Abstract:
A light emitting drive device has an output voltage supply unit, and an emergency drive unit. The output voltage supply unit generates an output voltage from an input voltage on the basis of a control signal transmitted from a control unit, and supplies at least one light emitting element with the output voltage. In the cases where the emergency drive unit received a signal indicating abnormality of the control unit, the emergency drive unit lights the whole or a part of the at least one light emitting element irrespective of the control signal transmitted from the control unit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a light-emitting element driving device for driving a light-emitting element, and to a light-emitting device and a vehicle that employ a light-emitting element driving device. 
       BACKGROUND ART 
       [0002]      FIG. 16  is a diagram showing one example of a light-emitting device. The light-emitting device of this example includes at least one light-emitting element (in  FIG. 16 , light-emitting diodes) Z 1 , a light-emitting element driver IC (integrated circuit)  100  which drives the light-emitting element Z 1 , a P-channel MOS (metal-oxide-semiconductor) transistor P 1  which is inserted in the power feed path from the light-emitting element driver IC  100  to the light-emitting element Z 1 , a controller IC  200  which feeds the light-emitting element driver IC  100  with a PWM (pulse-width modulation) dimming signal S 1 , and a monitor IC  300  which checks for a fault in the controller IC  200 . 
         [0003]    The light-emitting element driver IC  100  is a semiconductor integrated circuit device that has integrated into it an output voltage generator  101 , which generates a constant output voltage Vo from an input voltage Vi to feed the output voltage Vo to the light-emitting element Z 1 , and an output current controller  102 , which turns the transistor P 1  ON and OFF according to the PWM dimming signal S 1  output from the controller IC  200  to control the duty of an output current Io through the light-emitting element Z 1  (thereby to control the luminance of the light-emitting element Z 1 ). To increase the luminance of the light-emitting element Z 1 , the ON duty of the PWM dimming signal S 1  (the proportion of the ON period Ton in the period T shown in  FIG. 17 ) is set at a larger value; reversely, to decrease the luminance of the light-emitting element Z 1 , the ON duty of the PWM dimming signal S 1  is set at a smaller value. The light-emitting element driver IC  100  also has external terminals T 11  to T 14 . To the external terminal T 11 , the input voltage Vi is applied. From the external terminal T 12 , the output voltage Vo is output. To the external terminal T 13 , the PWM dimming signal S 1  is fed. From the external terminal T 14 , a gate control signal for turning the transistor P 1  ON and OFF is output. 
         [0004]    The controller IC  200  is a semiconductor integrated circuit device that has integrated into it a clock circuit  201 , which generates a clock signal S 2 , and a PWM circuit  202 , which generates the PWM dimming signal S 1  based on the clock signal S 2 . The controller IC  200  also has external terminals T 21  to T 23 . From the external terminal T 21 , the PWM dimming signal S 1  is output. From the external terminal T 22 , the clock signal S 2  is output. To the external terminal T 23 , the monitoring result signal S 3  is fed. There is no particular restriction on how the controller IC  200  uses the monitoring result signal S 3 ; for example, in one conceivable configuration, when the monitoring result signal S 3  is in a state that indicates a fault in the controller IC  200 , the operation of the controller IC  200  can be stopped completely for increased safety. 
         [0005]    The monitor IC  300  has external terminals T 31  and T 32 . Based on the clock signal S 2  fed to the external terminal T 31 , the monitor IC  300  checks whether or not the controller IC  200  is faulty to output the check result, as the monitoring result signal S 3 , from the external terminal T 32 . In this example, when the controller IC  200  is not faulty, the monitoring result signal S 3  is at HIGH level, and when the controller IC  200  is faulty, the monitoring result signal S 3  is at LOW level. That is, a LOW-level monitoring result signal S 3  serves as a signal that indicates a fault in the controller IC  200 . 
         [0006]    Although in  FIG. 16  the monitoring result signal S 3  is fed only to the controller IC  200 , this is not meant as any limitation; for example, instead of the controller IC  200 , or in addition to the controller IC  200 , a controller that controls the entire appliance that incorporates the light-emitting device shown in  FIG. 16  may be fed with the monitoring result signal S 3 . Although in  FIG. 16  the monitor IC  300  checks for only a fault in the controller IC  200 , a configuration is also possible where the controller IC  200  and the monitor IC  300  each check for a fault in the other. 
       LIST OF CITATIONS 
     Patent Literature 
       [0007]    Patent Document 1: Japanese Patent Application published as No. 2013-217213 
         [0008]    Patent Document 2: Japanese Patent Application published as No. 2007-218196 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0009]    One conceivable application of the light-emitting device shown in  FIG. 16  is in lights mounted on vehicles. 
         [0010]    It is preferable that vehicles be furnished with a limp-home capability that enables them to move over a short distance to a safe place in case a failure prevents them from travelling normally. For example, Patent Document 1 discloses a limp-home capability related to a common-rail fuel injection control device in vehicles, and Patent Document 2 discloses a limp-home capability related to a DPF (diesel particulate filter) in vehicles. 
         [0011]    Inconveniently, however, the light-emitting device shown in  FIG. 16  has the following drawback. When a fault in the controller IC  200  causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 17 ), no pulses appear in the PWM dimming signal S 1  any longer (see broken lines in the PWM dimming signal S 1  shown in  FIG. 17 ); this makes the ON duty of the PWM dimming signal S 1  zero, and makes the output current Io zero. This means that it is impossible to furnish lights for vehicles that incorporate the light-emitting device shown in  FIG. 16  with a limp-home capability for night travel and the like. 
         [0012]    Against the background discussed above, an object of the present invention is to provide a light-emitting element driving device that operates based on a control signal from a controller so long as the controller is operating normally but that turns ON a light-emitting element irrespective of the control signal from the controller when the controller is operating abnormally, and another object of the present invention is to provide a light-emitting device and a vehicle that employ such a light-emitting element driving device. 
       Means for Solving the Problem 
       [0013]    To achieve the above objects, according to one aspect of the present invention, a light-emitting element driving device includes: an output voltage feeder configured to generate an output voltage from an input voltage based on a control signal from a controller to feed the output voltage to at least one light-emitting element; and an emergency driver configured to turn ON all or part of the at least one light-emitting element irrespective of the control signal from the controller on receiving a signal indicating a fault in the controller. (A first configuration.) 
         [0014]    In the light-emitting element driving device of the first configuration described above, preferably, the value of the current that the emergency driver outputs to all or the part of the at least one light-emitting element is smaller than the value of the current that the output voltage feeder outputs to the at least one light-emitting element. (A second configuration.) 
         [0015]    In the light-emitting element driving device of the first or second configuration described above, preferably, the output voltage feeder and the emergency driver share the following circuit blocks between them: a high-side transistor and a low-side transistor connected between a terminal to which the input voltage is applied and a ground terminal, the connection node between the high-side and low-side transistors being connected via a coil to an output capacitor; a high-side driver and a low-side driver configured to generate drive control signals for the high-side and low-side transistors respectively; an amplifier configured to generate a feedback voltage commensurate with the current through the at least one light-emitting element; and a driver controller configured to drive the high-side and low-side drivers so as to turn the high-side and low-side transistors ON and OFF according to the feedback voltage. Here, the high-side and low-side transistors, the high-side and low-side drivers, the amplifier, and the driver controller are all integrated in a single semiconductor device. (A third configuration.) 
         [0016]    In the light-emitting element driving device of the third configuration described above, preferably, the emergency driver includes a selector configured to select and output the control signal when no signal indicating a fault in the controller is received and to select and output a constant voltage when a signal indicating a fault in the controller is received. Here, the driver controller is configured to switch whether or not to drive the high-side and low-side drivers according to the output of the selector. (A fourth configuration.) 
         [0017]    In the light-emitting element driving device of the fourth configuration described above, preferably, the selector is integrated in the single semiconductor device. (A fifth configuration.) 
         [0018]    According to another aspect of the present invention, a light-emitting device includes: a light-emitting element driving device of any one of the first to fifth configurations described above; at least one light-emitting element configured to be driven by the light-emitting element driving device; a controller configured to feed a control signal to the light-emitting element driving device; and a monitor configured to monitor the controller to transmit, on detecting a fault in the controller, a signal indicating the fault in the controller to the light-emitting element driving device. (A sixth configuration.) 
         [0019]    In the light-emitting device of the sixth configuration described above, preferably, the light-emitting element is a light-emitting diode or an organic EL element. (A seventh configuration.) 
         [0020]    In the light-emitting device of the seventh configuration described above, preferably, the light-emitting device is used as a vehicle-mounted lamp. (An eighth configuration.) 
         [0021]    In the light-emitting device of the eighth configuration described above, preferably, the light-emitting device is mounted as a head light module, a turn lamp module, or a rear lamp module on a vehicle. (A ninth configuration.) 
         [0022]    According to yet another aspect of the present invention, a vehicle includes a light-emitting device of the eighth or ninth configuration described above. (A tenth configuration.) 
         [0023]    In the vehicle of the tenth configuration described above, preferably, the light-emitting device is used as one of a head light, a light source for daytime running light, a tail lamp, a stop lamp, and a turn lamp. (An eleventh configuration.) 
       Advantageous Effects of the Invention 
       [0024]    According to the present invention, it is possible to provide a light-emitting element driving device that operates based on a control signal from a controller so long as the controller is operating normally but that turns ON a light-emitting element irrespective of the control signal from the controller when the controller is operating abnormally, and to provide a light-emitting device and a vehicle that employ such a light-emitting element driving device. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a diagram showing a light-emitting device according to a first embodiment; 
           [0026]      FIG. 2  is a timing chart illustrating an example of the operation of a light-emitting device according to the first or a third embodiment; 
           [0027]      FIG. 3  is a diagram showing a light-emitting device according to a second embodiment; 
           [0028]      FIG. 4  is a timing chart illustrating an example of the operation of a light-emitting device according to the second or a fourth embodiment; 
           [0029]      FIG. 5  is a diagram showing a light-emitting device according to the third embodiment; 
           [0030]      FIG. 6  is a diagram showing a light-emitting device according to the fourth embodiment; 
           [0031]      FIG. 7  is a diagram showing a light-emitting device according to a fifth embodiment; 
           [0032]      FIG. 8  is a timing chart illustrating an example of the operation of a light-emitting device according to the fifth embodiment; 
           [0033]      FIG. 9  is a diagram showing a light-emitting device according to a sixth embodiment; 
           [0034]      FIG. 10  is a timing chart illustrating an example of the operation of a light-emitting device according to the sixth embodiment; 
           [0035]      FIG. 11  is an exterior view (front side) of a vehicle having light-emitting devices mounted on it; 
           [0036]      FIG. 12  is an exterior view (rear side) of a vehicle having light-emitting devices mounted on it; 
           [0037]      FIG. 13  is an exterior view of an LED head light module; 
           [0038]      FIG. 14  is an exterior view of an LED turn lamp module; 
           [0039]      FIG. 15  is an exterior view of an LED rear lamp module; 
           [0040]      FIG. 16  is a diagram showing an example of a light-emitting device; and 
           [0041]      FIG. 17  is a timing chart illustrating an example of the operation of the light-emitting device shown in  FIG. 16 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0042]      FIG. 1  is a diagram showing a light-emitting device according to a first embodiment. In  FIG. 1 , such parts as are found also in  FIG. 16  are identified by common reference numerals, and no detailed description will be repeated as to them. 
         [0043]    The light-emitting device shown in  FIG. 1  includes at least one light-emitting element (in  FIG. 1 , light-emitting diodes) Z 1 , a light-emitting element driver IC  110  which drives the light-emitting element Z 1 , a transistor P 1  which is inserted in the power feed path from the light-emitting element driver IC  110  to the light-emitting element Z 1 , a controller IC  200  which feeds the light-emitting element driver IC  110  with a PWM dimming signal S 1 , and a monitor IC  300  which checks for a fault in the controller IC  200 . 
         [0044]    The light-emitting element driver IC  110 , as compared with the light-emitting element driver IC  100  shown in  FIG. 16 , additionally includes a switch SW 1  and an external terminal T 15 . To the external terminal T 15 , a monitoring result signal S 3  is fed. The switch SW 1  operates according to the monitoring result signal S 3  fed to the external terminal T 15 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when no signal indicating a fault in the controller IC  200  is received, the switch SW 1  selects the PWM dimming signal S 1  fed to the external terminal T 13  to feed it to the output current controller  102 . On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when a signal indicating a fault in the controller IC  200  is received, the switch SW 1  selects the terminal to which a constant voltage Vreg (a voltage corresponding to the HIGH level of the PWM dimming signal S 1 ) is applied to feed it to the output current controller  102 . 
         [0045]    The output current controller  102  keeps the transistor P 1  ON when fed with a HIGH-level PWM dimming signal S 1  from the switch SW 1 , keeps the transistor P 1  OFF when fed with a LOW-level PWM dimming signal S 1  from the switch SW 1 , and keeps the transistor P 1  ON when fed with the constant voltage Vreg from the switch SW 1 . Thus, the output voltage generator  101 , the output current controller  102 , the transistor P 1 , and the switch SW 1  serve both as an output voltage feeder which generates the output voltage Vo from the input voltage Vi based on the PWM dimming signal S 1  from the controller IC  200  to feed the output voltage Vo to the light-emitting element Z 1  and as an emergency driver which, on receiving a signal indicating a fault in the controller IC  200 , turns the light-emitting element Z 1  ON irrespective of the PWM dimming signal S 1  from the controller IC  200 . 
         [0046]      FIG. 2  is a timing chart illustrating an example of the operation of the light-emitting device shown in  FIG. 1 , and depicts, from top down, the clock signal S 2 , the monitoring result signal S 3 , the PWM dimming signal S 1 , and the output current Io. 
         [0047]    When a fault in the controller IC  200  causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 2 ), no pulses appear in the PWM dimming signal S 1  any longer (see broken lines in the PWM dimming signal S 1  shown in  FIG. 2 ), and the ON duty of the PWM dimming signal S 1  becomes zero. However, when a fault in the controller IC  200  causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 2 ), the monitoring result signal S 3  turns to LOW level, and as described above, the switch SW 1  selects the constant voltage Vreg. Thus, an output current Io is obtained that is similar to that obtained when the PWM dimming signal S 1  with an ON duty of 100% is fed to the output current controller  102 . In this way, the light-emitting device shown in  FIG. 1  can turn the light-emitting element Z 1  ON irrespective of the PWM dimming signal S 1  from the controller IC  200  when the controller IC  200  is faulty. 
       Second Embodiment 
       [0048]      FIG. 3  is a diagram showing a light-emitting device according to a second embodiment. In  FIG. 3 , such parts as are found also in  FIG. 1  are identified by common reference numerals, and no detailed description will be repeated as to them. 
         [0049]    The light-emitting device shown in  FIG. 3  includes at least one light-emitting element (in  FIG. 3 , light-emitting diodes) Z 1 , a light-emitting element driver IC  120  which drives the light-emitting element Z 1 , a transistor P 1  which is inserted in the power feed path from the light-emitting element driver IC  120  to the light-emitting element Z 1 , a controller IC  200  which feeds the light-emitting element driver IC  120  with a PWM dimming signal S 1 , and a monitor IC  300  which checks for a fault in the controller IC  200 . 
         [0050]    In the light-emitting element driver IC  120 , an output voltage generator  101 ′ substitutes for the output voltage generator  101  in the light-emitting element driver IC  110  shown in  FIG. 1 . The output voltage generator  101 ′ varies the set value of the output voltage Vo according to the monitoring result signal S 3  fed to the external terminal T 15 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when no signal indicating a fault in the controller IC  200  is received, the output voltage generator  101 ′ keeps the set value of the output voltage Vo at a standard value. On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when a signal indicating a fault in the controller IC  200  is received, the output voltage generator  101 ′ keeps the set value of the output voltage Vo at a value smaller than the standard value. Accordingly, also the value of the output current Io is then smaller than when the set value of the output voltage Vo equals the standard value. 
         [0051]      FIG. 4  is a timing chart illustrating an example of the operation of the light-emitting device shown in  FIG. 3 , and depicts, from top down, the clock signal S 2 , the monitoring result signal S 3 , the PWM dimming signal S 1 , and the output current Io. 
         [0052]    When a fault in the controller IC  200  causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 4 ), no pulses appear in the PWM dimming signal S 1  any longer (see broken lines in the PWM dimming signal S 1  shown in  FIG. 4 ), and the ON duty of the PWM dimming signal S 1  becomes zero. However, when a fault in the controller IC  200  causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 4 ), the monitoring result signal S 3  turns to LOW level, and as described above, the switch SW 1  selects the constant voltage Vreg, and in addition the output voltage generator  101 ′ turns the set value of the output voltage Vo to a value smaller than the standard value. Thus, an output current Io continues to be obtained with a smaller value than when the controller IC  200  is not faulty. In this way, the light-emitting device shown in  FIG. 3  can turn the light-emitting element Z 1  ON irrespective of the PWM dimming signal S 1  from the controller IC  200  when the controller IC  200  is faulty. 
         [0053]    Moreover, the light-emitting device shown in  FIG. 3  can keep the value of the output current Io smaller when the controller IC  200  is faulty than when the controller IC  200  is not faulty. 
         [0054]    When the controller IC  200  is faulty, the electric power source from which the input voltage Vi is derived may be in danger of approaching depletion. However, as described above, the light-emitting device shown in  FIG. 3  makes the value of the output current Io smaller when the controller IC  200  is faulty, and this reduces electric power consumption in the light-emitting device shown in  FIG. 3  when the controller IC  200  is faulty. In this way, it is possible to make the electric power source from which the input voltage Vi is derived less likely to deplete when the controller IC  200  is faulty. 
         [0055]    When the controller IC  200  is faulty, as a result of the output current Io continuing to be fed to the light-emitting element Z 1 , the light-emitting element Z 1  may break depending on its specifications. However, as described above, the light-emitting device shown in  FIG. 3  makes the value of the output current Io smaller when the controller IC  200  is faulty, and this prevents the light-emitting element Z 1  from breaking. 
       Third Embodiment 
       [0056]      FIG. 5  is a diagram showing a light-emitting device according to a third embodiment. In  FIG. 5 , such parts as are found also in  FIG. 16  are identified by common reference numerals, and no detailed description will be repeated as to them. 
         [0057]    The light-emitting device shown in  FIG. 5  includes at least one light-emitting element (in  FIG. 5 , light-emitting diodes) Z 1 , a light-emitting element driver IC  130  which drives the light-emitting element Z 1 , a coil L 1 , an output capacitor C 1 , a sense resistor Rs, a capacitor C 2 , a controller IC  200  which feeds the light-emitting element driver IC  130  with a PWM dimming signal S 1 , and a monitor IC  300  which checks for a fault in the controller IC  200 . 
         [0058]    The light-emitting element driver IC  130  is a semiconductor integrated circuit device (a so-called LED driver IC) that has integrated into it N-channel MOS field-effect transistors  1 H and  1 L (hereinafter referred to as high-side and low-side transistors  1 H and  1 L respectively), a high-side driver  2 H and a low-side driver  2 L, a diode D 1 , a driver controller  3 , an amplifier  4 , and a switch SW 1 . The light-emitting element driver IC  130  also has external terminals T 1  to T 7  for establishing electrical connection with the outside. 
         [0059]    Outside the light-emitting element driver IC  130 , the external terminal T 3  is connected to a terminal to which the input voltage Vi is applied. To the external terminal T 1 , the PWM dimming signal S 1  is fed. The external terminal T 2  is connected to the first terminal of the coil L 1 . The second terminal of the coil L 1  (i.e., the terminal to which the output voltage Vo is applied) is connected to the first terminal (anode) of the light-emitting element Z 1 . The second terminal (cathode) of the light-emitting element Z 1  is connected to the first terminal of the sense resistor Rs. The second terminal of the sense resistor Rs is connected to a ground terminal. The first terminal of the output capacitor C 1  is connected to the second terminal of the coil L 1 . The second terminal of the output capacitor C 1  is connected to the ground terminal. The external terminal T 4  is connected to the ground terminal. The external terminal T 5  is connected via the capacitor C 2  to the first terminal of the coil L 1 . The external terminal T 6  is connected to the first terminal of the sense resistor Rs. To the external terminal T 7 , the monitoring result signal S 3  is fed. 
         [0060]    Inside the light-emitting element driver IC  130 , the drain of the high-side transistor  1 H is connected to the external terminal T 3 . The source of the high-side transistor  1 H is connected to the external terminal T 2 . The gate of the high-side transistor  1 H is connected to the output terminal of the high-side driver  2 H. The drain of the low-side transistor  1 L is connected to the external terminal T 2 . The source of the low-side transistor  1 L is connected to the external terminal T 4 . The gate of the low-side transistor  1 L is connected to the output terminal of the low-side driver  2 L. Thus, the high-side and low-side transistors  1 H and  1 L are connected in series between the terminal to which the input voltage Vi is applied and the ground terminal, and the connection node between them (i.e., the terminal to which a switching voltage Vsw is applied) is connected via the coil L 1  to the output capacitor C 1 . 
         [0061]    Based on an instruction from the driver controller  3 , the high-side driver  2 H generates a control signal GH for the high-side transistor  1 H. The high-side transistor  1 H is ON when the control signal GH is at HIGH level, and is OFF when the control signal GH is at LOW level. Based on an instruction from the driver controller  3 , the low-side driver  2 L generates a control signal GL for the low-side transistor  1 L. The low-side transistor  1 L is ON when the control signal GL is at HIGH level, and is OFF when the control signal GL is at LOW level. 
         [0062]    The diode D 1  and the capacitor C 2 , which is externally fitted to the light-emitting element driver IC  130 , constitute a bootstrap circuit. The bootstrap circuit generates a boost voltage Vbst. The anode of the diode D 1  is connected to a terminal to which the constant voltage Vreg is applied. The cathode of the diode D 1  is connected to the external terminal T 5 . 
         [0063]    The first power terminal of the high-side driver  2 H and the first power terminal of the driver controller  3  are connected to the external terminal T 5  (i.e., the terminal to which the boost voltage Vbst is applied). The second power terminal of the high-side driver  2 H is connected to the external terminal T 2  (i.e., the terminal to which the switching voltage Vsw is applied). Thus, the control signal GH applied to the gate of the high-side transistor  1 H, when at HIGH level, equals the boost voltage Vbst and, when at LOW level, equals the switching voltage Vsw. 
         [0064]    The first power terminal of the low-side driver  2 L is connected to the terminal to which the constant voltage Vreg is applied. The second power terminal of the low-side driver  2 L is connected to the external terminal T 4  (i.e., the terminal to which a ground voltage GND is applied). Thus, the control signal GL applied to the gate of the low-side transistor  1 L, when at HIGH level, equals the constant voltage Vreg and, when at LOW level, equals the ground voltage GND. 
         [0065]    The operation of the bootstrap circuit configured as described above will now be described. When the high-side transistor  1 H is OFF and the low-side transistor  1 L is ON so that the switching voltage Vsw is at LOW level (GND), a current that passes from the constant voltage Vreg application terminal via the diode D 1  into the capacitor C 2  charges the capacitor C 2 . At this time, the boost voltage Vbst approximately equals the constant voltage Vreg (more precisely, the constant voltage Vreg minus the forward voltage drop Vf across the diode D 1 , i.e., Vreg−Vf). 
         [0066]    On the other hand, when, with the capacitor C 2  charged, the high-side transistor  1 H is turned ON and the low-side transistor  1 L is turned OFF so that the switching voltage Vsw rises from LOW level (GND) to HIGH level (Vi), the boost voltage Vbst is raised to a value (Vi+Vreg) that is higher than the HIGH level (Vi) of the switching voltage Vsw by the charge voltage (approximately Vreg) across the capacitor C 2 . Applying this boost voltage Vbst to the first power terminal of the high-side driver  2 H makes it possible to turn the high-side transistor  1 H ON and OFF reliably. 
         [0067]    The switch SW 1  operates according to the monitoring result signal S 3  fed to the external terminal T 7 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when no signal indicating a fault in the controller IC  200  is received, the switch SW 1  selects the PWM dimming signal S 1  fed to the external terminal T 1  to feed it to the driver controller  3 . On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when a signal indicating a fault in the controller IC  200  is received, the switch SW 1  selects the terminal to which the constant voltage Vreg (a voltage corresponding to the HIGH level of the PWM dimming signal S 1 ) is applied to feed it to the driver controller  3 . 
         [0068]    When fed with a HIGH-level PWM dimmer signal S 1  from the switch SW 1 , the driver controller  3  drives the high-side and low-side drivers  2 H and  2 L so as to turn the high-side and low-side transistors  1 H and  1 L ON and OFF according to a feedback voltage Vfb. Also, when fed with the constant voltage Vreg from the switch SW 1 , the driver controller  3  drives the high-side and low-side drivers  2 H and  2 L so as to turn the high-side and low-side transistors  1 H and  1 L ON and OFF according to the feedback voltage Vfb. By contrast, when fed with a LOW-level PWM dimmer signal S 1  from the switch SW 1 , the driver controller  3  drives the high-side and low-side drivers  2 H and  2 L so as to stop the operation for generating the output voltage Vo. 
         [0069]    The amplifier  4  generates the feedback voltage Vfb by amplifying the voltage across the sense resistor Rs that is applied between the non-inverting input terminal (+) and the inverting input terminal (−) of the amplifier  4 . Accordingly, the feedback voltage Vfb is a voltage signal that increases and decreases according to the output current Io through the sense resistor Rs. 
         [0070]    The high-side and low-side transistors  1 H and  1 L, the high-side and low-side drivers  2 H and  2 L, the driver controller  3 , and the amplifier  4  thus generate the output voltage Vo from the input voltage Vi such that the output current Io through the light-emitting element Z 1  remains equal to the target value during the ON period (HIGH-level period) of the PWM dimming signal S 1 , and stop the operation for generating the output voltage Vo during the OFF period of the PWM dimming signal S 1 . 
         [0071]    The light-emitting element driver IC  130 , the coil L 1 , the output capacitor C 1 , the sense resistor Rs, and the capacitor C 2  serve both as an output voltage feeder which generates the output voltage Vo from the input voltage Vi based on the PWM dimming signal S 1  from the controller IC  200  to feed the output voltage Vo to the light-emitting element Z 1  and as an emergency driver which, on receiving a signal indicating a fault in the controller IC  200 , turns the light-emitting element Z 1  ON irrespective of the PWM dimming signal S 1  from the controller IC  200 . 
         [0072]    The timing chart ( FIG. 2 ) illustrating an example of the operation of the light-emitting device shown in  FIG. 1  illustrates an example of the operation of the light-emitting device shown in  FIG. 5  as well, and therefore no overlapping description will be repeated. 
       Fourth Embodiment 
       [0073]      FIG. 6  is a diagram showing a light-emitting device according to a fourth embodiment. In  FIG. 6 , such parts as are found also in  FIG. 5  are identified by common reference numerals, and no detailed description will be repeated as to them. 
         [0074]    The light-emitting device shown in  FIG. 6  includes at least one light-emitting element (in  FIG. 6 , light-emitting diodes) Z 1 , a light-emitting element driver IC  130  which drives the light-emitting element Z 1 , a coil L 1 , an output capacitor C 1 , sense resistors Rs and Rs′, a switch SW 2 , a capacitor C 2 , a controller IC  200  which feeds the light-emitting element driver IC  130  with a PWM dimming signal S 1 , and a monitor IC  300  which checks for a fault in the controller IC  200 . 
         [0075]    The sense resistor Rs′ is inserted between the ground terminal and the second terminal of the sense resistor Rs, and the switch SW 2  is connected in parallel with the sense resistor Rs′. The switch SW 2  operates according to the monitoring result signal S 3 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when the switch SW 2  receives no signal indicating a fault in the controller IC  200 , the switch SW 2  is ON so as to short-circuit across the sense resistor Rs′. On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when the switch SW 2  receives a signal indicating a fault in the controller IC  200 , the switch SW 2  is OFF so as not to short-circuit across the sense resistor Rs′. Thus, when the switch SW 2  receives a signal indicating a fault in the controller IC  200 , the gain of the feedback voltage Vfb with respect to the output current Io is higher and hence the value of the output current Io is smaller than when the switch SW 2  receives no signal indicating a fault in the controller IC  200 . 
         [0076]    The timing chart ( FIG. 4 ) illustrating an example of the operation of the light-emitting device shown in  FIG. 3  illustrates an example of the operation of the light-emitting device shown in  FIG. 6  as well, and therefore no overlapping description will be repeated. 
       Fifth Embodiment 
       [0077]      FIG. 7  is a diagram showing a light-emitting device according to a fifth embodiment. In  FIG. 7 , such parts as are found also in  FIG. 5  are identified by common reference numerals, and no detailed description will be repeated as to them. 
         [0078]    The light-emitting device shown in  FIG. 7  includes at least one light-emitting element (in  FIG. 7 , light-emitting diodes) Z 1 , a light-emitting element driver IC  140  which drives the light-emitting element Z 1 , a coil L 1 , an output capacitor C 1 , a sense resistor Rs, a capacitor C 2 , a controller IC  200 ′ which feeds the light-emitting element driver IC  140  with a PWM dimming signal S 1 , and a monitor IC  300  which checks for a fault in the controller IC  200 ′. 
         [0079]    The controller IC  200 ′ additionally includes an enable circuit  203  and an external terminal T 24  as compared with the controller IC  200  shown in  FIG. 5 . The enable circuit  203  generates an enable signal S 4  based on the clock signal S 2 . From the external terminal T 24 , the enable signal S 4  is output. In this example, the enable signal S 4  is used to enable the light-emitting element driver IC  140 . A HIGH-level enable signal S 4  serves as a signal that enables the light-emitting element driver IC  140 , and a LOW-level enable signal S 4  serves as a signal (disable signal) that disables the light-emitting element driver IC  140 . 
         [0080]    In the light-emitting element driver IC  140 , a driver controller  3 ′ substitutes for the driver controller  3  in the light-emitting element driver IC  130  shown in  FIG. 5 , and a switch SW 3  and an external terminal T 8  are additionally provided. To the external terminal T 8 , the enable signal S 4  is fed. The switch SW 3  operates according to the monitoring result signal S 3  fed to the external terminal T 7 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when no signal indicating a fault in the controller IC  200 ′ is received, the switch SW 3  selects the enable signal S 4  fed to the external terminal T 8  to feed it to the driver controller  3 ′. On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when a signal indicating a fault in the controller IC  200 ′ is received, the switch SW 3  selects the terminal to which the constant voltage Vreg (a voltage corresponding to the HIGH level of the enable signal S 4 ) is applied to feed it to the driver controller  3 ′. 
         [0081]    The switch SW 1  operates according to the monitoring result signal S 3  fed to the external terminal T 7 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when no signal indicating a fault in the controller IC  200 ′ is received, the switch SW 1  selects the PWM dimming signal S 1  fed to the external terminal T 1  to feed it to the driver controller  3 ′. On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when a signal indicating a fault in the controller IC  200 ′ is received, the switch SW 1  selects the terminal to which the constant voltage Vreg (a voltage corresponding to the HIGH level of the PWM dimming signal S 1 ) is applied to feed it to the driver controller  3 ′. 
         [0082]    The driver controller  3 ′ is enabled when fed with a HIGH-level enable signal S 4  or the constant voltage Vreg from the switch SW 2 , and is disabled when fed with a LOW-level enable signal S 4  from the switch SW 2 . 
         [0083]    When enabled and fed with a HIGH-level PWM dimmer signal S 1  from the switch SW 1 , the driver controller  3 ′ drives the high-side and low-side drivers  2 H and  2 L so as to turn the high-side and low-side transistors  1 H and  1 L ON and OFF according to the feedback voltage Vfb. Also, when enabled and fed with the constant voltage Vreg from the switch SW 1 , the driver controller  3 ′ drives the high-side and low-side drivers  2 H and  2 L so as to turn the high-side and low-side transistors  1 H and  1 L ON and OFF according to the feedback voltage Vfb. By contrast, when enabled and fed with a LOW-level PWM dimmer signal S 1  from the switch SW 1 , the driver controller  3 ′ drives the high-side and low-side drivers  2 H and  2 L so as to stop the operation for generating the output voltage Vo. 
         [0084]    The light-emitting element driver IC  140 , the coil L 1 , the output capacitor C 1 , the sense resistor Rs, and the capacitor C 2  serve both as an output voltage feeder which generates the output voltage Vo from the input voltage Vi based on the PWM dimming signal S 1  and the enable signal S 4  from the controller IC  200 ′ to feed the output voltage Vo to the light-emitting element Z 1  and as an emergency driver which, on receiving a signal indicating a fault in the controller IC  200 ′, turns the light-emitting element Z 1  ON irrespective of the PWM dimming signal S 1  and the enable signal S 4  from the controller IC  200 ′. 
         [0085]      FIG. 8  is a timing chart illustrating an example of the operation of the light-emitting device shown in  FIG. 7 , and depicts, from top down, the clock signal S 2 , the monitoring result signal S 3 , the PWM dimming signal S 1 , the enable signal S 4 , and the output current Io. 
         [0086]    When a fault in the controller IC  200 ′ causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 8 ), no pulses appear in the PWM dimming signal S 1  any longer (see broken lines in the PWM dimming signal S 1  shown in  FIG. 8 ), with the result that the ON duty of the PWM dimming signal S 1  becomes zero and the enable signal S 4  falls to LOW level (see broken lines in the enable signal S 4  shown in  FIG. 8 ). However, when a fault in the controller IC  200 ′ causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 8 ), the monitoring result signal S 3  turns to LOW level, and as described above, the switches SW 1  and SW 3  both select the constant voltage Vreg. Thus, an output current Io is obtained that is similar to that obtained when the PWM dimming signal S 1  with an ON duty of 100% is fed along with a HIGH-level enable signal S 4  to the driver controller  3 ′. In this way, the light-emitting device shown in  FIG. 7  can turn the light-emitting element Z 1  ON irrespective of the PWM dimming signal S 1  and the enable signal S 4  from the controller IC  200 ′ when the controller IC  200 ′ is faulty. 
       Sixth Embodiment 
       [0087]      FIG. 9  is a diagram showing a light-emitting device according to a sixth embodiment. In  FIG. 9 , such parts as are found also in  FIG. 7  are identified by common reference numerals, and no detailed description will be repeated as to them. 
         [0088]    The light-emitting device shown in  FIG. 9  includes a plurality of light-emitting elements (in  FIG. 9 , light-emitting diodes) Z 1 , a light-emitting element driver IC  150  which drives the light-emitting elements Z 1 , a coil L 1 , an output capacitor C 1 , a sense resistor Rs, a capacitor C 2 , a controller IC  200 ′ which feeds the light-emitting element driver IC  150  with a PWM dimming signal S 1  and feeds a voltage booster IC  400  with an enable signal S 4 , a monitor IC  300  which checks for a fault in the controller IC  200 ′, and a voltage booster IC  400  which boosts the input voltage Vi to feed the boosted voltage to the light-emitting element driver IC  150 . In a case where the total forward voltage of a plurality of light-emitting elements Z 1  is higher than the input voltage Vi, a voltage boosting circuit (voltage booster IC  400 ) is provided to boost the input voltage Vi as in this example. For example, in a case where the output voltage of a battery mounted on a vehicle is used as the input voltage Vi, the input voltage Vi can be so low that the total forward voltage of a plurality of light-emitting elements Z 1  tends to be higher than the input voltage Vi. 
         [0089]    In the light-emitting element driver IC  150 , as compared with the light-emitting element driver IC  140  shown in  FIG. 7 , the switches SW 1  and SW 3  and the external terminals T 7  and T 8  are omitted. 
         [0090]    In this example, the enable signal S 4  is used to enable the voltage booster IC  400 . A HIGH-level enable signal S 4  serves as a signal that enables the voltage booster IC  400 , and a LOW-level enable signal S 4  serves as a signal (disable signal) that disables the voltage booster IC  400 . 
         [0091]    The voltage booster IC  400  has external terminals T 41  to T 43 . To the external terminal T 41 , the input voltage Vi is applied. When the voltage booster IC  400  is enabled, a voltage that results from boosting the input voltage Vi is output from the external terminal T 42  to be fed to the external terminal T 3  of the light-emitting element driver IC  150  and to the first terminal of a switch SW 4 . The second terminal of the switch SW 4  is connected via a resistor R 1  to the connection node between the cathode of a first light-emitting element  5  among the plurality of light-emitting elements Z 1  and the anode of a second light-emitting element  6  among the plurality of light-emitting elements Z 1 . The switch SW 4  operates according to the monitoring result signal S 3 . Specifically, when the monitoring result signal S 3  is at HIGH level, that is, when the switch SW 4  receives no signal indicating a fault in the controller IC  200 ′, the switch SW 4  is OFF. On the other hand, when the monitoring result signal S 3  is at LOW level, that is, when the switch SW 4  receives a signal indicating a fault in the controller IC  200 ′, the switch SW 4  is ON, so that part of the plurality of light-emitting elements Z 1  (the second light-emitting element  6  and those light-emitting elements which are provided on the cathode side of the second light-emitting element  6 ) turn ON. The current Isw 4  through the switch SW 4  is limited by the resistor R 1 . In this way, it is possible to prevent part of the plurality of light-emitting elements Z 1  (the second light-emitting element  6  and those light-emitting elements which are provided on the cathode side of the second light-emitting element  6 ) from being broken by the current Isw 4 . 
         [0092]    The voltage booster IC  400 , the light-emitting element driver IC  150 , the coil L 1 , the output capacitor C 1 , the sense resistor Rs, and the capacitor C 2  serve as an output voltage feeder which generates the output voltage Vo from the input voltage Vi based on the PWM dimming signal S 1  from the controller IC  200 ′ to feed the output voltage Vo to the light-emitting elements Z 1 . On the other hand, the voltage booster IC  400 , the switch SW 4 , and the resistor R 1  serve as an emergency driver which, on receiving a signal indicating a fault in the controller IC  200 ′, turns part of the plurality of light-emitting elements Z 1  ON irrespective of the PWM dimming signal S 1  from the controller IC  200 ′. Thus, the output voltage feeder and the emergency driver share the voltage booster IC  400  between them. 
         [0093]      FIG. 10  is a timing chart illustrating an example of the operation of the light-emitting device shown in  FIG. 9 , and depicts, from top down, the clock signal S 2 , the monitoring result signal S 3 , the PWM dimming signal S 1 , the enable signal S 4 , the output current Io, and the current Isw 4  through the switch SW 4 . 
         [0094]    When a fault in the controller IC  200 ′ causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 10 ), no pulses appear in the PWM dimming signal S 1  any longer (see broken lines in the PWM dimming signal S 1  shown in  FIG. 10 ), with the result that the ON duty of the PWM dimming signal S 1  becomes zero and the enable signal S 4  falls to LOW level (see broken lines in the enable signal S 4  shown in  FIG. 10 ). However, when a fault in the controller IC  200 ′ causes the clock signal S 2  to cease to be generated (after time point t 1  in  FIG. 10 ), the monitoring result signal S 3  turns to LOW level, and as described above, the switch SW 4  turns ON; thus, a current Isw 4  passes through part of the light-emitting elements Z 1  so that this part of the light-emitting elements Z 1  turn ON. In this way, the light-emitting device shown in  FIG. 9  can turn some of the light-emitting elements Z 1  ON irrespective of the PWM dimming signal S 1  and the enable signal S 4  from the controller IC  200 ′ when the controller IC  200 ′ is faulty. 
       Application 
       [0095]    The light-emitting devices according to the embodiments described above can be suitably used as various lights on vehicles as shown in  FIGS. 11 and 12 , such as head lights (including high-beam lamps, low-beam lamps, small lamps, fog lamps, and the like as necessary) X 11 , light sources for daytime running lights (DRLs) X 12 , tail lamps (including small lamps, back lamps, and the like as necessary) X 13 , stop lamps X 14 , and turn lamps X 15 . 
         [0096]    A light-emitting element driver IC may be provided as a module (like an LED head light module Y 10  as shown in  FIG. 13 , an LED turn lamp module Y 20  as shown in  FIG. 14 , or an LED rear lamp module Y 30  as shown in  FIG. 15 ) in which it is incorporated along with externally fitted components (such as an output capacitor C 1 , a capacitor C 2 , a coil L 1 , a sense resistor Rs, etc.) and a light-emitting element Z 1  as the driving target, or may be provided as an IC by itself (like the light-emitting element driver ICs  110  to  150 ) that is a half-finished product independent of externally fitted components (such as an output capacitor C 1 , a capacitor C 2 , coil L 1 , a sense resistor Rs, etc.) and a light-emitting element Z 1  as the driving target. 
       Other Modifications 
       [0097]    Although the embodiments described above deal with configurations where light-emitting diodes are used as light-emitting elements, this is not meant to limit how the invention should be implemented. Instead, for example, organic electroluminescence elements may be used as light-emitting elements. 
         [0098]    The various technical features disclosed herein may be implemented in any other manners than as in the embodiments described above, and allow for any modifications within the spirit of the technical ingenuity involved. For example, bipolar transistors and MOS field-effect transistors can be substituted for each other as necessary, and the logic levels of various signals can be inverted as necessary. For another example, the fifth and sixth embodiments can be modified to omit the dimming function. That is, the embodiments described above should be considered to be in every aspect illustrative and not restrictive, and it should be understood that the technical scope of the present invention is defined not by the description of embodiments given above but by the appended claims and encompasses any modifications made in the sense and scope equivalent to those of the claims. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           1 H N-channel MOS field-effect transistor (high-side transistor) 
           1 L N-channel MOS field-effect transistor (low-side transistor) 
           2 H high-side driver 
           2 L low-side driver 
           3  controller 
           4  amplifier 
           5  first light-emitting element 
           6  second light-emitting element 
           110 ,  120 ,  130 ,  140 ,  150  light-emitting element driver IC 
           101  output voltage generator 
           102  output current controller 
           200 ,  200 ′ controller IC 
           201  clock circuit 
           202  PWM circuit 
           203  enable circuit 
           300  monitor IC 
           400  voltage booster IC 
         C 1  output capacitor 
         C 2  capacitor 
         D 1  diode 
         L 1  coil 
         P 1  P-channel MOS field-effect transistor 
         R 1  resistor 
         Rs, Rs′ sense resistor 
         SW 1 -SW 4  switch 
         T 1 -T 8 , T 11 -T 15  external terminal 
         T 21 -T 24 , T 31 , T 32 , T 41 -T 43  external terminal 
         X 10  vehicle 
         X 11  head light 
         X 12  light source for daytime running light (DRL) 
         X 13  tail lamp 
         X 14  stop lamp 
         X 15  turn lamp 
         Y 10  LED head light module 
         Y 20  LED turn lamp module 
         Y 30  LED rear lamp module 
         Z 1  light-emitting element (light-emitting diode)