FIG. 10  is a circuit diagram according to a ninth embodiment of the present invention. The ninth embodiment is different from the sixth embodiment in a position in which the switching element  105  is inserted. That is, the switching element  105  is inserted between the power supply  101  and a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other.  
         [0059]     The circuit according to the ninth embodiment has a configuration in which the ground is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the rectifying device  103 , the other terminal of the rectifying device  103  is connected to one terminal of the smoothing capacity  104 , and the other terminal of the smoothing capacity  104  is connected to the ground. The power supply  101  is connected to one terminal of the switching element  105 , the other terminal of the switching element  105  is connected to a connecting point at which the inductor  102  and the rectifying device  103  are connected to each other. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, is connected to one terminal of one of the LEDs  108 , the other terminal of one of the LEDs  108  is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to one terminal of the other of the LEDs  108 , and the other terminal of the other of the LEDs  108  is connected to a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0060]     A feature of a configuration of  FIG. 10  resides in that both terminals of the inductor  102  and the rectifying device  103 , which are connected in series to each other, are connected to both terminals of the smoothing capacity  104 , and both terminals of the current sense element  107  and the LED  108 , which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104  through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0061]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to 0 V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Tenth Embodiment  
       [0062]      FIG. 11  is a circuit diagram according to a tenth embodiment of the present invention. The tenth embodiment is different from the second embodiment in a position in which the switching element  105  is inserted. That is, the switching element  105  is inserted between the power supply  101  and a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other.  
         [0063]     The circuit according to the tenth embodiment has a configuration in which the ground is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the smoothing capacity  104 , the other terminal of the smoothing capacity  104  is connected to one terminal of the rectifying device  103 , and the other terminal of the rectifying device  103  is connected to the ground. The power supply  101  is connected to one terminal of the switching element  105 , the other terminal of the switching element  105  is connected to a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to one terminal of the LED  108 , the other terminal of the LED  108  is connected to a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0064]     A feature of a configuration of  FIG. 11  resides in that the inductor  102  and the rectifying device  103  are connected in series to each other at each one terminal thereof, and the other terminals of the inductor  102  and the rectifying device  103  are connected to both terminals of the smoothing capacity  104 , and to both terminals each of the current sense element  107  and the LED  108  which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104 , through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0065]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a power current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Eleventh Embodiment  
       [0066]      FIG. 12  is a circuit diagram according to an eleventh embodiment of the present invention. The eleventh embodiment is different from the first embodiment in a position in which the switching element  105  is inserted. That is, the switching element  105  is inserted between the power supply  101  and a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other.  
         [0067]     The circuit according to the eleventh embodiment has a configuration in which the ground is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the smoothing capacity  104 , the other terminal of the smoothing capacity  104  is connected to one terminal of the rectifying device  103 , and the other terminal of the rectifying device  103  is connected to the ground. The power supply  101  is connected to one terminal of the switching element  105 , the other terminal of the switching element  105  is connected to a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, is connected to one terminal of the LED  108 , the other terminal of the LED  108  is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0068]     A feature of a configuration of  FIG. 12  resides in that the inductor  102  and the rectifying device  103  are connected in series to each other at each one terminal thereof, and therefore terminals of the inductor  102  and the rectifying device  103  are connected to both terminals of the smoothing capacity  104 , and to both terminals each of the current sense element  107  and the LED  108  which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104 , through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0069]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.  
       Twelfth Embodiment  
       [0070]      FIG. 13  is a circuit diagram according to a twelfth embodiment of the present invention. The twelfth embodiment is different from the third embodiment in a position in which the switching element  105  is inserted. That is, the switching element  105  is inserted between the power supply  101  and a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other.  
         [0071]     The circuit according to the twelfth embodiment has a configuration in which the ground is connected to one terminal of the inductor  102 , the other terminal of the inductor  102  is connected to one terminal of the smoothing capacity  104 , the other terminal of the smoothing capacity  104  is connected to one terminal of the rectifying device  103 , and the other terminal of the rectifying device  103  is connected to the ground. The power supply  101  is connected to one terminal of the switching element  105 , the other terminal of the switching element  105  is connected to a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other. In the switching element  105 , short-circuiting and open-circuiting are repeated, thereby causing the inductor  102  to charge/discharge electric power to cause the smoothing capacity  104  to generate a voltage through the rectifying device  103 . Further, a connecting point at which the rectifying device  103  and the smoothing capacity  104  are connected to each other, is connected to one terminal of one of the LEDs  108 , the other terminal of one of the LEDs  108  is connected to one terminal of the current sense element  107 , the other terminal of the current sense element  107  is connected to one terminal of the other one of the LEDs  108 , the other terminal of the other one of the LEDs  108  is connected to a connecting point at which the inductor  102  and the smoothing capacity  104  are connected to each other, thereby making it possible to drive the LED  108  at a voltage generated in the smoothing capacity  104 . Further, the control circuit  106  is connected to both terminals of the current sense element  107 , and a timing of short-circuiting and open-circuiting of the switching element  105  is adjusted in the control circuit  106 , thereby making it possible to control a current flowing through the LED  108  at a proper value to cause the LED to emit light properly.  
         [0072]     A feature of a configuration of  FIG. 13  resides in that the inductor  102  and the rectifying device  103  which are connected in series to each other at each one terminal thereof, and the other terminals of the inductor  102  and the rectifying device  103  are connected to both terminals of the smoothing capacity  104 , and to both terminals each of the current sense element  107  and the LED  108  which are connected in series to each other, in parallel with each other. With such the configuration, when the switching element  105  is short-circuited and the inductor  102  is charged with electric power, and then the switching element  105  is open-circuited and electric power is discharged from the inductor  102 , the inductor  102  emits electric power directly to the current sense element  107 , the LED  108 , and the smoothing capacity  104 , through the rectifying device  103 , thereby making it possible to drive the LED  108  irrespective of the voltage of the power supply  101 .  
         [0073]     Therefore, with such the configuration, assuming that a voltage generated in the rectifying device  103  is set to  0  V, even when a voltage of a power supply  101  is higher than a sum of a forward voltage generated due to a proper current caused to flow through the LED  108  and a voltage generated due to a proper current caused to flow through the current sense element  107 , it is possible to cause a proper current to flow without making the LED  108  to emit light excessively.