Patent Publication Number: US-2023164892-A1

Title: Lighting control device, lighting device, and vehicle

Description:
TECHNICAL FIELD 
     The present disclosure relates to lighting control devices, lighting devices, and vehicles. 
     BACKGROUND ART 
       FIG.  12    shows a lighting device  910  of a first reference configuration. In the lighting device  910 , a lighting control device  911  performs switching driving of an output transistor  913  constituting a boost (step-up) converter and thereby generates from an input voltage Vin an output voltage Vout, and a driving current Iled based on the output voltage Vout is supplied to a lighting unit  912  composed of one or more light-emitting diodes, thereby to make the lighting unit  912  emit light. Between a terminal to which the output voltage Vout is applied and the positive terminal  912 P of the lighting unit  912 , a load switch LDsw configured with a field-effect transistor is inserted so that, only when the load switch LDsw is on, the driving current Iled is supplied to the lighting unit  912 . The negative terminal  912 N of the lighting unit  912  is connected via a diode  914  to a terminal to which the input voltage Vin is applied. 
     In the lighting device  910 , the lighting unit  912  is PWM-driven base on a control signal CNT generated by the lighting control device  911 , and thereby the dimming of the lighting unit  912  is achieved.  FIG.  13    is a diagram illustrating the operation of the lighting device  910 . In the lighting device  910 , only during the high-level period of the control signal CNT, the switching driving of the output transistor  913  is performed and in addition the load switch LDsw is kept on. Through the adjustment of the duty of the control signal CNT, the dimming of the lighting unit  912  is achieved. The lighting device  910  in  FIG.  12    provides high dimming accuracy (fast response of the driving current Iled) while requiring the load switch LDsw. Moreover, in the event of a ground short in which the terminal  912 P or  912 N short-circuits to the ground, the path for the driving current Iled has to be shut off, and the load switch LDsw is required also for such shutting-off. 
       FIG.  14    shows a lighting device  930  of a second reference configuration. In the lighting device  930 , a lighting control device  931  performs switching driving of an output transistor  933  constituting a buck (step-down) converter and thereby generates from an input voltage Vin an output voltage Vout, and a driving current Iled based on the output voltage Vout is supplied to a lighting unit  932  composed of one or more light-emitting diodes, thereby to make the lighting unit  932  emit light. The lighting unit  932  is connected to a load switch LDsw in series so that, only when the load switch LDsw is on, the driving current Iled is supplied to the lighting unit  932 . In the lighting device  930 , the ground gnd 1  for the input voltage Vin is different from the ground gnd 2  for the output voltage Vout, and the potential of the output voltage Vout is equal to the potential of the ground gnd 1 . 
     In the lighting device  930 , the lighting unit  932  is PWM-driven base on a control signal CNT generated by the lighting control device  931 , and thereby the dimming of the lighting unit  932  is achieved.  FIG.  15    is a diagram illustrating the operation of the lighting device  930 . In the lighting device  930 , only during the high-level period of the control signal CNT, the switching driving of the output transistor  933  is performed and in addition the load switch LDsw is kept on. Through the adjustment of the duty of the control signal CNT, the dimming of the lighting unit  932  is achieved. The lighting device  930  in  FIG.  14    provides high dimming accuracy (fast response of the driving current Iled) while requiring the load switch LDsw. Here, however, in the configuration of the lighting device  930  in  FIG.  14   , as distinct from the configuration of the lighting device  910  in  FIG.  12   , coping with a ground short does not require the load switch LDsw. In the lighting device  930  in  FIG.  14   , even if the positive or negative terminal  932 P or  932 N in the lighting unit  932  short-circuits to the ground gnd 1 , simply the driving current Iled ceases to flow, and thus no extra protection is required. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP-A-2019-103289 
     Summary of Disclosure 
     Technical Problem 
     Eliminating the load switch LDsw would provide benefits such as reduced costs. Accordingly, a study will now be made on a lighting device  930   a  shown in  FIG.  16    resulting from omitting the load switch LDsw from the lighting device  930  in  FIG.  14   . In the lighting device  930   a  in  FIG.  16   , the operation of a buck converter configured to include an output transistor  933  is turned on and off and thereby dimming is achieved by PWM. Specifically, during the high-level period of the control signal CNT, the buck converter is kept on (the buck converter is kept in operation) to keep the lighting unit  932  lit; during the low-level period of the control signal CNT, the buck converter is kept off (the buck converter is kept out of operation) to keep the lighting unit  932  extinguished. 
     Inconveniently, with the scheme of the lighting device  930   a , the response of the driving current Iled suffers a delay from the control signal CNT.  FIG.  17    is a diagram illustrating the operation of the lighting device  930   a . In the lighting device  930   a , in response to the control signal CNT turning from low level to high level, the switching driving of the output transistor  933  is started (that is, the operation of the buck converter is turned on) but, immediately after the start of the switching by the output transistor  933 , the output voltage Vout has not risen to be high enough to let the driving current Iled flow; only when a delay time td has passed after the turning of the control signal CNT to high level does the driving current Iled reach the desired current value to permit the lighting unit  932  to emit light with the desired luminance. 
     As described above, in the lighting device  930   a , due to the delay time td, in each cycle of PWM driving, the light emission time of the lighting unit  932  is shorter than that specified by the control signal CNT, making it difficult to perform desired dimming. 
     An object of the present disclosure is to provide a lighting control device that permits desired dimming while achieving a reduced number of components, and to provide a lighting device and a vehicle that employ such a lighting control device. 
     Solution to Problem 
     According to one aspect of the present disclosure, a lighting control device that generates an output voltage by switching of an input voltage and that supplies a driving current based on the output voltage to a lighting unit to light the lighting unit includes: a control signal generator configured to generate as a raw control signal a control signal that specifies alternately a lighting period in which to keep the lighting unit lit and a extinction period in which to keep the lighting unit extinguished; a lighting unit current sensor configured to sense the current flowing through the lighting unit; a control signal corrector configured to generate a corrected control signal by correcting, based on a current state signal based on the result of sensing by the lighting unit current sensor, the raw control signal in a direction in which the lighting period is extended; and a switching controller configured to perform the switching of the input voltage during the lighting period specified by the corrected control signal and suspend the switching of the input voltage during the extinction period specified by the corrected control signal. (A first configuration.) 
     In the lighting control device of the first configuration described above, in each of the raw control signal and the corrected control signal, the lighting period and the extinction period may be specified alternately at a predetermined cycle, and in each cycle, the lighting period in the corrected control signal may be extended compared with the lighting period in the raw control signal by correction by the control signal corrector. (A second configuration.) 
     In the lighting control device of the first or second configuration described above, the control signal corrector may be configured to generate the corrected control signal by, while keeping the start timing of the lighting period the same before and after correction, correcting the raw control signal in the direction in which the lighting period is extended by the time required, after the start of the lighting period, for the current flowing through the lighting unit to reach a predetermined judgement current. (A third configuration.) 
     In the lighting control device of the third configuration described above, there may be further provided a current state signal generator configured to generate the current state signal. The current state signal generator may be configured such that, after the start of the switching of the input voltage, when the output voltage has risen until the current flowing through the lighting unit reaches the judgement current, the current state signal generator switches the current state signal from a first state to a second state. The control signal corrector may be configured to correct the raw control signal in the direction in which the lighting period is extended by the time after the start of the lighting period until the current state signal turns from the first state to the second state. (A fourth configuration.) 
     In the lighting control device of the fourth configuration described above, the control signal corrector may be configured to perform a first measurement process to measure a first time after the start of the lighting period until the current state signal turns from the first state to the second state, perform a second measurement process to measure the time lapse after the timing of a transition from the lighting period to the extinction period in the raw control signal, and switch the period specified by the current state signal from the lighting period to the extinction period when, after the first measurement process, the time lapse in the second measurement process reaches the first time. (A fifth configuration.) 
     In the lighting control device of the fifth configuration described above, the control signal corrector may include an up/down counter and may be configured to perform the first and second measurement processes using the up/down counter. (A sixth configuration.) 
     In the lighting control device of the sixth configuration described above, the up/down counter may be configured such that, after the start of the lighting period until the current state signal turns from the first state to the second state, synchronously with a predetermined clock signal, the up/down counter changes the count value, starting at a predetermined initial value, in a first direction and then, after the timing of the transition from the lighting period to the extinction period in the raw control signal until the count value returns to the initial value, the up/down counter changes, synchronously with the clock signal, the count value in a second direction opposite to the first direction. The first measurement process may be achieved by the count value being changed in the first direction and the second measurement process may be achieved by the count value being changed in the second direction. The control signal corrector may be configured to switch the period specified by the corrected control signal from the lighting period to the extinction period when the count value has returned to the initial value in the second measurement process. (A seventh configuration.) 
     In the lighting control device of any of the first to seventh configurations described above, the lighting unit may include one or more light-emitting elements that emit light by being supplied with a current. (An eighth configuration.) 
     According to another aspect of the present disclosure, a lighting device includes: the lighting control device according to any of the first to eighth configurations described above; and a lighting unit configured to be supplied with a driving current under the control of the lighting control device. (A ninth configuration.) 
     According to yet another aspect of the present disclosure, a vehicle includes: the lighting device according to the ninth configuration described above as a vehicle-mounted lighting apparatus; a voltage source configured to supply the lighting device with an input voltage; and a body in which the lighting apparatus is installed. (A tenth configuration.) 
     Advantageous Effects of Disclosure 
     According to the present disclosure, it is possible to provide a lighting control device that permits desired dimming while achieving a reduced number of components, and to provide a lighting device and a vehicle that employ such a lighting control device 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an overall configuration diagram of a lighting device according to a first embodiment of the present disclosure. 
         FIG.  2    is an external perspective view of a lighting control device according to the first embodiment of the present disclosure. 
         FIG.  3    is a diagram showing a relationship among a dimming signal, a ramp voltage, and a control signal according to the first embodiment of the present disclosure. 
         FIG.  4    is a diagram showing a relationship between control signals before and after correction according to the first embodiment of the present disclosure. 
         FIG.  5    is a diagram showing a relationship between a current through a lighting unit and a current state signal according to the first embodiment of the present disclosure. 
         FIG.  6    is a schematic operating waveform diagram of a lighting device according to the first embodiment of the present disclosure. 
         FIG.  7    is a detailed operating waveform diagram of the lighting device according to the first embodiment of the present disclosure. 
         FIG.  8    is a configuration diagram of a control signal corrector according to the first embodiment of the present disclosure. 
         FIG.  9    is a timing chart illustrating the operation of the control signal corrector in  FIG.  8   . 
         FIG.  10    is a configuration diagram of a lighting system according to a second embodiment of the present disclosure. 
         FIG.  11    is a diagram showing a lighting system incorporated in a vehicle according to the second embodiment of the present disclosure. 
         FIG.  12    is a configuration diagram of a lighting device of a first reference configuration. 
         FIG.  13    is a diagram illustrating the operation of the lighting device of the first reference configuration. 
         FIG.  14    is a configuration diagram of a lighting device of a second reference configuration. 
         FIG.  15    is a diagram illustrating the operation of the lighting device of the second reference configuration. 
         FIG.  16    is a configuration diagram of a lighting device of a third reference configuration. 
         FIG.  17    is a diagram illustrating the operation of the lighting device of the third reference configuration. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, examples of implementing the present disclosure will be described specifically with reference to the accompanying drawings. Among the diagrams referred to in the course, the same parts are identified by the same reference signs, and in principle no overlapping description of the same parts will be repeated. In the present description, for the sake of simplicity, symbols and reference signs referring to information, signals, physical quantities, elements, parts, and the like are occasionally used with omission or abbreviation of the names of the information, signals, physical quantities, elements, parts, and the like corresponding to those symbols and reference signs. For example, the control signal described later and identified by the reference sign “CNT 1 ” (see  FIG.  1   ) is sometimes referred to as the control signal CNT 1  and other times abbreviated to the signal CNT 1 , both referring to the same entity. 
     First, some of the terms used to describe embodiments of the present disclosure will be defined. “Level” denotes the level of a potential, and for any signal or voltage, “high level” has a higher potential than “low level”. For any signal or voltage, its being at high level means its level being equal to high level, and its being at low level means its level being equal to low level. A level of a signal is occasionally referred to as a signal level, and a level of a voltage is occasionally referred to as a voltage level. 
     For any signal or voltage, a transition from low level to high level is termed an up edge, and the timing of a transition from low level to high level is termed an up-edge timing. Likewise, for any signal or voltage, a transition from high level to low level is termed a down edge, and the timing of a transition from high level to low level is termed a down-edge timing. 
     For any transistor configured as an FET (field-effect transistor), which can be a MOSFET, “on state” refers to a state where the drain-source channel of the transistor is conducting, and “off state” refers to a state where the drain-source channel of the transistor is not conducting (cut off). Similar definitions apply to any transistor that is not classified as an FET. Unless otherwise stated, any MOSFET can be understood to be an enhancement MOSFET. “MOSFET” is an abbreviation of “metal-oxide-semiconductor field-effect transistor”. 
     Any switch can be configured with one or more FETs (field-effect transistors). When a given switch is in the on state, the switch conducts across its terminals; when a given switch is in the off state, the switch does not conduct across its terminals. 
     For any transistor or switch, its being in the on or off state is occasionally expressed simply as its being on or off respectively. For any transistor or switch, its switching from the off state to the on state is expressed as a turning-on, and its switching from the on state to the off state is expressed as a turning-off. For any transistor or switch, a period in which it is in the on state is occasionally referred to as an on period, and a period in which it is in the off state is occasionally referred to as an off period. For any signal that takes as its signal level high level or low level, a period in which the signal is at high level is referred to as a high-level period and a period in which the signal is at low level is referred to as a low-level period. The same applies to any voltage that takes as its voltage level high level or low level. 
     First Embodiment 
     A first embodiment of the present disclosure will be described.  FIG.  1    is an overall configuration diagram of a lighting device  1  according to the first embodiment. The lighting device  1  includes a lighting control device  10  and a lighting unit  20 , and also includes discrete components connected to the lighting control device  10  or the lighting unit  20 . The discrete components provided in the lighting device  1  include an output transistor M 1 , an inductor L 1 , a diode D 1 , capacitors C 1  and C 2 , and resistors R 1  and R 2 , and further include resistors R 11  to R 16  and capacitors C 11  and C 12 . The output transistor M 1  is configured as an N-channel MOSFET. A modification is however possible by using a P-channel MOSFET as the output transistor M 1 . 
     The lighting control device  10  is a control device that operates by being supplied with a positive direct-current input voltage V IN  and that switches the state of light-emitting elements constituting the lighting unit  20  between a lit state and an extinguished state. The lighting control device  10  is an electronic component (semiconductor device) as shown in  FIG.  2    that is formed by sealing a semiconductor integrated circuit into a housing (package) formed of resin, and has the circuits constituting the lighting control device  10  integrated on a semiconductor substrate. The housing of the lighting control device  10  has a plurality of external terminals formed so as to be exposed out of it, and those external terminals include terminals CSH, CSL, GH, BOOT, SW, PGND, SNSP, SNSN, GND_TM, IN_TM, VREFA, EN, VREFB, RT, COMP, and DSET shown in  FIG.  1   , and can further include other terminals that are not shown in  FIG.  1   . The number of external terminals on, and the exterior appearance of, the lighting control device  10  shown in  FIG.  2    are simply illustrative. 
     The lighting unit  20  is composed of one or more light-emitting elements that emit light by being supplied with a current. The lighting unit  20  is composed of a plurality of light-emitting diodes connected in series. The lighting unit  20  is provided between terminals  20 P and  20 N. In  FIG.  1   , a series circuit of light-emitting diodes  21  to  23  constitutes the lighting unit  20 , with the anode of the light-emitting diode  21  connected to the terminal  20 P, the cathode of the light-emitting diode  21  connected to the anode of the light-emitting diode  22 , the cathode of the light-emitting diode  22  connected to the anode of the light-emitting diode  23 , and the cathode of the light-emitting diode  23  connected to the terminal  20 N. The number of light-emitting diodes constituting the lighting unit  20  may be one or two or four or more. The lighting unit  20  may include a parallel circuit of a plurality of light-emitting diodes. 
     The lighting device  1  includes a ground GND 1  as one electrically conductive part at a reference potential (i.e., a reference potential part), and also includes a ground GND 2  as another electrically conductive part at a reference potential (i.e., a reference potential part). While there can be timings at which the potentials of the grounds GND 1  and GND 2  are equal, so long as the positive input voltage V IN  is fed to the lighting device  1  and the switching driving of the output transistor M 1  is performed, the grounds GND 1  and GND 2  have different potentials. 
     In the lighting device  1 , the input voltage V IN  applied to an input terminal IN can be bucked (stepped down) to generate an output voltage V OUT  at an output terminal OUT. The input voltage V IN  is a positive direct-current voltage with reference to the ground GND 1 . That is, the input voltage V IN  is applied to the input terminal IN with reference to the potential of the ground GND 1 . In other words, the input voltage V IN  is applied between the ground GND 1  and the input terminal IN with the potential of the ground GND 1  on the negative side. On the other hand, the output voltage V OUT  is a positive direct-current voltage with reference to the ground GND 2 . That is, the output voltage V OUT  is applied to the output terminal OUT with reference to the potential of the ground GND 2 . In other words, the output voltage V OUT  is applied between the ground GND 2  and the output terminal OUT with the potential of the ground GND 2  on the negative side. In the lighting device  1 , the output terminal OUT is connected to the ground GND 1 . 
     A description will now be given of the interconnection between the discrete components and the lighting control device  10  or the lighting unit  20 , all provided in the lighting device  1 . 
     One terminal of the resistor R 1  is connected to the input terminal IN and also to the terminal CSH. The other terminal of the resistor R 1  is connected to the drain of the output transistor M 1  and also to the terminal CSL. The gate of the output transistor M 1  is connected to the terminal GH. 
     The source of the output transistor M 1  is connected to a node NDsw and also to the terminal SW. To the node NDsw, one terminal of the inductor L 1  and the cathode of the diode D 1  are both connected, and the node NDsw is connected also via the capacitor C 2  to the terminal BOOT. The other terminal of the inductor L 1  is connected to the output terminal OUT, and is also connected via the capacitor C 1  to the ground GND 2 . The anode of the diode D 1  is connected to the ground GND 2 . The terminals PGND and GND_TM are connected to the ground GND 2 . The terminal  20 P is connected to the output terminal OUT, and the terminal  20 N is connected via the resistor R 2  to the ground GND 2 . The connection node between the resistor R 2  and the terminal  20 N is connected to the terminal SNSP, and the connection node between the resistor R 2  and the ground GND 2  is connected to the terminal SNSN. The circuits within the lighting control device  10  operate with reference to the potential at the terminal GND_TM (i.e., the potential of the ground GND 2 ). 
     The output transistor M 1 , the diode D 1 , the inductor L 1 , and the capacitor C 1  constitute a DC-DC converter (buck converter) that bucks the input voltage V IN  to generate the output voltage V OUT . Such blocks that control the switching of the output transistor M 1  (i.e., a switching controller  150  and an amplifier  130 , which will be described later) are included among the elements constituting the DC-DC converter. 
     The terminal IN_TM is connected to the input terminal IN. The terminal VREFA is connected via the capacitor C 11  to the ground GND 2 . The input terminal IN is connected to one terminal of the resistor R 11 , and the other terminal of the resistor R 11  is connected via the resistor R 12  to the ground GND 1 . The connection node between the resistors R 11  and R 12  is connected to the terminal EN. The terminal VREFB is connected to one terminal of the resistor R 13 , and the other terminal of the resistor R 13  is connected via the resistor R 14  to the ground GND 2 . The connection node between the resistors R 13  and R 14  is connected to the terminal DSET. The terminal RT is connected via the resistor R 15  to the ground GND 2 . The terminal COMP is connected to one terminal of the resistor R 16 , and the other terminal of the resistor R 16  is connected via the capacitor C 12  to the ground GND 2 . 
     In the following description, the current flowing through the inductor L 1  will occasionally be referred to as the inductor current and identified by the symbol “I L ”. The inductor current I L  flows from the node NDsw to the output terminal OUT. The current flowing through the lighting unit  20  will occasionally be referred to as the driving current and identified by the symbol “I LED ”. The driving current LED flows from the output terminal OUT to the ground GND 2 . 
     In the present description, the state of the lighting unit  20  or the light-emitting elements where they are not emitting light will be referred to as their being unlit or extinguished. As the contrary term, the state of the lighting unit  20  or the light-emitting elements where they are emitting light will sometimes be referred to as their being lit; for the lighting unit  20  or the light-emitting elements, their emitting light is synonymous with their being lit. Strictly speaking, even when a very low current is flowing through the lighting unit  20  or the light-emitting elements, they can emit light at a very low luminance; in the following description, however, it is assumed that, when a driving current I LED  of a predetermined threshold value or more is flowing in the lighting unit  20  or the light-emitting elements, they are lit (in a lit state) and that, when a driving current I LED  of a predetermined threshold value or more is not flowing in the lighting unit  20  or the light-emitting elements, they are extinguished (in an extinguished state). 
     Next, the internal configuration of the lighting control device  10  will be described. The lighting control device  10  includes a control signal generator  110 , a control signal corrector  120 , an amplifier  130 , a comparator  140 , a switching controller  150 , and blocks identified by the reference signs  171  to  175 . 
     The control signal generator  110  generates a control signal CNT 1  based on a dimming signal DIM fed in via the terminal DSET. In the lighting device  1 , the lighting unit  20  is PWM-driven. PWM is short for pulse width modulation. The control signal CNT 1  corresponds to a pulse width modulation signal in PWM driving. The control signal CNT 1 , and also the control signal CNT 2  described later, is a binary signal that takes as its signal level either high or low level. 
     Referring to  FIG.  3   , an additional description follows.  FIG.  3    shows the relationship among the dimming signal DIM, a ramp voltage Vr, and the control signal CNT 1 . In the configuration example of the lighting device  1  in  FIG.  1   , the dimming signal DIM is an analog voltage fed to the terminal DSET, and has a predetermined direct-current voltage value with the lighting control device  10  in a stable state. The control signal generator  110  generates a ramp voltage Vr of which the voltage value changes so as to describe sawtooth or triangular waves at a predetermined PWM frequency, namely a frequency f PWM . The reciprocal of the frequency f PWM  is the cycle of the ramp voltage Vr, and a period with a length corresponding to one cycle of the ramp voltage Vr will be referred to as a unit period. A plurality of unit periods occur one after another at the frequency f PWM  as time passes. The end timing of one unit period coincides with the start timing of the next unit period. In each unit period, the ramp voltage Vr equals a predetermined lower-limit voltage at the start of the unit period and increases monotonically from the lower-limit voltage to a predetermined higher-limit voltage over the length of one unit period. The lower-limit voltage is, for example, equal to the potential of ground GND 2 . The higher-limit voltage is higher than the lower-limit voltage. The control signal generator  110  keeps the control signal CNT 1  at high level during a period in which the level of the dimming signal DIM is higher than the level of the ramp voltage Vr, and keeps the control signal CNT 1  at low level during a period in which the level of the dimming signal DIM is lower than the level of the ramp voltage Vr. 
     The control signal CNT 1  specifies, within each unit period, a lighting period and an extinction period. The lighting period is a period in which the lighting unit  20  is supposed to be lit; the extinction period is a period in which the lighting unit  20  is supposed to be extinguished. Each unit period comprises a combination of one lighting period and one extinction period, and the control signal CNT 1  specifies the lighting period and the extinction period alternately at a cycle corresponding to the reciprocal of the frequency f PWM . While it is here assumed that the start timing of the unit period coincides with the start timing of the lighting period, a modification is possible where they do not coincide. 
     The control signal corrector  120  corrects the control signal CNT 1  based on a current state signal SG, which will be described later, and outputs the control signal CNT 1  having undergone correction as a control signal CNT 2 . The control signals CNT 1  and CNT 2  can be called a raw control signal and a corrected control signal respectively.  FIG.  4    shows an outline of the correction by the control signal corrector  120 . Like the control signal CNT 1 , the control signal CNT 2  is a control signal that specifies the lighting period and the extinction period alternately at a cycle corresponding to the reciprocal of the frequency f PWM . In the control signal CNT 2 , a period in which it is at high level corresponds to the lighting period and a period in which it is at low level corresponds to the extinction period. 
     In each unit period, the start timing of the lighting period specified by the control signal CNT 1  coincides with the start timing of the lighting period specified by the control signal CNT 2 . However, in each unit period, the lighting period in the control signal CNT 2  is longer than the lighting period in the control signal CNT 1 . That is, the control signal CNT 1  is corrected in a direction in which the lighting period is extended, and thereby the control signal CNT 2  is generated. 
     In  FIG.  4   , time T ON1  represents the length of the high-level period of the control signal CNT 1  in one unit period (i.e., the length of the lighting period specified by the control signal CNT 1 ), and time T OFF1  represents the length of the low-level period of the control signal CNT 1  in one unit period (i.e., the length of the extinction period specified by the control signal CNT 1 ). Time T ON2  represents the length of the high-level period of the control signal CNT 2  in one unit period (i.e., the length of the lighting period specified by the control signal CNT 2 ), and time T OFF2  represents the length of the low-level period of the control signal CNT 2  in one unit period (i.e., the length of the extinction period specified by the control signal CNT 2 ). 
     Suppose that, in each unit period, the lighting unit  20  is lit for time T ON1  and is extinguished for time T OFF1 . Then the ratio T ON1 /(T ON1 +T OFF1 ) represents the duty of lighting in the lighting unit  20 . Suppose that, in each unit period, the lighting unit  20  is lit for time T ON2  and is extinguished for time T OFF2 . Then the ratio T ON2 /(T ON2 +T OFF2 ) represents the duty of lighting in the lighting unit  20 . 
     The amplifier  130  is an example of a lighting unit current sensor that senses the driving current I LED  that flows through the lighting unit  20 . The resistor R 2  may be understood to be included among the elements constituting the lighting unit current sensor. Specifically, a voltage proportional to the driving current I LED  appears across the resistor R 2  as a voltage drop across it (i.e., the terminal-to-terminal voltage across the resistor R 2 ), and the voltage drop across the resistor R 2  is fed via the terminals SNSP and SNSN to the amplifier  130 . The amplifier  130  amplifies the signal reflecting the voltage drop across the resistor R 2 , and outputs the result of the amplification as a voltage signal V SNS . The voltage signal V SNS  has a voltage value proportional to the driving current I LED . In the following description, the voltage signal V SNS  will occasionally be referred to also as the sense voltage V SNS . The sense voltage V SNS  has a potential with reference to the ground GND 2 , and increases as the driving current I LED  increases. 
     The comparator  140  is an example of a current state signal generator. The comparator  140  is fed with the sense voltage V SNS  from the amplifier  130  and, based on the sense voltage V SNS , generates and outputs a current state signal SG commensurate with the driving current I LED . Specifically, the non-inverting input terminal of the comparator  140  is fed with the sense voltage V SNS  from the amplifier  130 , and the inverting input terminal of the comparator  140  is fed with a predetermined judgment voltage V LED_SG . The judgment voltage V LED_SG  has a predetermined potential higher than the ground GND 2 . 
     For the driving current I LED  that flows through the lighting unit  20  in the lighting period, a target current I TG  is set (see  FIG.  5   ). The target current I TG  may be set at any current value, and is set, for example, at 1 A (ampere). The judgment voltage V LED_SG  is set beforehand such that when the driving current I LED  is equal to a predetermined judgment current I LED_SG  lower than the target current I TG , the sense voltage V SNS  is equal to the judgment voltage V LED_SG . 
       FIG.  5    shows the relationship of the current state signal SG with other relevant currents. As shown in  FIG.  5   , the judgment current I LED_SG  is lower than the target current I TG , and is set at, for example, 10% or 20% of the target current I TG . The current state signal SG is at low level when the sense voltage V SNS  is lower than the judgment voltage V LED_SG  (i.e., when I LED &lt;I LED_SG ), and is at high level when the sense voltage V SNS  is higher than the judgment voltage V LED_SG  (i.e., when I LED &gt;I LED_SG ). When the sense voltage V SNS  is equal to the judgment voltage V LED_SG , the current state signal SG is at either high or low level. 
     The switching controller  150  performs the switching driving of the output transistor M 1  during the lighting period specified by the control signal CNT 2 . Through the switching driving of the output transistor M 1 , the input voltage V IN  is switched to generate the output voltage V OUT  sufficiently high to light the lighting unit  20 . The switching controller  150  suspends the switching driving of the output transistor M 1  during the extinction period specified by the control signal CNT 2 . Suspending the switching driving of the output transistor M 1  is equivalent to keeping the output transistor M 1  in the off state, and thus to suspending the switching of the input voltage Vin. When the switching driving of the output transistor M 1  is suspended, the supply of electric power from the input terminal IN to the output terminal OUT is suspended, and thus the lighting unit  20  is extinguished, though, for a while after a switch from the lighting period to the extinction period, the lighting unit  20  may keep lighting based on the electric charge stored in the capacitor C 1 . 
     In the configuration example shown in  FIG.  1   , the switching controller  150  includes a slope voltage generator  151 , an adder  152 , an error amplifier  153 , an oscillator  154 , a comparator  155 , and a driver  156 . 
     The slope voltage generator  151  is fed with the voltage drop across the resistor R 1  (i.e., the terminal-to-terminal voltage across the resistor R 1 ) via the terminals CSH and CSL, thereby to sense the current flowing through the output transistor M 1  (in other words, the inductor current I L ), and generates a slope voltage V SLP  commensurate with the current that flows through the output transistor M 1  (in other words, the inductor current I L ) during its on period. The slope voltage V SLP  increases as the voltage drop across the resistor R 1  increases. 
     The adder  152  adds a predetermined positive voltage V SFT  (e.g., 200 mV) to the voltage V SNS  from the amplifier  130 , and outputs the resulting sum voltage (V SNS +V SFT ). 
     The error amplifier  153  is a conductance amplifier. The inverting terminal of the error amplifier  153  is fed with the voltage (V SNS +V SFT ), and the non-inverting input terminal of the error amplifier  153  is fed with a predetermined positive reference voltage V REF . The output terminal of the error amplifier  153  is connected to a conductor  157 . The error amplifier  153  lets pass, in and out between its output terminal and the conductor  157 , a current commensurate with the error between the voltage (V SNS +V SFT ) and the reference voltage V REF . The output terminal of the error amplifier  153  is connected via the conductor  157  to the terminal COMP, and an error voltage V ERR  commensurate with the error between the voltage (V SNS +V SFT ) and the reference voltage V REF  appears on the conductor  157 . The resistor R 16  and the capacitor C 12  compensate the phase of the error voltage V ERR . The error amplifier  153 , when V SNS  V SFT &lt;V REF , outputs a current to the conductor  157  so as to raise the error voltage V ERR  and, when V SNS  V SFT &gt;V REF , draws in a current from the conductor  157  so as to lower the error voltage V ERR . 
     The oscillator  154  generates and outputs a signal SET of which the signal level changes between low and high levels cyclically. The frequency f SW  of the signal SET corresponds to the switching frequency of the output transistor M 1 . The frequency f SW  is significantly higher than the frequency f PWM  of the control signals CNT 1  and CNT 2 . For example, whereas the frequency f PWM  is about several hundred hertz, the frequency f SW  is several tens of kilohertz to several hundred kilohertz. The frequency f SW  is set variably according to the resistance value of the resistor R 15  connected to the terminal RT, though the frequency f SW  may be a fixed frequency. 
     The non-inverting input terminal and the inverting input terminal of the comparator  155  are fed with the error voltage V ERR  and the slope voltage V SLP  respectively. The comparator  155  compares the error voltage V ERR  and the slope voltage V SLP  and outputs a signal RST according to their magnitude relationship. When V ERR &gt;V SLP , the signal RST is at high level, and when V ERR &lt;V SLP , the signal RST is at low level. When V ERR =V SLP , the signal RST is at either high or low level. 
     The driver  156  is fed with the signals SET and RST and the control signal CNT 2 . During the high-level period of the control signal CNT 2  (i.e., the lighting period specified by the control signal CNT 2 ), based on the signals SET and RST the driver  156  controls the gate potential of the output transistor M 1  and thereby performs the switching driving of the output transistor M 1 . When the gate potential of the output transistor M 1  is at high level, the output transistor M 1  is in the on state; when the gate potential of the output transistor M 1  is at low level, the output transistor M 1  is in the off state. Accordingly, in the switching driving of the output transistor M 1 , synchronously with an up edge (a transition from low level to high level) in the signal SET, the gate potential of the output transistor M 1  is turned from low level to high level and thereby the output transistor M 1  is turned on; thereafter, synchronously with a down edge (a transition from high level to low level) in the signal RST, the gate potential of the output transistor M 1  is turned from high level to low level and thereby the output transistor M 1  is turned off. The signal SET is a rectangular-wave signal of the frequency f SW , and thus, during the high-level period of the control signal CNT 2 , the output transistor M 1  is subjected to switching driving at the frequency f SW . 
     Through the switching driving of the output transistor M 1 , the input voltage V IN  is switched so that a voltage with a rectangular waveform appears at the node NDsw. The inductor L 1  and the capacitor C 1  constitute a rectifying-smoothing circuit that rectifies and smooths the voltage with a rectangular waveform appearing at the node NDsw to generate the output voltage V OUT . 
     According to the reference voltage V REF  fed to the error amplifier  153 , the target current I TG  is set, and feedback control is performed such that, during the high-level period of the control signal CNT 2 , the driving current I LED  is equal to the target current I TG . 
     For the inductor current I L , a limit current I OCL  is set as an upper-limit current. When, with the output transistor M 1  in the on state, the voltage drop across the resistor R 1  (i.e., the terminal-to-terminal voltage across the resistor R 1 ) reaches a predetermined voltage that corresponds to the resistance value of the resistor R 1  multiplied by the limit current I OCL , the driver  156  turns off the output transistor M 1  regardless of the level of the signal RST. The driver  156  is connected to the terminal GH as well as to the terminals BOOT and SW and, by operating a bootstrap circuit configured to include the capacitor C 2 , acquires a voltage with which to turn on the output transistor M 1 . 
     During the low-level period of the control signal CNT 2  (i.e., the extinction period specified by the control signal CNT 2 ), the driver  156  keeps the gate potential of the output transistor M 1  at low level regardless of the signals SET and RST and thereby holds the output transistor M 1  in the off state (i.e., suspends the switching driving of the output transistor M 1 ). As a result, the switching of the input voltage V IN  is suspended. During the low-level period of the control signal CNT 2 , once the output transistor M 1  in the off state for certain, the operation of the switching controller  150  may be suspended. 
     In the lighting control device  10  in  FIG.  1   , the blocks identified by the reference signs “ 171 ” to “ 175 ” operate based on the input voltage V IN  fed in via the terminal IN_TM, or based on an internal supply voltage generated from the input voltage V IN . An internal power supply circuit  171  generates a predetermined internal supply voltage (e.g., a voltage of 5 V with reference to the ground GND 2 ) at the terminal VREFA based on the input voltage V IN . An undervoltage detection circuit  172  detects an undervoltage condition in which the input voltage V IN  is too low. An overheat detection circuit  173  performs protective operation in accordance with the temperature of the lighting control device  10 . An internal power supply circuit  174  generates a predetermined internal supply voltage (e.g., a voltage of 3 V with reference to the ground GND 2 ) at the terminal VREFB based on the input voltage V IN  or the voltage at the terminal VREFA. An enable detection circuit  175  enables or disables the lighting control device  10  according to the voltage at the terminal EN (with reference to the ground GND 2 ). Only when the voltage at the terminal EN (with reference to the ground GND 2 ) is equal to or higher than a predetermined enable voltage is the lighting control device  10  in an enabled state, and the switching driving described above is performed only in the enabled state. In the following description, unless otherwise stated, it is assumed that the lighting control device  10  is in the enabled state. 
       FIG.  6    schematically shows the relationship among the driving current I LED , the output voltage V OUT , and the control signal CNT 2  along with the operating state of the DC-DC converter configured to include the output transistor M 1 . While  FIG.  6    shows as if, synchronously with an up edge in the control signal CNT 2 , the driving current I LED  starts to flow with no delay, in reality the driving current I LED  starts to flow with a delay.  FIG.  7    shows waveforms observed in the lighting device  1 , with the delay taken into consideration. 
     In  FIG.  7   , the waveforms  611 ,  612 ,  613 ,  615  and  616  indicated by polygonal solid lines depict the driving current I LED , the output voltage V OUT , the inductor current I L , the control signal CNT 1 , and the control signal CNT 2  respectively. In  FIG.  7   , identified by the reference sign “614” is the operating state of the DC-DC converter configured to include the output transistor M 1 . 
     Immediately before timing T A1 , the control signals CNT 1  and CNT 2  are at low level, the output transistor M 1  is kept in the off state (i.e., the operation of the DC-DC converter is kept off), and the driving current I LED  is substantially zero. At timing T A1 , the control signal CNT 1  turns from low level to high level; thus, at timing T A1 , the control signal CNT 2  too turns from low level to high level. In response to the control signal CNT 2  turning to high level, the switching controller  150  starts to perform the switching driving of the output transistor M 1  (i.e., the operation of the DC-DC converter is turned on). As a result, at timing T A1 , the output voltage V OUT  starts to rise. 
     When the output voltage V OUT  has risen to a certain point, the driving current I LED  starts to increase; when at timing T A2  the driving current I LED  reaches the target current I TG , feedback control acts in a direction to reduce the on duty of the output transistor M 1 . Through this feedback control, until the control signal CNT 2  thereafter turns to low level (until timing T A4  mentioned later), the driving current I LED  is kept around the target current I TG  and the output voltage V OUT  is kept around a voltage suited to keep I LED =I TG . During the rise of the output voltage V OUT  between timings T A1  and T A2 , the inductor current I L  remains approximately equal to the limit current I OCL  (precisely, as a pulsating current of the frequency f SW ); when the feedback control that tends to reduce the on duty of the output transistor M 1  acts, the inductor current I L  becomes lower than the limit current I OCL  under the feedback control. 
     After timing T A2 , at timing T A3 , the control signal CNT 1  turns from high level to low level; at this timing, the control signal CNT 2  is kept at high level. After timing T A3 , at timing T A4 , the control signal CNT 2  turns from high level to low level. In response to the control signal CNT 2  turning to low level, the switching controller  150  suspends the switching driving of the output transistor M 1  (i.e., the operation of the DC-DC converter is turned off). Thus, at timing T A4 , the output voltage V OUT  and the driving current I LED  start to fall. After timing T A4 , owing to the electric charge stored in the capacitor C 1 , the driving current I LED  flows for a length of time while decreasing until eventually becoming substantially equal to zero. After timing T A4 , the output voltage V OUT  falls to a potential higher than the ground GND 2  by a voltage Vf 20 . The voltage Vf 20  is the forward voltage across the lighting unit  20  with a minute current (e.g., 1 μA) flowing through it. After timing T A4 , at timing T A5 , the control signals CNT 1  and CNT 2  turn back to high level. The operation during the unit period starting at timing T A5  is the same as the operation during the unit period starting at timing TAL 
     For the sake of discussion, if after timing T A4  the control signal CNT 2  is kept at low level for a sufficiently long time, the output voltage V OUT  will eventually fall to the potential of the ground GND 2 . Here, however, it is assumed that the capacitor C 1  has a sufficient capacitance to keep the output voltage V OUT  from falling below the voltage Vf 20 . 
     Let the difference between the output voltage V OUT  between timings T A2  and T A4  and the voltage Vf 20  be ΔV OUT . Then, the time lag between timings T A1  and T A2  is given by (C 1 ×ΔV OUT )/(I OCL ×DUTY OFF ). Here, DUTY OFF  represents the off duty of the output transistor M 1  between timings T A1  and T A2  (the proportion of the off period of the output transistor M 1  in the sum of the on and off periods of the output transistor M 1 ). 
     At a particular timing after timing T A1  but before timing T A2 , the driving current I LED  in the process of rising reaches a predetermined judgment current I LED_SG . The time from timing T A1  to that particular timing is referred to as the delay time t D . The control signal corrector  120  generates the control signal CNT 2  such that timing T A4 , at which the control signal CNT 2  turns from high level to low level, is delayed by the delay time t D  from timing T A3 , at which the control signal CNT 1  turns from high level to low level. That is, the control signal CNT 2  is generated by correcting, in each of the cycles recurring at the frequency f PWM , the control signal CNT 1  in a direction in which the lighting period is extended by the time (t D ) after the start of the lighting period (after an up edge in the control signals CNT 1  and CNT 2 ) until the driving current I LED  reaches the predetermined judgment current I LED_SG . 
     In this way, the light emission time of the lighting unit  20  becomes substantially equal to the time specified by the control signal CNT 1  (the length of the high-level period of the control signal CNT 1 ), and this makes it possible to achieve dimming as set by the dimming signal DIM. 
       FIG.  8    shows a configuration example of the control signal corrector  120 . The control signal corrector  120  in  FIG.  8    includes an EXOR circuit  121 , which is a two-input exclusive logical disjunction circuit; an AND circuit  122 , which is a two-input logical conjunction circuit; a FF  123 , which is an asynchronous set/reset-type flip-flop circuit; an OR circuit  124 , which is a two-input logical disjunction circuit; and a counter  125 , which is an up/down counter. 
     The EXOR circuit  121  has a first input terminal, a second input terminal, and an output terminal, and outputs via the output terminal a signal Sig 121 . The first and second input terminals of the EXOR circuit  121  are fed with the control signal CNT 1  and the current state signal SG respectively. The EXOR circuit  121  outputs a low-level signal Sig 121  if the control signal CNT 1  and the current state signal SG are both at high level or if the control signal CNT 1  and the current state signal SG are both at low level. The EXOR circuit  121  outputs a high-level signal Sig 121  if, of the control signal CNT 1  and the current state signal SG, one is at high level and the other is at low level. 
     The AND circuit  122  has a first input terminal, a second input terminal, and an output terminal, and outputs via the output terminal a signal Sig 122 . The first and second input terminals of the AND circuit  122  are fed with the signal Sig 121  and a clock signal CLK respectively. The clock signal CLK is a rectangular-wave signal of which the signal level changes between low and high levels cyclically, and is generated by a clock signal generator (not shown) incorporated in the lighting control device  10 . The clock signal CLK has a frequency f CLK  significantly higher than the frequency f PWM  of the control signals CNT 1  and CNT 2 . For example, whereas the frequency f PWM  is about several hundred hertz, the frequency f CLK  is several tens of kilohertz to several hundred kilohertz. The signal SET generated in the oscillator  154  may be used as the clock signal CLK. The AND circuit  122  outputs a high-level signal Sig 122  only if the signal Sig 121  and the clock signal CLK are both at high level, and otherwise keeps the signal Sig 121  at low level. 
     The FF  123  has a set terminal, a reset terminal, and an inverting output terminal, and outputs via the inverting output terminal a signal Sig 123 . The set terminal and reset terminal of the FF  123  are fed with a signal Sig 125  and the control signal CNT 1  respectively. The FF  123  holds the logical value “1” if the signal Sig 125  is at high level and in addition the control signal CNT 1  is at low level, and holds the logical value “0” if the signal Sig 125  is at low level and in addition the control signal CNT 1  is at high level. The FF  123  does not change the logical value it holds if the signal Sig 125  is at low level and in addition the control signal CNT  1  is at low level. When holding the logical value “1”, the FF  123  outputs a low-level signal Sig 123  and, when holding the logical value “0”, the FF  123  outputs a high-level signal Sig 123 . In the configuration in  FIG.  8   , it does not occur that the signal Sig 125  and the control signal CNT 1  are simultaneously at high level. 
     The OR circuit  124  has a first input terminal, a second input terminal, and an output terminal, and outputs via the output terminal the control signal CNT 2 . The first and second input terminals of the OR circuit  124  are fed with the control signal CNT 1  and the signal Sig 123  respectively. The OR circuit  124  keeps the control signal CNT 2  at high level if at least one of the control signal CNT 1  and the signal Sig 123  is at high level, and keeps the control signal CNT 2  at low level only if the control signal CNT 1  and the signal Sig 123  are both at low level. 
     The counter  125  has input terminals  125   a ,  125   b , and  125   c  and an output terminal  125   d . The input terminals  125   a ,  125   b , and  125   c  are fed with the control signal CNT 1 , the signal Sig 122 , and the signal Sig 123  respectively, and from the output terminal  125   d , the signal Sig 125  is output. 
     The counter  125  has the function of counting upward or downward a count value VAL CNT  it manages. The count value VAL CNT  has an initial value of zero. 
     In the high-level period of the signal Sig 123 , the counter  125  performs upward counting if the control signal CNT 1  is at high level and performs downward counting if the control signal CNT 1  is at low level. The counter  125 , in upward counting, increments the count value VAL CNT  by one every time an up edge occurs in the signal Sig 122  and, in downward counting, decrements the count value VAL CNT  by one every time an up edge occurs in the signal Sig 122 . The counter  125  may instead, in upward counting, increment the count value VAL CNT  by one every time a down edge occurs in the signal Sig 122  and, in downward counting, decrement the count value VAL CNT  by one every time a down edge occurs in the signal Sig 122 . The lower limit of the count value VAL CNT  is zero, and it does not occur that the count value VAL CNT  falls below zero. 
     During the low-level period of the signal Sig 123 , the counter  125  performs neither upward nor downward counting, and keeps the count value VAL CNT  unchanged. 
     Moreover, the counter  125 , if the count value VAL CNT  is zero, keeps the signal Sig 125  at high level and, if the count value VAL CNT  is not zero (hence when VAL CNT &gt;0), keeps the signal Sig 125  at low level. 
       FIG.  9    is a timing chart of the control signal corrector  120 . In  FIG.  9   , the period between timings T B1  and T B2  corresponds to the high-level period (lighting period) of the control signal CNT 2  within one unit period. Timings T B1 , T B2 , and T B4  in  FIG.  9    correspond to T A1 , T A2 , and T A4  in  FIG.  7   . Timing T B2  in  FIG.  9    corresponds to the timing that the delay time t D  mentioned above has elapsed after the timing T B1 . It is assumed that, immediately before timing T B1 , the count value VAL CNT  equals zero. 
     At timing T B1 , an up edge occurs in the control signal CNT 1 . Immediately before timing T B1 , the driving current km equals substantially zero, and thus, immediately before and at timing T B1 , the current state signal SG is at low level. Accordingly, the EXOR circuit  121  so operates that, synchronously with the up edge in the control signal CNT 1  at timing T B1 , an up edge occurs in the signal Sig 121 . After the signal Sig 121  turns to high level at timing T B1 , until the signal Sig 121  turns to low level, a signal identical with the clock signal CLK appears as the signal Sig 122 . On the other hand, the FF  123  and the OR circuit  124  so operate that, synchronously with an up edge in the control signal CNT 1  at timing T B1 , an up edge occurs also in the signal Sig 123  and in the control signal CNT 2 . At timing T B1 , the signal Sig 123  turns to high level, and this enables the above-mentioned upward or downward counting. From timing T B1  to timing T B2 , the control signal CNT 1  is at high level, and thus upward counting is performed; after upward counting is started, when VAL CNT &gt;0, a down edge occurs in the signal Sig 125 . 
     In response to the control signal CNT 2  turning to high level at timing T B1 , the switching controller  150  starts the switching driving of the output transistor M 1  (i.e., the operation of the DC-DC converter is turned on). Thus, at timing T B1 , the output voltage V OUT  starts to rise. When the output voltage V OUT  has risen to a certain point, the driving current I LED  starts to increase; at timing T B2 , the driving current I LED  reaches the judgment current I LED _so, and thus an up edge occurs in the current state signal SG. The control signal CNT 1  is at high level from timing T B1  to timing T B3  after timing T B2 , and thus, synchronously with the up edge in the current state signal SG at timing T B2 , a down edge occurs in the signal Sig 121 . This causes a transition to a state where the signal Sig 122  is held at low level; thus, the incrementing of the count value VAL CNT  by upward counting is suspended until timing T B2 . 
     Thereafter, at timing T B3 , a down edge occurs in the control signal CNT 1 . After timing T B2 , the driving current I LED  is continuously kept higher than the judgment current I LED_SG , and thus at timing T B3  the current state signal SG is at high level. Accordingly, synchronously with the down edge in the control signal CNT 1  at timing T B3 , an up edge occurs in the signal Sig 121 . This restarts the state where a signal equivalent to the clock signal CLK appears as the signal Sig 122 , but here, since after timing T B3  the control signal CNT 1  is at low level, from the timing T B3  downward counting is performed synchronously with the signal Sig 122 . 
     By the downward counting that starts at timing T B3 , the count value VAL CNT  is decremented starting with the count value VAL CNT  at timing T B3  until, at timing T B4 , the count value VAL CNT  falls down to zero. Then, at timing T B4 , an up edge occurs in the signal Sig 125 . In synchronization with the up edge in the signal Sig 125 , a down edge occurs in the signal Sig 123 , and, since at this timing the control signal CNT 1  is at low level, at timing T B4  a down edge occurs in the control signal CNT 2 . In response to the down edge in the control signal CNT 2  at timing T B4 , the switching driving of the output transistor M 1  is suspended (i.e., the operation of the DC-DC converter is turned off). Now the output voltage V OUT  and the driving current I LED  decreases until, when the driving current I LED  falls below the judgment current I LED_SG , a down edge occurs in the current state signal SG and a down edge occurs also in the signal Sig 121 , causing a return to the state where the signal Sig 122  is kept at low level. 
     Since the frequency f CLK  of the clock signal CLK is fixed, the time between timings T B3  and T B4  is equal to the time between timings T B1  and T B2  (i.e., the delay time t D ). 
     As described above, within each of the cycles recurring at the frequency f PWM , the start of the switching of the input voltage V IN  causes the output voltage V OUT  to rise; as the output voltage V OUT  rises, when the driving current I LED  flowing through the lighting unit  20  reaches the predetermined judgment current I LED_SG , the comparator  140  (current state signal generator) turns the current state signal SG from a first state (here, low level) to a second state (here, high level). Within each of the cycles recurring at the frequency f PWM , after the start of the lighting period (from timing TO until the current state signal SG is turned from the first state to the second state, the control signal corrector  120  corrects the control signal CNT 1  (raw control signal) in a direction in which the lighting period is extended by the time (t D ) and thereby generates the control signal CNT 2  (corrected control signal). 
     While here the first and second states of the current state signal SG corresponding to low and high levels respectively, the correspondence may be reversed. 
     More specifically, within each of the cycles recurring at the frequency f PWM , the control signal corrector  120  performs a first measurement process to measure a first time (the time between timings T B1  and T B2 ) after the start of the lighting period (from timing TO until a current state signal turns from the first state to the second state; the control signal corrector  120  then performs a second measurement process to measure the time lapse after the timing (timing T B3 ) of the transition from the lighting period to the extinction period in the control signal CNT 1 ; and, after the first measurement process, when the time lapse measured in the second measurement process reaches the first time (i.e., at timing T B4 ), the control signal corrector  120  changes the period specified by the control signal CNT 2  from the lighting period to the extinction period (here, turns the control signal CNT 2  from high level to low level). 
     The first measurement process described above is achieved by the upward counting between timings T B1  and T B2 , and the second measurement process described above is achieved by the downward counting between timings T B3  and T B4 . 
     In the configuration in  FIG.  8   , the counter  125  as an upward counter operates as follows. Within each of the cycles recurring at the frequency f PWM , after the start of the lighting period (from timing TO until the current state signal SG turns from the first state (here, low level) to the second state (here, high level), synchronously with a predetermined clock signal CLK, the counter  125  changes the count value VAL CNT , starting at a predetermined initial value (here, zero), in a first direction (here, in the incrementing direction). Thereafter, from the timing (timing T B3 ) of the transition from the lighting period to the extinction period in the control signal CNT 1  until the count value VAL CNT  returns to the initial value, synchronously with the clock signal CLK, the counter  125  changes the count value VAL CNT  in a second direction (here, in the decrementing direction) opposite to the first direction. Changing the count value VAL CNT  in the first direction achieves the first measurement process, and changing the count value VAL CNT  in the second direction achieves the second measurement process. When the count value VAL CNT  returns to the initial value in the second measurement process, the control signal corrector  120  changes the period specified by the control signal CNT 2  from the lighting period to the extinction period (here, the control signal CNT 2  is turned from high level to low level). 
     In a case where the first measurement process is achieved by upward counting, the first direction corresponds to the incrementing direction, and in a case where the second measurement process is achieved by downward counting, the second direction corresponds to the decrementing direction. Instead, the first measurement process may be achieved by downward counting and the second measurement process by upward counting, in which case the first direction corresponds to the decrementing direction and the second direction to the incrementing direction. 
     While in this embodiment the low-level period of the control signals CNT 1  and CNT 2  is associated with the extinction period and the high-level period of the control signals CNT 1  and CNT 2  is associated with the lighting period, a modification the other way around is possible where the high-level period of the control signals CNT 1  and CNT 2  is associated with the extinction period and the low-level period of the control signals CNT 1  and CNT 2  is associated with the lighting period. 
     Second Embodiment 
     A second embodiment of the present disclosure will be described. The second embodiment deals with applied technologies applicable to the first embodiment, modified technologies derived from the first embodiment, and the like. 
     In the configuration example in  FIG.  1   , the DC-DC converter that generates the output voltage V OUT  from the input voltage V IN  employs diode rectification; instead, the DC-DC converter may employ synchronous rectification. In a configuration that employs synchronous rectification, the lighting device  1  can include, as a rectification element, a synchronous rectification transistor (not shown) instead of the diode D 1 . In that case, the synchronous rectification transistor can be configured as an N-channel MOSFET, with its drain and the source connected to the node NDsw and the ground GND 2  respectively, and the driver  156  can control the gate potential of the synchronous rectification transistor to turn it on and off. Specifically, during the high-level period of the control signal CNT 2 , the driver  156  can keep the synchronous rectification transistor off when the output transistor M 1  is on and keep the synchronous rectification transistor on when the output transistor M 1  is off. 
     In configuration example in  FIG.  1   , some of the components provided outside the lighting control device  10  may be incorporated in the lighting control device  10 . For example, the resistor R 1 , the output transistor M 1 , and the diode D 1  in  FIG.  1    may be incorporated in the lighting control device  10 . In that case, the terminals CSH, CSL, and GH are internal terminals of the lighting control device  10 . In a configuration that employs synchronous rectification, the resistor R 1 , the output transistor M 1 , and the synchronous rectification transistor may be incorporated in the lighting control device  10 . 
       FIG.  10    shows the configuration of a lighting system SYS that incorporates the lighting device  1 . The lighting system SYS includes the lighting device  1 , a voltage source  2 , a system controller  3 , and a switch  4 . The voltage source  2  has a negative terminal and a positive terminal. The negative terminal of the voltage source  2  is connected to the ground GND 1 , and the voltage source  2  outputs via the positive terminal a positive direct-current voltage Vs with reference to the potential of the ground GND 1 . Between the positive terminal of the voltage source  2  and the input terminal IN of the lighting device  1 , the switch  4  is inserted in series; thus, only when the switch  4  is on is the output voltage Vs of the voltage source  2  supplied as the input voltage V IN  to the input terminal IN. When the switch  4  is off, the potential at the input terminal IN is equal to the potential of the ground GND 1  (a transient state ignored). The system controller  3  controls the switch  4  to turn it on and off. In the lighting system SYS, only when the switch  4  is on does the lighting unit  20  PWM-driven light. 
     As shown in  FIG.  11   , the lighting system SYS can be mounted on a vehicle CR. The vehicle CR is, for example, any car that can run on a road. While  FIG.  11    assumes a four-wheel automobile as the vehicle CR, the vehicle CR may be any vehicle such as a motorbike. The vehicle CR includes a body BDY formed of metal or the like and the lighting system SYS installed in the body BDY. The voltage source  2  in the lighting system SYS may be a battery provided in the vehicle CR, or may be a power supply circuit that generates the direct-current voltage Vs by performing DC-DC conversion on the output voltage of the battery. The lighting device  1  can be used as any lighting apparatus on the vehicle CR (i.e., a vehicle-mounted lighting apparatus), and the lighting apparatus implemented with the lighting device  1  is provided at a predetermined place in the body BDY. For example, the lighting device  1  can be used as a headlamp (headlight) of the vehicle CR. The lighting device  1  can be used as any lighting apparatus other than a headlamp (such as a tail lamp, brake lamp, fog lamp, direction indicator lamp, or the like) on the vehicle CR. 
     The present disclosure may be applied to any uses other than vehicle onboard uses. That is, the lighting device  1  or the lighting system SYS may be incorporated in any equipment other than vehicles. 
     The average luminance of the PWM-driven lighting unit  20  is set based on the dimming signal DIM. While in the configuration example in  FIG.  1    the dimming signal DIM is a fixed analog voltage, the dimming signal DIM may instead be a variable analog voltage that is fed from a circuit (e.g., the system controller  3  in  FIG.  10   ) provided outside the lighting control device  10 . Or, in a configuration where communication via a serial peripheral interface or the like is possible between the lighting control device  10  and a circuit (e.g., the system controller  3  in  FIG.  10   ) provided outside the lighting control device  10 , the dimming signal DIM may be set according to a digital signal communicated from the circuit provided outside. 
     The light-emitting elements constituting the lighting unit  20  may be any light-emitting elements other than light-emitting diodes, and may be, for example, light-emitting elements employing organic electroluminescence. 
     For any signal or voltage, the relationship between its high and low levels may be reversed so long as that can be done without departure from what has been described above. 
     Any of the transistors mentioned above may be of any type. For example, any of the transistors mentioned above as a MOSFET may be replaced with a junction FET, an IGBT (insulated-gate bipolar transistor), or a bipolar transistor. 
     Embodiments of the present disclosure can be modified in many ways as necessary without departure from the scope of the technical concepts defined in the appended claims. The embodiments described herein are merely examples of how the present disclosure can be implemented, and what is meant by any of the terms used to describe the present disclosure and its constituent elements is not limited to what is specifically mentioned above in connection with the embodiments. The specific values mentioned in the above description are merely illustrative and needless to say can be modified to different values.