Abstract:
A lighting circuit for lighting a semiconductor light source includes: a DC/DC converter configured to receive a DC first voltage and a DC second voltage and generate a DC third voltage; a first connector including a first terminal, wherein the third voltage is applied to the first terminal, wherein the first connector connects the first terminal and one end of the semiconductor light source; and a control circuit that controls the DC/DC converter. The control circuit selects only the first voltage as a voltage applied to the other end of the semiconductor light source, when a voltage for emitting the semiconductor light source is less than an absolute value of a difference between the first and second voltages. The control circuit selects the first voltage or the second voltage as the voltage applied to the other end thereof, when the voltage is not less than the absolute value.

Description:
[0001]    This application claims priority from Japanese Patent Application No. 2011-141546, filed on Jun. 27, 2011, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a semiconductor light source lighting circuit. 
         [0004]    2. Related Art 
         [0005]    In recent years, an LED, which has a relatively long life and low power consumption, has been used in a vehicle lamp such as a headlight instead of a halogen lamp, which includes filaments. The degree of light emission of an LED, that is, the brightness of an LED, depends on the magnitude of flowing current. Accordingly, a lighting circuit, which adjusts current flowing in an LED, is needed when the LED is used as a light source. 
         [0006]    When a plurality of LEDs connected in series are to be lit, the decision whether to step up or step down a battery voltage in the lighting circuit is made according to the magnitude relationship between the battery voltage and the sum of forward-drop voltages of the LEDs. If dedicated lighting circuits should be designed for each of the respective cases, the variation of the lighting circuit is increased and manufacturing costs could increase. 
         [0007]    Accordingly, a step-up/step-down DC/DC converter, which can cope with a forward drop voltage over a wide range, has been proposed (see Japanese Patent Document No. JP-A-2010-98836). 
         [0008]    However, if the step-up/step-down DC/DC converter is used when the sum of forward drop voltages of the LEDs is higher than the battery voltage, it is disadvantageous in terms of the efficiency of divided electricity, which has a function of stepping a voltage down, as compared to a case where a step-up DC/DC converter is used. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    Some implementations of the present invention may address the foregoing issue as well as other issues. However, the present invention is not required to overcome the disadvantages described above and thus, some implementations of the present invention may not overcome these disadvantages. 
         [0010]    In one aspect, the present disclosure describes a semiconductor light source lighting circuit having high electrical efficiency and capable of allowing a semiconductor light source to emit light over a wide range of a light emission voltage. 
         [0011]    According to one or more aspects, a lighting circuit ( 100 ,  200 ,  300 ) for lighting a semiconductor light source ( 4 ) is described. The circuit includes: a DC/DC converter configured to receive a DC first voltage (V bat ) and a DC second voltage different from the first voltage so as to generate a DC third voltage (V boost ) such that a difference between the third and second voltages is more than a difference between the first and second voltages. The circuit includes a first connector comprising a first terminal (Boost), wherein the third voltage (V boost ) is applied to the first terminal, and the first connector is configured to connect the first terminal and one end of the semiconductor light source. A control circuit is configured to control the DC/DC converter such that a value of current flowing between the DC/DC converter and the first terminal is set to a certain value. The control circuit is configured to select only the first voltage (V bat ) as a voltage applied to the other end of the semiconductor light source, when a light emission voltage (V F ) for emitting the semiconductor light source is less than an absolute value of the difference between the first and second voltages. The control circuit is configured to select the first voltage or the second voltage as the voltage applied to the other end of the semiconductor light source, when the light emission voltage is not less than the absolute value. 
         [0012]    Other aspects, features and advantages of the present invention will be apparent from the following description, the drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a circuit diagram showing the configuration of a semiconductor light source lighting circuit according to a first embodiment, and an in-vehicle battery and an LED connected to the light source lighting circuit. 
           [0014]      FIG. 2  is a graph showing a change over time of a boost voltage when a forward drop voltage is lower than a battery voltage and when a forward drop voltage is equal to or higher than a battery voltage. 
           [0015]      FIG. 3  is a schematic view showing a relationship between an LED-side connector and a three-terminal circuit-side connector of a semiconductor light source lighting circuit according to a second embodiment. 
           [0016]      FIG. 4  is a circuit diagram showing the configuration of the semiconductor light source lighting circuit according to the second embodiment, and an in-vehicle battery and an LED connected to the light source lighting circuit. 
           [0017]      FIG. 5  is a circuit diagram showing the configuration of a semiconductor light source lighting circuit according to a third embodiment, and an in-vehicle battery, a first LED package, a second LED package, and a vehicle ECU (Engine Control Unit) connected to the light source lighting circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Hereinafter, the same or equivalent components, members, and signals, which are shown in the respective drawings, are denoted by the same reference numerals, and the repeated description thereof will be appropriately omitted. Further, some of members, which are not important in the description, will be omitted in the respective drawings. 
       First Embodiment 
       [0019]    A semiconductor light source lighting circuit according to a first embodiment drives an LED that is a light source of a vehicle lamp such as a headlight. The semiconductor light source lighting circuit applies an output voltage of a DC/DC converter to an anode of the LED. The semiconductor light source lighting circuit switches a voltage, which is applied to a cathode of the LED, between a battery voltage and a ground potential, which is a reference potential, according to the magnitude relationship between a battery voltage of an in-vehicle battery and a forward drop voltage of the LED, that is, a light emission voltage that is required to make the LED emit light. Accordingly, it is possible to drive the LED even when the forward drop voltage is lower than the battery voltage, and it is possible further to improve electrical efficiency in driving the LED when the forward drop voltage is not lower than the battery voltage. 
         [0020]      FIG. 1  is a circuit diagram showing the configuration of a semiconductor light source lighting circuit  100  according to a first embodiment, and an in-vehicle battery  2  and an LED  4  connected to the light source lighting circuit  100 . In the illustrated example, the semiconductor light source lighting circuit  100  includes a DC/DC converter  6 , a current detecting resistor  18 , a second switching element  20 , a third switching element  22 , a control circuit  102 , a battery voltage input terminal BATIN, a battery voltage output terminal BATOUT, a ground potential input terminal GNDIN, a ground potential output terminal GNDOUT, and a boost voltage output terminal BOOST. The battery voltage input terminal BATIN is connected to a positive terminal of the in-vehicle battery  2 , and a battery voltage V bat  is applied to the battery voltage input terminal BATIN. A negative terminal of the in-vehicle battery  2  and the ground potential input terminal GNDIN are grounded, and a ground potential is applied to the ground potential input terminal GNDIN. 
         [0021]    The LED  4  is formed of eight in-vehicle LEDs that are connected in series. A forward drop voltage V F  of the LED  4  is the sum of forward drop voltages of the eight in-vehicle LEDs. Current flowing in the LED  4  is referred to as LED current. The semiconductor light source lighting circuit  100  and the LED  4  are mounted on a vehicle lamp. 
         [0022]    The DC/DC converter  6  is a step-up/non-isolated switching regulator that receives the DC battery voltage V bat  and the DC ground potential different from each other and generates a DC boost voltage V boost  by converting the battery voltage V bat  so that a difference between the ground potential and the battery voltage V bat  is increased. The DC/DC converter  6  includes a first capacitor  8 , an inductor  10 , a first switching element  12 , a diode  14  and a second capacitor  16 . 
         [0023]    One end of the first capacitor  8  and one end of the inductor  10  are connected to the battery voltage input terminal BATIN. The other end of the first capacitor  8  is grounded by being connected to the ground potential input terminal GNDIN. The first switching element  12  is composed, for example, of an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The other end of the inductor  10  is connected to the anode of the diode  14  and the drain of the first switching element  12 . The source of the first switching element  12  is grounded. The cathode of the diode  14  is connected to one end of the second capacitor  16  and is connected to one end of the current detecting resistor  18 . The other end of the second capacitor  16  is grounded. The gate of the first switching element  12  receives a pulse-width modulated PWM (Pulse Width Modulation) signal S 1  from the control circuit  102 . The PWM signal S 1  is a signal used to control the LED current that is output to the LED  4  from the DC/DC converter  6 . 
         [0024]    The other end of the current detecting resistor  18  is connected to the boost voltage output terminal BOOST. A resistance value of the current detecting resistor  18  is small, and the voltage drop, which is caused by the LED current flowing in the current detecting resistor  18 , can be detected. However, hereinafter, it is assumed that the voltage drop is negligible compared to the boost voltage V boost . Accordingly, the boost voltage V boost  is applied to the boost voltage output terminal BOOST. 
         [0025]    The second switching element  20  and the third switching element  22  are a P-channel MOSFET and an N-channel MOSFET, respectively. Sources of the second and third switching elements  20  and  22  are connected to the battery voltage input terminal BATIN and the ground potential input terminal GNDIN, respectively. Drains of the second and third switching elements  20  and  22  are connected to the battery voltage output terminal BATOUT and the ground potential output terminal GNDOUT, respectively. The second and third switching elements  20  and  22  are controlled by a battery input control signal S 2  and a ground input control signal S 3  that are input to gates thereof from the control circuit  102 , respectively. 
         [0026]    The boost voltage output terminal BOOST, the battery voltage output terminal BATOUT, and the ground potential output terminal GNDOUT form one three-terminal circuit-side connector. When the three-terminal circuit-side connector is engaged with a corresponding LED-side connector of the LED  4 , the boost voltage output terminal BOOST is connected to the anode side of the LED  4 , and the battery voltage output terminal BATOUT and the ground potential output terminal GNDOUT are connected to the cathode side of the LED  4 . Accordingly, the boost voltage V boost  output from the DC/DC converter  6  is applied to the anode side of the LED  4 . 
         [0027]    The control circuit  102  controls the DC/DC converter  6  so that current flowing between the DC/DC converter  6  and the boost voltage output terminal BOOST, that is, the LED current has a desired value. Further, when the forward drop voltage V F  of the LED  4  is lower than the battery voltage V bat  (i.e., V F &lt;V bat ), the control circuit  102  selects the battery voltage V bat  as a voltage to be applied to the cathode side of the LED  4 . When the battery voltage V bat  is not selected, the control circuit  102  selects the ground potential as a voltage to be applied to the cathode side of the LED  4 . An example in which the battery voltage V bat  is not selected may be a situation in which the forward drop voltage V F  of the LED  4  is equal to or higher than the battery voltage V bat  (i.e., V F &gt;V bat ). 
         [0028]    Accordingly, the battery voltage V bat  is applied to the cathode side of the LED  4  if V F &lt;V bat , and the ground potential is applied to the cathode side of the LED  4  if V F ≧V bat . 
         [0029]    In the illustrated example, the control circuit  102  includes a drive unit  104 , a first differential amplifier  106 , a delay generator  108 , a second differential amplifier  110 , an error amplifier  112 , a comparator  114 , a first buffer  116 , a second buffer  118 , a third buffer  120 , and a reference voltage source  122 . 
         [0030]    The first differential amplifier  106  generates a detection voltage V d , which corresponds to the magnitude of a voltage drop of the current detecting resistor  18 , that is, LED current, by amplifying a difference between a voltage at one end of the current detecting resistor  18  and a voltage at the other end of the current detecting resistor  18 . The first differential amplifier  106  applies the generated detection voltage V d  to an inverting input terminal of the error amplifier  112 . 
         [0031]    The reference voltage source  122  generates a reference voltage V ref  corresponding to a target value of the magnitude of the LED current, and applies the reference voltage V ref  to a non-inverting input terminal of the error amplifier  112 . 
         [0032]    The error amplifier  112  compares the detection voltage V d  and the reference voltage V ref . That is, the error amplifier  112  compares the magnitude of the LED current, which is indicated by the detection voltage V d , with a target value that is indicated by the reference voltage V ref . The error amplifier  112  generates an error voltage V e  that corresponds to a difference between a target value and the magnitude of the LED current, and outputs the error voltage V e  to the drive unit  104 . 
         [0033]    The drive unit  104  controls an on/off duty ratio of the first switching element  12  on the basis of the error voltage V. The drive unit  104  generates the PWM signal S 1  and outputs the PWM signal S 1  to the gate of the first switching element  12  through the third buffer  120 . The drive unit  104  sets a duty ratio of the PWM signal S 1  according to the error voltage V e  so that the magnitude of the LED current approaches a target value. 
         [0034]    The second differential amplifier  110  generates a difference between a voltage applied to the boost voltage output terminal BOOST and a voltage applied to the ground potential output terminal GNDOUT, as an LED voltage V LED . The LED voltage V LED  is a voltage across the LED  4 . When the LED  4  usual emits light, the value of the LED voltage V LED  is the same as the value of the forward drop voltage V F  of the LED  4 . The second differential amplifier  110  applies the generated LED voltage V LED  to a non-inverting input terminal of the comparator  114 . 
         [0035]    The battery voltage V bat  is applied to an inverting input terminal of the comparator  114 . The comparator  114  generates a switching signal S 4 . When the battery voltage V bat  is higher than the LED voltage V LED , the switching signal S 4  is negated, that is, set to a low level. When the battery voltage V bat  is not higher than the LED voltage V LED , the switching signal S 4  is asserted, that is, set to a high level. 
         [0036]    The delay generator  108  prevents the control circuit  102  from selecting a voltage to be applied to the cathode side of the LED  4  until a predetermined delay period passes after power is supplied to the semiconductor light source lighting circuit  100 . In the delay period, the delay generator  108  maintains a state where the battery voltage V bat  is applied to the cathode side of the LED  4 . 
         [0037]    The delay generator  108  is fixed at a low level until the delay period passes after power is supplied to the semiconductor light source lighting circuit  100 . After that, the delay generator  108  generates a delay switching signal S 5  that is equivalent to the switching signal S 4 . The delay switching signal S 5  corresponds to a signal that is obtained by masking the switching signal S 4  at a low level during the delay period. The delay generator  108  outputs the generated delay switching signal S 5  to the gates of the third and second switching elements  22  and  20  through the first and second buffers  116  and  118 , respectively. 
         [0038]    Operation of the semiconductor light source lighting circuit  100  having the foregoing configuration is described next. 
         [0039]      FIG. 2  is a graph showing a change over time of the boost voltage V boost  in the respective cases of V F &lt;V bat  and V F ≧V bat . The solid line of  FIG. 2  represents the change over time of the boost voltage V boost  when V F ≧V bat , and the dashed-dotted line of  FIG. 2  represents a change over time of the boost voltage V boost  when V F &lt;V bat . 
         [0040]    The battery voltage V bat  is applied to the battery voltage input terminal BATIN at a time t 1 , so that power is supplied to the semiconductor light source lighting circuit  100 . Since the delay switching signal S 5  is fixed at a low level during a delay period DP of which a starting point is the time t 1 , the second switching element  20  is in a conducting state and the third switching element  22  is in a non-conducting state. Accordingly, the battery voltage V bat  is applied to the cathode side of the LED  4 . 
         [0041]    The boost voltage V boost  starting to rise at the time t 1  is stabilized near a value, which is obtained by adding the forward drop voltage V F  of the LED  4  to the battery voltage V bat , when the LED  4  emits light. Hereinafter, the forward drop voltage V F , when V F ≧V bat , is referred to as a first forward drop voltage V F1 , and the forward drop voltage V F , when V F &lt;V bat , is referred to as a second forward drop voltage V F2 . When V F ≧V bat , a stabilized value of the boost voltage V boost  is a voltage that is obtained by adding the first forward drop voltage V F1  to the battery voltage V bat . When V F &lt;V bat , a stabilized value of the boost voltage V boost  is a voltage that is obtained by adding the second forward drop voltage V F2  to the battery voltage V bat . 
         [0042]    When V F ≧V bat , at a time t 2  when the delay period DP has passed from the time t 1 , the delay switching signal S 5  is turned to a high level, the second switching element  20  is in a non-conducting state, and the third switching element  22  is in a conducting state. Accordingly, the ground potential is applied to the cathode side of the LED  4 . Then, the boost voltage V boost  drops to the vicinity of the first forward drop voltage V F1  and is stabilized. 
         [0043]    Meanwhile, a dead time may be provided when the voltage applied to the cathode side of the LED  4  is switched. 
         [0044]    When V F &lt;V bat , the delay switching signal S 5  is maintained at a low level even after the time t 2 . Accordingly, the battery voltage V bat  is applied to the cathode side of the LED  4 . 
         [0045]    According to some implementations of the semiconductor light source lighting circuit  100 , it is possible to use the same semiconductor light source lighting circuit  100 , particularly, the same step-up DC/DC converter  6  in any one of the situations, i.e., V F &lt;V bat  and V F ≧V bat . Accordingly, since different semiconductor light source lighting circuits or DC/DC converters do not need to be used depending on, for example, the number or specifications of LEDs and a value of the battery voltage, it is possible to reduce manufacturing costs. 
         [0046]    Further, in some implementations, the semiconductor light source lighting circuit  100  performs driving of the LED  4 , which is caused by a drop in voltage, by applying the battery voltage V bat  to the cathode side of the LED  4  if V F &lt;V bat  and switches the voltage, which is applied to the cathode side of the LED  4 , to the ground potential if V F ≧V bat . Accordingly, it is possible to make the boost voltage at the time of usual lighting lower compared to the case in which the battery voltage V bat  is steadily applied to the cathode side of the LED  4  without the above-mentioned switching function. Therefore, it is possible further to improve the electrical efficiency of the semiconductor light source lighting circuit  100  at the time of usual lighting. As a result, the amount of heat generated can be reduced, and it is possible to use elements that are more compact and inexpensive. 
         [0047]    Furthermore, the voltage applied to the cathode side of the LED  4  can be switched automatically in the semiconductor light source lighting circuit  100  according to this embodiment. Accordingly, even though the forward drop voltage V F  of the LED  4  fluctuates as a result of the variation in temperature material characteristics of the LED, it is possible to select an optimum driving state adaptively. The same also applies to fluctuations of the battery voltage V bat . 
         [0048]    Moreover, a delay period can be provided after the supply of power and an operation for selecting a voltage, which is to be applied to the cathode side of the LED  4 , is stopped during the delay period. Accordingly, it is possible to prevent the voltage, which is applied to the cathode side of the LED  4 , from being switched until the boost voltage V boost  rises and is sufficiently stabilized. As a result, since the determination of whether or not to switch a voltage is made through comparison on the basis of the sufficiently stabilized boost voltage V boost , it is possible to improve the reliability of the determination. Further, even when a voltage is to be switched, it is possible to more smoothly switch the voltage since the DC/DC converter  6  is stabilized sufficiently after the delay period. 
         [0049]    Furthermore, the battery voltage V bat  can be applied to the cathode side of the LED  4  during the delay period. Accordingly, it is possible to prevent a voltage, which significantly exceeds the forward drop voltage V F , from being applied to the LED  4  during the delay period when V F &lt;V bat . 
       Second Embodiment 
       [0050]    In the first embodiment described above, control circuit  102  automatically switches a voltage to be applied to the cathode side of the LED  4  on the basis of the magnitude relationship between the forward drop voltage V F  of the LED  4  and the battery voltage V bat . According to a second embodiment, an LED-side connector corresponding to a three-terminal circuit-side connector  250  of the semiconductor light source lighting circuit  200  includes two terminals, and a corresponding relationship between the two terminals and three terminals of the circuit is then decided on the basis of the magnitude relationship between a known forward-drop voltage V F  and a battery voltage V bat . 
         [0051]      FIG. 3  is a schematic view showing a relationship between the LED-side connector  252  and the three-terminal circuit-side connector  250  of the semiconductor light source lighting circuit  200  according to the second embodiment. The three-terminal circuit-side connector  250  includes a boost voltage output terminal BOOST, a battery voltage output terminal BATOUT, and a ground potential output terminal GNDOUT. A boost voltage V boost  generated by a DC/DC converter  6  is applied to the boost voltage output terminal BOOST, the battery voltage V bat  is applied to the battery voltage output terminal BATOUT, and a ground potential is applied to the ground potential output terminal GNDOUT. 
         [0052]    A module of an LED includes an LED-side connector  252  corresponding to the three-terminal circuit-side connector  250 , LED-side cable harnesses  254 , and an LED. The LED-side connector  252  includes an anode terminal  258  and a cathode terminal  260 , and the anode terminal  258  and the cathode terminal  260  are connected to the anode and cathode of the LED through the LED-side cable harness  254 , respectively. 
         [0053]    The forward-drop voltage V F  of the LED is known in the second embodiment. 
         [0054]    When a forward-drop voltage V F  of an LED  262  is lower than the battery voltage V bat , the LED-side connector  252  is formed so that the boost voltage output terminal BOOST and the anode terminal  258  correspond to each other and the battery voltage output terminal BATOUT and the cathode terminal  260  correspond to each other. Accordingly, when the three-terminal circuit-side connector  250  is engaged with the LED-side connector  252 , the boost voltage output terminal BOOST is connected to an anode of the LED  262  and the battery voltage output terminal BATOUT is connected to a cathode of the LED  262 . 
         [0055]    When a forward-drop voltage V F  of an LED  256  is equal to or higher than the battery voltage V bat , the LED-side connector  252  is formed so that the boost voltage output terminal BOOST and the anode terminal  258  correspond to each other and the ground potential output terminal GNDOUT and the cathode terminal  260  correspond to each other. Accordingly, when the three-terminal circuit-side connector  250  is engaged with the LED-side connector  252 , the boost voltage output terminal BOOST is connected to the anode of the LED  256  and the ground potential output terminal GNDOUT is connected to the cathode of the LED  256 . 
         [0056]    The three-terminal circuit-side connector  250  may be a receptacle that includes, for example, three terminal pins and a housing including three slots in which the terminal pins are held. The LED-side connector  252  may be, for example, a plug that includes two terminal pins and a housing including three slots in which the terminal pins are held. Depending on the magnitude relationship between the forward-drop voltage V F  and the battery voltage V bat , it is decided in which two slots of the three slots of the housing of the plug the terminal pins are held. 
         [0057]      FIG. 4  is a circuit diagram showing the configuration of the semiconductor light source lighting circuit  200  according to the second embodiment, and an in-vehicle battery  2  and an LED  270  connected to the light source lighting circuit  200 .  FIG. 4  shows an example in which a forward-drop voltage V F  of the LED  270  is equal to or higher than the battery voltage V bat , and the cathode side of the LED  270  is connected to the ground potential output terminal GNDOUT. 
         [0058]    The semiconductor light source lighting circuit  200  corresponds to the semiconductor light source lighting circuit  100  according to the first embodiment from which an automatic switching function is excluded. The semiconductor light source lighting circuit  200  includes a DC/DC converter  6 , a current detecting resistor  18 , a control circuit  202 , a battery voltage input terminal BATIN, a battery voltage output terminal BATOUT, a ground potential input terminal GNDIN, a ground potential output terminal GNDOUT, and a boost voltage output terminal BOOST. The control circuit  202  has the same current feedback function as the current feedback function of the control circuit  102  of the first embodiment. 
         [0059]    According to the semiconductor light source lighting circuit  200  of this embodiment, the same advantages as described with respect to the semiconductor light source lighting circuit  100  according to the first embodiment can be obtained in terms of the sharing and electrical efficiency of a semiconductor light source lighting circuit. 
       Third Embodiment 
       [0060]    The second embodiment described a situation in which the ground potential output terminal GNDOUT is connected to the cathode of the LED  256  when the forward drop voltage V F  of the LED  256  is equal to or higher than the battery voltage V bat . A semiconductor light source lighting circuit  300  according to a third embodiment switches a voltage, which is applied to a ground potential output terminal GNDOUT, to a battery voltage V bat  from a ground potential and generates an interruption detection signal S 6  if a predetermined short-circuit condition is satisfied when the ground potential output terminal GNDOUT is connected to the cathode of an LED. 
         [0061]    The short-circuit may be, for example, a condition that an actual measured value of an electrical parameter is within a range of a value of the electrical parameter, a condition that an actual measured value of a forward-drop voltage V F  of the LED is smaller than a known value, or a condition that the actual measured value of the forward-drop voltage V F  of the LED is smaller than a predetermined short-circuit threshold value that is smaller than a known value and larger than the battery voltage V bat , when a short-circuit occurs in the LED. 
         [0062]      FIG. 5  is a circuit diagram showing the configuration of a semiconductor light source lighting circuit  300  according to the third embodiment, and an in-vehicle battery  2 , a first LED package  350 , a second LED package  352 , and a vehicle ECU  358  connected to the light source lighting circuit  300 .  FIG. 5  shows a case that a forward-drop voltage V F  of both the first and second LED packages  350  and  352  is equal to or higher than the battery voltage V bat . The anode side of the first LED package  350 , which is a package including two LEDs connected in series, is connected to a boost voltage output terminal BOOST. The cathode side of the first LED package  350  is connected to the anode side of the second LED package  352 , which includes four LEDs connected in series. The cathode side of the second LED package  352  is connected to the ground potential output terminal GNDOUT. 
         [0063]    The semiconductor light source lighting circuit  300  includes a DC/DC converter  6 , a current detecting resistor  18 , a control circuit  302 , a switching diode  354 , a fourth switching element  356 , a battery voltage input terminal BATIN, a battery voltage output terminal BATOUT, a ground potential input terminal GNDIN, a ground potential output terminal GNDOUT, and a boost voltage output terminal BOOST. 
         [0064]    The anode of the switching diode  354  is connected to the ground potential output terminal GNDOUT, and the cathode of the switching diode  354  is connected to the battery voltage output terminal BATOUT. 
         [0065]    The fourth switching element  356  is an N-channel MOSFET, and the drain of the fourth switching element  356  is connected to the anode of the switching diode  354  and the ground potential output terminal GNDOUT, and a source of the fourth switching element  356  is connected to the ground potential input terminal GNDIN. The fourth switching element  356  is controlled by a short-circuit switching signal S 7  that is provided to its gate thereof from the control circuit  302 . 
         [0066]    The control circuit  302  has the same current feedback function as the current feedback function of the control circuit  102  of the first embodiment. The control circuit  302  monitors the forward-drop voltage V F  of both the first and second LED packages  350  and  352 . When the actual measured value of the forward-drop voltage V F  is smaller than a short-circuit threshold value, the control circuit  302  selects the battery voltage V bat  as a voltage, which is to be applied to the ground potential output terminal GNDOUT, and generates the interruption detection signal S 6 . In particular, when the actual measured value of the forward-drop voltage V F  is smaller than the short-circuit threshold value, the control circuit  302  switches the fourth switching element  356  to a non-conducting state from a conducting state by converting the short-circuit switching signal S 7  to a low level from a high level. Accordingly, a voltage, which is obtained by adding a forward-drop voltage of the switching diode  354  to the battery voltage V bat , instead of a ground potential is applied to the ground potential output terminal GNDOUT. 
         [0067]    The control circuit  302  sends the generated interruption detection signal S 6  to an external vehicle ECU  358 . 
         [0068]    According to the semiconductor light source lighting circuit  300  of this embodiment, the same advantages as the advantages of the semiconductor light source lighting circuit  100  according to the first embodiment can be obtained in terms of the sharing and electrical efficiency of a semiconductor light source lighting circuit. 
         [0069]    Further, when any one of the first and second LED packages  350  and  352  is short-circuited, there is a possibility that the forward-drop voltage V F  of both the first and second LED packages  350  and  352  is lower than the battery voltage V bat . Accordingly, in the semiconductor light source lighting circuit  300  according to this embodiment, a voltage applied to the ground potential output terminal GNDOUT is switched to the battery voltage V bat  from a ground potential when such a short-circuit of the package is detected. For this reason, it is possible to maintain the lighting of the LED. Furthermore, the vehicle ECU  358  can perform appropriate processing in accordance with the interruption detection signal S 6 . 
         [0070]    Various semiconductor light source lighting circuits have been described. These embodiments are illustrative, and it is understood by those skilled in the art that each of the components of the embodiments or the combination of the respective processing may have various modifications, and the modifications are also included in the scope of the invention. Moreover, the embodiments may be combined with each other. For example, the short-circuit detecting/switching function of the semiconductor light source lighting circuit  300  according to the third embodiment may be introduced into the semiconductor light source lighting circuit  100  according to the first embodiment. 
         [0071]    A situation in which the boost voltage output terminal BOOST, the battery voltage output terminal BATOUT, and the ground potential output terminal GNDOUT form one three-terminal circuit-side connector was described in connection with the first embodiment, but the invention is not limited to such an arrangement. For example, new terminals, which are connected in the semiconductor light source lighting circuit, are provided at both the drains of the second and third switching elements  20  and  22  instead of the battery voltage output terminal BATOUT and the ground potential output terminal GNDOUT, and the new terminals and the boost voltage output terminal BOOST may form a two-terminal circuit-side connector. 
         [0072]    In the first embodiment, the second switching element  20  may be substituted with a diode. In this case, the anode of the diode is connected to the battery voltage output terminal BATOUT, and the cathode of the diode is connected to the battery voltage input terminal BATIN. According to this modification, one switch of an object to be controlled is reduced as compared to the first embodiment. Accordingly, it is possible to simplify control. However, the electrical efficiency of the first embodiment may be better than that of this modification due to a forward-drop voltage of the diode. 
         [0073]    A situation in which the battery voltage V bat  is used as a threshold value of a forward-drop voltage of an LED used to select a voltage, which is to be applied to the cathode side of an LED of an object to be driven, was described in connection with the first to third embodiments, but the invention is not limited to such arrangements. For example, a voltage higher than the battery voltage V bat  may be used as the threshold value. 
         [0074]    A situation in which the positive boost voltage V boost  is generated to drive the LED was described in the first to third embodiments, but the invention is not limited to such arrangements. The technical idea of the first, second, or third embodiment also may be applied to a situation in which a negative boost voltage is generated to drive the LED. 
         [0075]    While aspects of embodiments of the present invention have been shown and described above, other implementations are within the scope of the claims. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.