Abstract:
A light-emitting-element-driving-control circuit comprising: a control circuit to turn on or off a transistor based on an input-control signal, the transistor being connected in series with a light-emitting element and an inductor connected in series and controlling increase and decrease of a driving current of the light-emitting element; a maximum-value-detection circuit to detect a maximum value of the driving current; and a control-signal-generation circuit to generate the control signal for turning on the transistor to increase the driving current at a speed corresponding to a level of a power-supply voltage when the driving current is smaller than the maximum value and turning off the transistor to be kept for a predetermined period to decrease the driving current at a speed corresponding to a level of a forward voltage of the light-emitting element when the driving current reaches the maximum value, based on a detection result of the maximum-value-detection circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to Japanese Patent Application No. 2008-244589, filed Sep. 24, 2008, of which full contents are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a light-emitting element driving control circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    In order to efficiently drive an LED (Light Emitting Diode), which is recently used in various electronic equipment, an LED driving control circuit employing a switching control method might be used (See Japanese Patent Laid-Open Publication No. 2006-230133, for example.) 
         [0006]      FIG. 4  is an example of the LED driving control circuit for controlling driving of a white LED for illumination. An LED driving control circuit  100  performs switching for an NMOS transistor  300  to control a driving current Is of white LEDs  310  to  319  (hereinafter referred to as LEDs  310  to  319 .) The LED driving control circuit  100  includes a pulse generation circuit  200 , a comparator  210 , a reference voltage circuit  220 , and an SR flip-flop  230 . 
         [0007]    The pulse generation circuit  200  generates an output signal Vp including a pulse of a high level (hereinafter referred to as H level) in every predetermined cycle TA. 
         [0008]    The comparator  210  detects whether or not the driving current Is has reached a predetermined current value I 1 . Specifically, the comparator  210  compares a detection voltage Vs, which is generated at one end of a detection resistor  340  and generated according to a current value of the driving current Is, with a reference voltage Vref of a reference voltage circuit  220 . When the detection voltage Vs becomes higher than the reference voltage Vref, it is considered that the driving current Is has reached the predetermined current value I 1 , and the comparator  210  changes an output signal Vc from a low level (hereinafter referred to as L level) to the H level. 
         [0009]    The SR flip-flop  230  changes a Q output to the H level to turn on the NMOS transistor  300  when the output signal Vp from the pulse generation circuit  200  is changed to the H level. On the other hand, the SR flip-flop  230  changes the Q output to the L level to turn off the NMOS transistor  300  when the output signal Vc of the comparator  210  is change to the H level. 
         [0010]    A change of the driving current Is will now be described referring to an upper side of a timing chart shown in  FIG. 5 . First, when the output signal Vp is changed to the H level at a time T 0 , the Q output of the SR flip-flop  230  is changed to the H level, and thus, the NMOS transistor  300  is turned on. As a result, the driving current Is is increased at a speed corresponding to an inductance L of an inductor  320  and a level of a power supply voltage VDD. Since the driving current Is is supplied to the detection resistor  340  through the NMOS transistor  300  which has been turned on, the detection voltage Vs is also raised according to the increase of the driving current Is. When the current value of the driving current Is becomes equal to the predetermined current value I 1  at a time T 1 , that is, when the detection voltage Vs becomes equal to the reference voltage Vref, the output signal Vc of the comparator  210  is changed to the H level, and thus, the Q output of the SR flip-flop  230  is changed to the L level. As a result, the NMOS transistor  300  is turned off, and the energy stored in the inductor  320  is released through a loop of the LEDs  310  to  319 , the inductor  320 , and a diode  330 . The energy stored in the inductor  320  is released by the driving current Is at a speed corresponding to the inductance L and respective levels of forward voltages of the LEDs  310  to  319  and the diode  330 . As above, the predetermined current value I 1  is the maximum value of the driving current Is, and the LED driving control circuit  100  controls the NMOS transistor  300  so that the driving current Is does not exceed the maximum value. Since the driving current Is is decreased at the time T 1 , the output signal Vc of the comparator  210  is changed to the L level. 
         [0011]    At a time T 3  at which one cycle of the output signal Vp has elapsed from the time T 0 , the output signal Vp of the pulse generation circuit  200  is changed to the H level, and thus, the NMOS transistor  300  is turned on and the driving current Is is increased as in the case with the time T 0 . In this way, a change from the time T 0  to the time T 3  is repeated at the time T 3  and thereafter. Since the driving current Is is changed in the cycle TA, an average value of the driving current Is is a predetermined value, and thus, the LEDs  310  to  319  are driven by a constant current. If the power supply voltage VDD is increased and the speed of increase of the driving current Is is increased, for example, a period of ON-time of the NMOS transistor  300  is reduced, but a cycle during which the transistor  300  is turned on is not changed. That is, the LED driving control circuit  100  is a switching circuit, which employs a pulse-width modulation method, for changing a pulse width of ON-time when the NMOS transistor  300  is turned on in the cycle TA. 
         [0012]    As described above, the LED driving control circuit  100  performs switching for the NMOS transistor  300  in the cycle TA so that the LEDs  310  to  319  are driven by a constant current. As a result, the cycle of the driving current Is also becomes equal to the cycle TA similarly to a switching cycle. 
         [0013]    However, as shown in a lower side of the timing chart in  FIG. 5 , when the driving current Is, which is changed in the cycle TA before the time T 0 , is reduced due to transitional fluctuations of the power supply voltage VDD, for example, the cycle of the driving current Is does not become equal to the cycle TA even if the power supply voltage VDD is not changed from a desired level at the time T 0  and thereafter. Specifically, when the NMOS transistor  300  is turned on at the time T 0 , the actual driving current Is indicated by a solid line is increased at a speed equivalent to the speed of increase of the driving current Is in the cycle TA indicated by a dotted line, that is, the speed corresponding to the inductance L of the inductor  320  and the level of the power supply voltage VDD. As a result, the actual driving current Is reaches the current value I 1  at the time T 2  later than the above-mentioned time T 1 . Then, when the NMOS transistor  300  is turned off at the time T 2 , the actual driving current Is is decreased at a speed equivalent to the speed of decrease of the driving current Is in the cycle TA, that is, the speed corresponding to the inductance L and the forward voltage level of the LEDs  310  to  319  and the diode  330 . At the time T 3  at which the output signal Vp is changed to the H level, the NMOS transistor  300  is turned on, and thus, the actual driving current Is is increased. Since the actual driving current Is at the time T 3  is greater in current value than the driving current Is in the cycle TA, the actual driving current Is reaches the current value I 1  at a time T 4  earlier than a time T 5 . When the NMOS transistor  300  is turned off at the time T 4 , the actual driving current Is is decreased until a time T 6  at which one cycle of the output signal Vp has elapsed from the time T 3 . The actual driving current Is at the time T 6  is much lower in current value than the driving current Is in the cycle TA. Therefore, even if the NMOS transistor  300  is turned on at the time T 6 , the actual driving current Is will not reach the current value I 1  by a time T 7  at which one cycle of the output signal Vp has elapsed from the time T 6 , but reaches the current value I 1  at a time T 8  within a period from the time T 7  to the time at which one cycle of the output signal Vp has elapsed. 
         [0014]    As described above, even if the switching cycle TA of the NMOS transistor  300 , the speed of increase and the speed of decrease of the driving current Is, and the current value I 1  for detecting the maximum value of the driving current Is are constant, the cycle of the actual driving current Is may not be equal to the cycle TA. That is, when the NMOS transistor  300  is turned on in the cycle TA and the maximum value of the driving current Is is detected to control the driving current Is as mentioned above, sub-harmonic oscillation which oscillates in a cycle longer than the cycle TA may be generated. 
       SUMMARY OF THE INVENTION 
       [0015]    A light-emitting element driving control circuit according to an aspect of the present invention, comprises: a control circuit configured to turn on or off a transistor based on an input control signal, the transistor being connected in series with a light-emitting element and an inductor connected in series, the transistor being configured to control increase and decrease of a driving current of the light-emitting element; a maximum-value detection circuit configured to detect a maximum value of the driving current; and a control signal generation circuit configured to generate the control signal for turning on the transistor to increase the driving current at a speed corresponding to a level of a power supply voltage when the driving current is smaller than the maximum value and turning off the transistor to be kept for a predetermined period to decrease the driving current at a speed corresponding to a level of a forward voltage of the light-emitting element when the driving current reaches the maximum value, based on a detection result of the maximum-value detection circuit. 
         [0016]    Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a diagram illustrating a configuration of an LED driving control circuit  10  according to an embodiment of the prevent invention; 
           [0019]      FIG. 2  is a timing chart for explaining an example of an operation of an LED driving control circuit  10 ; 
           [0020]      FIG. 3  is a timing chart for explaining an example of an operation of an LED driving control circuit  10 ; 
           [0021]      FIG. 4  is a diagram illustrating a configuration of an LED driving control circuit  100 ; and 
           [0022]      FIG. 5  is a timing chart for explaining an example of an operation of an LED driving control circuit  100 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    At least the following details will become apparent from descriptions of this specification and of the accompanying drawings. 
         [0024]      FIG. 1  is a diagram illustrating a configuration of an LED driving control circuit  10  according to an embodiment of the present invention. The LED driving control circuit  10  controls switching of an NMOS transistor  30  so that white LEDs  20  to  29  for illumination (hereinafter referred to as LEDs  20  to  29 ) are driven by a desired constant current, for example. 
         [0025]    The LEDs  20  to  29  are 10 pieces of white LEDs connected in series, in which an anode of the LED  20  is connected to a power supply voltage VDD and a cathode of the LED  29  is connected to one end of an inductor  31 . It is assumed that each forward voltage of the LEDs  20  to  29  according to an embodiment of the present invention is 3V, for example. Also, it is assumed that the power supply voltage VDD according to an embodiment of the present invention is at a sufficiently high level so that the ten LEDs  20  to  29  can be driven. 
         [0026]    The NMOS transistor  30  controls increase and decrease of the driving current Is for driving the LEDs  20  to  29  with use of the inductor  31  and a diode  32 . Specifically, when the NMOS transistor  30  is turned on, the driving current Is is increased at a speed corresponding to an inductance L of the inductor  31  and the power supply voltage VDD. Since the voltage across the inductor  31  is changed according to a difference between the power supply voltage VDD and the sum of the respective forward voltages of the LEDs  20  to  29 , that is 30V, a speed of increase in the driving current Is, i.e., S 1 =dIs/dt, is changed according to (VDD−30)/L. That is, the speed of increase S 1  of the driving current Is according to an embodiment of the present invention is increased according to rise in level of the power supply voltage VDD. When the NMOS transistor  30  is turned on, energy corresponding to the current value of the driving current Is is stored in the inductor  31 . Therefore, when the NMOS transistor  30  is turned off, the energy stored in the inductor  31  is released through a loop of the LEDs  20  to  29 , the inductor  31 , and the diode  32 . In this case, the driving current Is is decreased at a speed corresponding to the sum of the inductance L and the forward voltages of the LEDs  20  to  29  and the diode  32 . Here, assuming that the forward voltage of the diode  32  is 1V, for example, the voltage across the inductor  31  is equal to 31V, which is the sum of 30V, i.e., the sum of the forward voltages of the LEDs  20  to  29 , and the above-mentioned 1V. That is, a speed of decrease in the driving current Is when the NMOS transistor  30  is turned off, i.e., S 2 =dIs/dt, is changed according to  31 /L. Moreover, since the inductance L of the inductor  31  according to an embodiment of the present invention is constant in value, the speed of decrease S 2  of the driving current Is is constant regardless of the level of the power supply voltage VDD. 
         [0027]    A detection resistor  33  detects a current value of the driving current Is when the NMOS transistor  30  is turned on and is provided between a source of the NMOS transistor  30  and a ground GND. In an embodiment according to the present invention, it is assumed that a voltage generated at one end of the detection resistor  33  according to the current value of the driving current Is is a detection voltage Vs. Therefore, a speed of increase in the detection voltage Vs is equal to the speed of increase S 1  in the above-mentioned driving current Is. When the NMOS transistor  30  is turned off, the driving current Is does not flow through the detection resistor  33 , and thus, the detection voltage Vs becomes equal to the ground GND level. 
         [0028]    Circuits making up the LED driving control circuit  10  will now be described in outline. The LED driving control circuit  10  includes a filter  40 , a comparator  41 , a one-shot pulse circuit  42 , an AND circuit  43 , and a buffer circuit  44 . The LED driving control circuit  10  according to an embodiment of the present invention is assumed to be integrated. The filter  40  and the comparator  41  (comparison circuit) correspond to a maximum-value detection circuit according to the present invention, and the AND circuit  43  and the buffer circuit  44  correspond to a control circuit according to the present invention. 
         [0029]    The filter  40  suppresses noise of the detection voltage Vs generated at one end of the detection resistor  33  and outputs the voltage as an output voltage Vf. Since parasitic capacitance (not shown) is present in the inductor  31  according to an embodiment of the present invention, when the NMOS transistor  30  is turned on, electrical charge charged in the parasitic capacitance of the inductor  31  is discharged into the detection resistor  33  through the NMOS transistor  30 . Thus, a surge current corresponding to a capacitance value of the parasitic capacitance transitionally flows through the detection resistor  33 , and a surge voltage is generated as noise in the detection resistor  33 . The filter  40  according to an embodiment of the present invention is assumed to be a low-pass filter for which such a time constant is set that the surge voltage is suppressed and the detection voltage Vs, which changes at the speed of increase S 1 , is output as an output voltage Vf. 
         [0030]    The comparator  41  detects whether or not the driving current Is has reached a predetermined current value I 1 . Specifically, the comparator  41  compares the output voltage Vf output from the filter  40  and a reference voltage Vref output from a microcomputer (not shown), for example. When the output voltage Vf becomes higher than the reference voltage Vref, it is considered that the driving current Is has reached the predetermined current value I 1 , and an output signal Vc of the comparator  41  is changed from the H level to the L level. 
         [0031]    The one-shot pulse circuit  42  (control signal generation circuit) changes an output signal Vp (control signal) to the L level to be kept only for a predetermined period Tx corresponding to a resistance value of a resistor  50  and a capacitance value of a capacitor  51  when the output signal Vc of the comparator  41  is changed to the L level. That is, the one-shot pulse circuit  42  generates a pulse of the L level only for the predetermined period Tx when the output signal Vc is changed to the L level. 
         [0032]    The AND circuit  43  changes an output based on the output signal Vp so as to perform the switching for the NMOS transistor  30  when an enable signal ENB output from the microcomputer (not shown) is at the H level, and outputs a signal for stopping the switching of the NMOS transistor when the enable signal ENB is at the L level. Specifically, when the enable signal ENB is at the H level, the output signal Vp is output as an output of the AND circuit  43 , and when the enable signal is at the L level, the signal of the L level is output. 
         [0033]    The buffer circuit  44  directly drives the NMOS transistor  30  based on the output from the AND circuit  43 . Specifically, when the output from the AND circuit  43  is at the H level, a driving signal Vdr at the H level is output so as to turn on the NMOS transistor  30 . On the other hand, when the output from the AND circuit  43  is at the L level, the driving signal Vdr at the L level is output so as to turn off the NMOS transistor  30 . 
         [0034]    There will now be described an example of an operation of the LED driving control circuit  10  when the LEDs  20  to  29  are driven by a constant current, referring to a timing chart shown in  FIG. 2 . It is assumed here that pulse generation in the one-shot pulse circuit  42  is finished and the output signal Vp is changed from the L level to the H level at a time T 0 . Hereinafter, it is also assumed that the enable signal ENB output from the microcomputer (not shown) is at the H level and the power supply voltage VDD is 33V. Thus, when the NMOS transistor  30  is turned on, the speed of increase S 1 =dIs/dt of the driving current Is is changed according to (33−30)/L=3/L. On the other hand, the speed of decrease S 2 =dIs/dt of the driving current Is when the NMOS transistor  30  is turned off is changed according to 31/L as described above. Therefore, in an embodiment according to the present invention, the speed of decrease S 2  of the driving current Is is faster than the speed of increase S 1 . 
         [0035]    First, when the one-shot pulse circuit  42  changes the output signal Vp to the H level at the time T 0 , the output of the AND circuit  43  is changed to the H level, and as a result, the driving signal Vdr is also changed to the H level. Thus, the NMOS transistor  30  is turned on. When the NMOS transistor  30  is turned on, the surge current is superimposed on the driving current Is due to influence of the parasitic capacitance of the inductor  31 . As a result, the surge voltage is generated as noise in the detection voltage Vs at one end of the detection resistor  33 . As described above, the filter  40  suppresses the surge voltage in the detection voltage Vs as well as increases the output voltage Vf at the same speed as the speed of increase S 1  of the detection voltage Vs. When the driving current Is is increased to reach the current value I 1  at a time T 1 , that is, when the output voltage Vf of the filter  40  reaches the reference voltage Vref, the comparator  41  changes the output signal Vc to the L level. When the output signal Vc is changed to the L level, the one-shot pulse circuit  42  changes the output signal Vp to the L level, and thus, the output of the AND circuit  43  is changes to the L level and the driving signal Vdr of the buffer circuit  44  is changes to the L level as well. As a result, at the time T 1 , the NMOS transistor  30  is turned off. When the NMOS transistor  30  is turned off, the inductor  31  releases the energy accumulated by the driving current Is through the loop of the LEDs  20  to  29 , the inductor  31 , and the diode  32 , and thus, the driving current Is is decreased at the speed of decrease S 2 . The current flowing through the detection resistor  33  at the time T 1  becomes equal to zero, and the detection voltage Vs becomes equal to the ground GND level. Since the one-shot pulse circuit  42  stops generating a pulse at a time T 2  at which the predetermined period Tx has elapsed from the time T 1 , the output signal Vp is changed to the H level. Since the output of the AND circuit  43  is changed to the H level based on the output signal Vp of the H level, the driving signal Vdr of the buffer circuit  44  is changed to the H level as well. Therefore, at the time T 2 , the NMOS transistor  30  is turned on, and the driving current Is is increased at the speed of increase S 1 . At the time T 2  and thereafter, the operation from the time T 0  to the time T 2  is repeated. 
         [0036]    As described above, the period Tx, during which the NMOS transistor  30  is off and the driving current Is is decreased, and the speed of decrease S 2  are constant. Therefore, an amount is also constant of change ΔIA of the driving current Is when decreasing at the speed of decrease S 2  only for the period Tx. When the power supply voltage VDD is constant in level, the speed of increase S 1  of the driving current Is is constant, and thus, the period is also constant during which the driving current Is is changed by ΔIA at the speed of increase S 1 . Therefore, the LED driving control circuit  10  according to an embodiment of the present invention can change the driving current Is in a predetermined cycle based on the speed of increase S 1 , the speed of decrease S 2 , and the period Tx. In an embodiment according to the present invention, when the supply voltage VDD is 33V, the period is referred to as a period Ty during which the driving current Is is changed by ΔIA at the speed of increase S 1 , and a cycle of the driving current Is is referred to as a cycle Tz. Since the driving current Is is changed in the predetermined cycle Tz as above, an average value of the driving current Is is a predetermined value, and thus, the LEDs  20  to  29  are driven by a constant current. 
         [0037]    There will now be described an example of an operation of the LED driving control circuit  10  when the power supply voltage VDD is transitionally fluctuated and the current value of the driving current Is, which changes in the cycle Tz, is changed, for example, referring to a timing chart shown in  FIG. 3 . Here, it is assumed that pulse generation of the one-shot pulse circuit  42  is finished and the output signal Vp is changed from the L level to the H level at a time T 10 . A waveform shown by a broken line in an upper side in  FIG. 3  indicates a driving current Is 1 , which changes in the cycle Tz, and a waveform shown by a solid line indicates a driving current Is 2  whose current value is reduced to be lower than the driving current Is 1  due to transitional fluctuations of the power supply voltage VDD, for example, before the time T 10 . It is also assumed that the power supply voltage VDD is 33V and constant at the time T 10  and thereafter. That is, it is assumed that the speed of increase S 1  and the speed of decrease S 2  of the driving currents Is 1  and Is 2  are not changed at the time T 10  and thereafter. 
         [0038]    When the one-shot pulse circuit  42  changes the output signal Vp to the H level at the time T 10 , the driving signal Vdr is also changed to the H level, and thus, the NMOS transistor  30  is turned on. As a result, the driving current Is 2  on which the surge current is superimposed flows through the detection resistor  33 . The filter  40  suppresses the surge voltage of the detection voltage Vs and increases the output voltage Vf at the speed of increase S 1 . A current value of the driving current Is 2  at the time T 10  is smaller than the driving current Is 1  when there are no transitional fluctuations in the power supply voltage VDD. Therefore, at a time T 12  later than a time T 11  at which the driving current Is 1  reaches the current value I 1 , the driving current Is 2  reaches the current value I 1 . When the driving current Is 2  reaches the current value I 1 , the comparator  41  changes the output signal Vc to the L level, and thus, the one-shot pulse circuit  42  changes the output signal Vp to the L level to be kept only for the predetermined period Tx so as to turn off the NMOS transistor  30 . Therefore, the driving current Is 2  is decreased at the speed of decrease S 2  until a time T 13  at which the period Tx has elapsed from the time T 12 . An amount of decrease of the driving current Is 2  from the time T 12  to the time T 13  is equal to the above-mentioned amount of change ΔIA, since the speed of decrease S 2  and the period Tx are constant. At the time T 13 , the one-shot pulse circuit  42  stops the pulse generation to change the output signal Vp to the H level. Thus, the NMOS transistor  30  is turned on, and the driving current Is 2  is increased at the speed of increase S 1 . The period until when the driving current Is 2  reaches the current value I 1  again is determined according to the above-mentioned amount of change ΔIA and the speed of increase S 1 . At the time T 10  and thereafter, since the power supply voltage VDD is assumed to be constant, the period until when the driving current Is 2  reaches the current value I 1  again is equal to the above-mentioned period Ty. At a time T 14 , at which the period Ty has elapsed from the time T 13 , the driving current Is 2  reaches the current value I 1 , and thus, the one-shot pulse circuit  42  changes the output signal Vp to the L level. The operation of the LED driving control circuit  10  from the time T 14  to a time T 15  at which the period Tx has elapsed is the same as the operation from the time T 12  to the time T 13 . Also, the operation from the time T 13  to the time T 15  is repeated at the time T 15  and thereafter. Therefore, even if the power supply voltage VDD is transitionally fluctuated and the driving current Is 2  having a current value lower than the driving current Is 1  is generated, for example, the LED driving control circuit  10  can continue to change the driving current Is 2  in the cycle Tz. Even if the driving current I 2  is increased to become greater than the driving current I 1  before the time T 10 , for example, since the amount of change ΔIA of the driving current Is 2  and the speed of increase S 1  of the driving current Is 2  are constant, the LED driving control circuit  10  can continue to change the driving current Is 2  in the cycle Tz. 
         [0039]    In the LED driving control circuit  10  according to an embodiment of the present invention with a configuration described above, the comparator  41  detects that the driving current Is reaches the current value I 1 , which is the predetermined maximum value. The one-shot pulse circuit  42  outputs the output signal Vp of the H level so as to turn on the NMOS transistor  30  based on the output signal Vp of the comparator  41  when the driving current Is is smaller than the current value I 1 . When the driving current Is reaches the current value I 1 , the output signal Vp of the L level is output for the period Tx so as to turn off the NMOS transistor  30 . Since the speed of decrease S 2  of the driving current Is and the period Tx when the NMOS transistor  30  is turned off are constant, the amount of change ΔIA of the driving current Is is constant. Moreover, when the power supply voltage VDD is constant in level, the speed of increase S 1  of the driving current Is is constant, and thus, the period is also constant during which the driving current Is is changed by ΔIA at the speed of increase S 1 . Therefore, the LED driving control circuit  10  according to an embodiment of the present invention can change the driving current Is in the predetermined cycle Tz, and can suppress subharmonic oscillation. In general, in a circuit that detects a maximum value of a driving current of a load such as an LED and controls increase and decrease of the driving current by performing switching of a transistor, slope compensation for imparting predetermined inclination to the maximum value of the driving current may be performed in order to suppress the subharmonic oscillation. In an embodiment according to the present invention, there is no need to use a circuit for compensating the slope as above in order to suppress the subharmonic oscillation, thereby preventing a configuration of the LED driving control circuit  10  from becoming complicated. 
         [0040]    Moreover, in an embodiment according to the present invention, the one-shot pulse circuit  42  is employed in order to bring the output signal Vp to the L level only for the period Tx. Thus, it is possible to reliably bring the output signal Vp to the L level only for the period Tx when the comparator  41  detects that the driving current Is has reached the current I 1 . That is, in an embodiment according to the present invention, every time the driving current Is reaches the current value I 1 , the driving current Is can be reliably decreased in current amount by ΔIA. Therefore, if the speed of increase S 1  of the driving current Is is constant, the cycle of the driving current Is can be made constant. 
         [0041]    Furthermore, in an embodiment according to the present invention, the detection voltage Vs is processed in the filter  40 , to be output as the output voltage Vf to the comparator  41 . In a configuration without the filter  40 , if the surge voltage becomes so great that the detection voltage Vs exceeds the level of the reference voltage Vref, there might occur such a malfunction that the output signal Vc is changed to the L level even if the driving current Is has not reached the maximum value yet. In an embodiment according to the present invention, noise caused by the surge voltage of the detection voltage Vs is suppressed by the filter  40  when the maximum value of the driving current Is is detected, and thus, the malfunction can be prevented. 
         [0042]    The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in anyway to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof. 
         [0043]    In an embodiment according to the present invention, the NMOS transistor  30  is employed in order to suppress increase and decrease of the driving current Is, however, an NPN transistor may be employed, for example. 
         [0044]    Moreover, in an embodiment according to the present invention, the inductor  31  is provided between the cathode of the LED  29  and a drain of the NMOS transistor  30 , however, the inductor may be provided between the power supply voltage VDD and the anode of the LED  20 . 
         [0045]    Furthermore, in an embodiment according to the present invention, the diode  32  is provided in order to regenerate the driving current Is when the NMOS transistor  30  is turned off, however, this is not limitative. The same effect can be obtained as in an embodiment according to the present invention by providing a switch circuit that is turned on or off in a complementary manner with the NMOS transistor  30  instead of the diode  32 , for example.