Abstract:
An LED driver circuit capable of overcoming the issues in that a brightness change is perceived as a stepped change in a very low light amount region or light abruptly goes out by using PWM control and of realizing smooth dimming even at a very low amount of light is provided. In the LED driver circuit, a first circuit including a first resistor and a first power transistor connected in series and a second circuit including a second resistor and a second power transistor connected in series are connected in parallel with each other and are connected to an LED. First and second PWM signal generator circuits drive the first and second power transistors, respectively. When the first and second power transistors are in an on-state, currents flow through the LED via the first and second resistors, respectively, which enable a smooth brightness change even in a low illuminance region.

Description:
FIELD OF THE INVENTION 
     The present invention relates to LED (light-emitting diode) driver circuits for adjusting luminance of an LED, and more specifically to an LED driver circuit suitable for, for example, a light dimmer circuit enabling expression of a subtle change in background brightness on a dome screen at a planetarium without causing an unnatural sensation. 
     BACKGROUND OF THE INVENTION 
     Unlike incandescent lamps and electric-discharge lamps, LEDs are readily switched on with a direct-current power supply and have a long lifespan. Recently, blue and white LEDs have been put to practical use. Efficiency of LEDs also has reached a level that is better than efficiency of not only incandescent lamps but also fluorescent lamps. LEDs are increasingly widely used not only as display light sources of indicators or the like but also as ordinary illumination light sources or the like. Constant brightness is sufficient for ordinary illumination, whereas dimming for changing the brightness as desired is desirable for some types of interior illumination, stage illumination, and effects illumination at facilities such as planetariums. Changing the brightness of interior illumination as desired allows people to enjoy a change in atmosphere. At bedtime, dim illumination can substitute a function provided by traditional night light bulbs or the like. Stage illumination not only can express a brightness change corresponding to a scene but also can create various scenes by changing the brightness of light sources of individual colors such as red, green, and blue to control hue and saturation. At planetariums, the color and the brightness of illumination are changed as desired not only to use the illumination as houselights when an audience enters or exits but also to express daytime and nighttime scenes. A subtle change in brightness from daytime to dusk and to nighttime is expressed by changing the brightness of illumination. 
     To dim an LED, a method of changing drive current, a method based on PWM (pulse width modulation) control, and other methods are used. In general, the method based on PWM control is often used because of simple circuitry. Since the amount of LED light responds to current in 1 ms or less, which is extremely fast, LEDs have a property in which the brightness and the pulse width are directly proportional to each other. This property enables accurate brightness control. 
     However, in the case where LEDs are used in the aforementioned illumination applications, an issue that is not caused by the hitherto used light sources such as incandescent lamps may occur. The issue is that the brightness change is perceived as a stepped change at low illuminance. In PWM control, a command regarding a pulse width can be numerically given, and the pulse width can be accurately specified using a digital circuit. For example, in the case of a PWM signal of 12-bit resolution, the minimum brightness is 1/4,096. Although this brightness seems extremely low, even this minimum brightness is sufficiently bright for humans because the dynamic range of the human eye for brightness is extremely wide. For example, suppose that the maximum brightness is 500 lx. In this case, the minimum brightness of 1/4,096 is equal to 0.12 lx, which is sufficiently bright to be sensed by eyes. When the brightness is increased by one step, the brightness is equal to 0.24 lx, that is, is steeply doubled. Consequently, the stepped brightness change is visually noticeable. 
     To address this issue, a solution for increasing the resolution of the PWM signal has been conceived. For example, in the case of 16-bit resolution, the minimum brightness is equal to 0.0076 lx, and a significant improvement is expected. However, increasing the resolution in this manner may cause another issue. Specifically, in the case where PWM control is performed at a brightness at which no flicker is noticeable, for example, at approximately 200 Hz, one period is 5 ms. When control is performed at 16-bit resolution, the minimum pulse width is equal to 5 ms/65,536=76.3 ns. Although this is not a speed that is difficult to handle as a speed of a signal of digital circuits, noise is likely to occur if current for causing an LED to emit light is repeatedly supplied and cut at this speed. Consequently, neighboring electronic devices may be affected by noise or noise suppression may become difficult. In addition, if the speed at a rise or fall of the current waveform changes due to a factor such as external noise, the brightness at the lowest illuminance may vary or a flicker may occur, making the brightness change unpleasant to see. 
     These issues are overlooked in ordinary illumination applications as requirements therefor are met even if such issues occur. However, particularly in applications in which brightness control at low illuminance is important, for example, illumination at planetariums or the like, a subtle nighttime brightness needs to be reproduced by an illumination device. Thus, the above-described issues hinder the stage effects. 
     Many proposals such as LED driver circuits including a LED dimmer circuit unit for changing the brightness of an LED by using PWM control have been made. Such driver circuits are used in illumination devices, on-vehicle illumination devices, or the like. However, none of many hitherto proposed LED driver circuits are capable of exerting effects assumed by the inventor of this application. Specific issues will be described by using some related-art literatures regarding configurations that functionally operate in a way similar to that of a configuration of an LED driver circuit proposed by the inventor herein, that is, regarding adjustment of the brightness of an LED by using PWM control. 
     JP 2007-317443(A) has proposed a circuit that enables continuous dimming from the lower limit to the upper limit of an output without complicating circuitry in an illumination system for which dimming control is performed by controlling on/off of power from a power supply based on a PWM signal. In this illumination system, a PWM signal is supplied to a gate of an FET (field effect transistor) that controls an LED from a microprocessor including a ROM (read-only memory) to control the brightness of the LED. In the case where a pulse-on time of the PWM signal based on a control command is finer than a resolution of the clock of the microprocessor, a plurality of pulses having different on times are combined together to create a combined PWM signal so that the average of the pulse-on times becomes equal to the pulse-on time based on the control command. In this way, the continuous dimming from the lower limit to the upper limit of an output is realized. However, in JP 2007-317443(A), pulse width modulation for a low illuminance region and pulse width modulation for a high illuminance region are not used as control signals in order to increase a dynamic range for a LED brightness change, which is aimed by the present invention. Thus, JP 2007-317443(A) does not aim to smooth the brightness change in the low illuminance region. 
     JP 2011-171231(A) aims to provide an LED lighting circuit capable of performing dimming control in a range below the lower limit of PWM-based dimming. To this end, the LED lighting circuit includes a step-down chopper circuit that supplies an LED current to an LED light source unit as a result of oscillation control of a switching element, and a dimming control unit that controls the LED current by performing oscillation control of the switching element. The dimming control unit includes an oscillation frequency control unit that controls an oscillation frequency, a PWM control unit that controls a PWM on-duty, and a driver unit that switches on/off the switching element on the basis of the oscillation frequency and the PWM on-duty. Operation details are as follows. In a range in which the dimming degree is at or above the lower limit of PWM-based dimming, the PWM control unit changes the PWM on-duty to perform dimming control. In a range in which the dimming degree is below the lower limit of PWM-based dimming, the PWM on-duty is kept unchanged and the oscillation frequency is set to be higher than that for the lower limit of PWM-based dimming. The brightness of an LED is controlled by inputting a signal to the control terminal of the switching element from the driver unit to which the PWM signal from the PWM control unit and the oscillation frequency controlled by the oscillation frequency control unit are input. As in JP 2007-317443(A), pulse width modulation for a low illuminance region and pulse width modulation for a high illuminance region are not used as control signals, and control for mainly changing a pulse width that is input to a driver that causes flow of a small current in the low illuminance region and for mainly changing a pulse width input to a driver that causes flow of a large current in the other illuminance region is not performed. 
     An apparatus for generating a drive signal for an illumination device as described in JP 2013-519988(A) aims to provide a concept for driving an LED or an LED spot for an HDTV (high-definition television) camera while making requirements for a drive signal generator for the LED or the LED spot lower than in the related art. To this end, the apparatus according to JP 2013-519988(A) includes a pulse generator that generates a first pulse train in response to a first brightness request for a first brightness and generates a second pulse train in response to a second brightness request for a second brightness. The first pulse train has a first frequency, and the second pulse train has a second frequency different from the first frequency. The second pulse train includes two neighboring pulses of the first pulse train and an additional pulse between the two neighboring pulses. The additional pulse is not included in in the first pulse train. Operation details are as follows. Two pulse trains having different frequencies, the additional pulse being inserted to one of the two pulse trains, are input to drive the LED or the LED spot of the HDTV camera. However, this configuration is different from the configuration of the LED driver circuit proposed by the inventor of this application. In addition, the object is not to address the operation in which a brightness change is perceived as a stepped change in a very low light amount region or light abruptly goes out when the brightness of the LED is changed by using PWM control. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention address various issues described above and aim to provide an LED driver circuit capable of overcoming the issues in that a brightness change is perceived as a stepped change in a very low light amount region or light abruptly goes out when the brightness of an LED is changed by using PWM control and of realizing smooth dimming even at a very low amount of light. 
     To this end, in accordance with a first aspect of the invention, an LED driver circuit includes at least one driver circuit connected to an LED. The at least one driver circuit includes a plurality of current-limiting circuits for which currents that flow therethrough are certain current set values that are different from one another, and a plurality of pulse width modulation circuits. The LED driver is configured to mainly control a pulse width for a current-limiting circuit having a small current set value, among the plurality of current-limiting circuits, in a low light amount region and mainly control a pulse width for a current-limiting circuit having a large current set value, among the plurality of current-limiting circuits, in a high light amount region, whereby an amount of light of the LED is smoothly changed in a wider dynamic range than in a case of using a single driver circuit. 
     In accordance with a second aspect of the invention, in the first aspect of the invention, the at least one driver circuit may be a plurality of driver circuits, and each of the plurality of driver circuits may be connected to a corresponding one of LEDs. 
     In accordance with a third aspect of the invention, in the first aspect of the invention, the at least one driver circuit may be a plurality of driver circuits, and the plurality of driver circuits may be connected in parallel to one another and be connected in common to the LED. 
     In accordance with a fourth aspect of the invention, in the first aspect of the invention, the current-limiting circuit may be a resister. 
     In accordance with a fifth aspect of the invention, in the first aspect of the invention, the current-limiting circuit may be a constant-current diode. 
     In accordance with a sixth aspect of the invention, an LED driver circuit used in an illumination device that illuminates background on a dome screen at a planetarium includes at least one driver circuit connected to an LED. The at least one driver circuit includes a plurality of current-limiting circuits for which currents that flow therethrough are certain current set values that are different from one another, and a plurality of pulse width modulation circuits. The LED driver circuit is configured to mainly control a pulse width for a current-limiting circuit having a small current set value, among the plurality of current-limiting circuits, in a low light amount region and mainly control a pulse width for a current-limiting circuit having a large current set value, among the plurality of current-limiting circuits, in a high light amount region, whereby an amount of light of the LED is smoothly changed in a wider dynamic range than in a case of using a single driver circuit. 
     In accordance with a seventh aspect of the invention, in the sixth aspect of the invention, the at least one driver circuit may be a plurality of driver circuits, and each of the plurality of driver circuits may be connected to a corresponding one of LEDs. 
     In accordance with an eighth aspect of the invention, in the sixth aspect of the invention, the at least one driver circuit may be a plurality of driver circuits, and the plurality of driver circuits may be connected in parallel to one another and be connected in common to the LED. 
     In accordance with a ninth aspect of the invention, in the sixth aspect of the invention, a plurality of the illumination devices may be provided in the vicinity of a periphery of the planetarium. 
     In accordance with a tenth aspect of the invention, in the sixth aspect of the invention, a brightness of the illumination device may be controlled by manual operation or be automatically controlled by a control system at the planetarium. 
     In accordance with an eleventh aspect of the invention, in the sixth aspect of the invention, a control system at the planetarium may store a real-time solar altitude, calculate a brightness on the basis of the solar altitude, and send a command input based on the calculated brightness to the illumination device to reproduce a change between daytime and nighttime. 
     With the configuration above, a change in background brightness on a dome screen can be smoothly expressed if the LED driver circuit according to the aspects of the invention is applied to an illumination device at planetariums. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an LED driver circuit according to an embodiment of the present invention; 
         FIG. 2  is a graph illustrating characteristics of a conversion function used to obtain, for a change in a dimming command value, an actual brightness change in a wide dynamic range implemented by the circuit illustrated in  FIG. 1 ; 
         FIG. 3  is a graph illustrating an example of an actual brightness change controlled based on a change in the dimming command value, the graph being an enlarged illustration of output characteristics implemented by a PWM circuit of 12-bit resolution in a low light flux region; 
         FIG. 4  is a graph illustrating examples of pulse widths respectively for PWM signal generators L and H of the circuit illustrated in  FIG. 1  in response to a dimming command value; 
         FIG. 5  is a graph illustrating an example of an actual brightness change controlled based on a change in the dimming command value, the graph being obtained by superimposing the value of the graph in  FIG. 4  on the value in  FIG. 3 ; 
         FIG. 6  is a circuit diagram of an LED driver circuit according to another embodiment of the present invention and illustrates an example in which a PWM_H circuit and a PWM_L circuit are assigned to different LEDs; 
         FIG. 7  is a diagram illustrating an overview of a planetarium apparatus that uses illumination devices each including the LED driver circuit according to the embodiments of the present invention; and 
         FIG. 8  is a graph illustrating an example of a brightness change controlled based on a change in the solar altitude in the case where the illumination devices illustrated in  FIG. 7  are automatically controlled. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
       FIG. 1  is a circuit diagram illustrating an LED driver circuit according to an embodiment of the present invention. Two driver circuits (i.e. a driver circuit including a register (hereinafter also referred to as current-limiting transistors) RH and a power transistor TRH, and a driver circuit including a resistor RL and a power transistor TRL) are connected to a light-emitting diode LED. A PWM signal generator circuit (hereinafter also referred to as a PWM signal generator) H outputs a PWM signal (pulse train A) having a certain pulse width to drive the power transistor TRH. Similarly, a PWM signal generator circuit (hereinafter also referred to as a PWM signal generator) L outputs a PWM signal (pulse train B) having a certain pulse width to drive the power transistor TRL. When the power transistor TRH is in an on-state, a current flows through the LED via the current-limiting resistor RH. When the power transistor TRL is in the on-state, a current flows through the LED via the current-limiting resistor RL. When both the power transistors TRH and TRL are in the on-state, a current that flows through the LED is equal to the sum of currents that flow through the current-limiting resistors RH and RL. The current that flows through the LED is set by the individual current-limiting resistors RH and RL. 
     For example, it is assumed that a power supply voltage VCC is 5V and a forward-direction voltage of the LED is 3V. In such a case, the current that flows through the current-limiting resistor RH is (5−3)/RH=2/RH, and the current that flows through the current-limiting resistor RL is (5−3)/RL=2/RL. A dimming command value has 8-bit resolution. That is, a value ranging from 0 to 255 is supplied to the PWM signal generators H and L. Each of the PWM signal generators H and L converts this dimming command value into a pulse width by using a certain conversion function stored therein and outputs the pulse width. The pulse width has 12-bit resolution. That is, the pulse width is specified by a value ranging from 0 to 4,095. When the pulse width is 0, the duty cycle is equal to 0%. When the pulse width is 4,095, the duty cycle is equal to 100%. Now, it is assumed that a resistance of the current-limiting resistor RH is 10 Ω and a resistance of the current-limiting resistor RL is 1 kΩ. In this case, the current that flows through the current-limiting resistor RH is 2/10=0.2 A, and the current that flows through the current-limiting resistor RL is 2/1,000=2 mA. A ratio between these currents is 100:1. That is, a ratio between amounts of lights to be emitted for the same pulse width given by the PWM signal generators L and H is 1:100. 
     A specific example of the conversion function for converting the dimming command value into the pulse width of the PWM signal will be described next. The sensitivity of the human eye is not necessarily linear. Thus, a smoother result is obtained in the case where a relationship between the dimming command value and the actual brightness is based on a certain function than in the case where the relationship is linear. For example, a function is set such that a brightness=an input raised to the power of 2.7 so as to obtain a brightness change in a wide dynamic range for a limited number of input command values denoted by 8 bits.  FIG. 2  illustrates an example of this function. The dimming command value is given as an 8-bit gradation ranging from 0 to 255, whereas the output changes according to a curve of a function of the power of 2.7.  FIG. 3  is an illustration obtained by enlarging output characteristics implemented, for example, by a PWM circuit of 12-bit resolution in a low light flux region. With 12-bit resolution, one step is equal to 1/4,096 of the maximum value (i.e. 1), that is, is equal to approximately 0.000244. In the case of illumination having the maximum illuminance of 100 lx, this value is equal to approximately 24 mlx. This brightness change is sufficiently noticeable as a stepped change when the eyes are adjusted to the dark. 
     Pulse widths respectively for the PWM signal generators L and H in the circuit illustrated in  FIG. 1  are set as illustrated in  FIG. 4 . In a low light amount region up to the dimming command value of approximately 50, the value for the PWM signal generator L is changed. In a region of a greater light amount, the value for the PWM signal generator H is changed. When the duty for the PWM signal generator L is equal to 1, 1% of the maximum light amount is obtained. When the duties for both the PWM signal generators L and H are equal to 1, the maximum light amount is obtained. Tables 1-1 and 1-2 illustrate an example of these values. The dimming command value takes a value ranging from 0 to 255. A value PWM_L denotes a pulse width output by the PWM signal generator L and takes a 12-bit gradation value ranging from 0 to 4,095. The same applies to a value PWM_H. The brightness of the LED is denoted by (PWM_L/4,095)*0.01+(PWM_H/4,095). Tables 1-1 and 1-2 illustrate this brightness value. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1-1 
               
               
                   
                   
               
               
                   
                 Command Value 
                 PWM_L 
                 PWM_H 
                 Brightness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 0 
                 0 
                 0.000000 
               
               
                   
                 1 
                 0 
                 0 
                 0.000000 
               
               
                   
                 2 
                 0 
                 0 
                 0.000000 
               
               
                   
                 3 
                 2 
                 0 
                 0.000005 
               
               
                   
                 4 
                 5 
                 0 
                 0.000012 
               
               
                   
                 5 
                 10 
                 0 
                 0.000024 
               
               
                   
                 6 
                 16 
                 0 
                 0.000039 
               
               
                   
                 7 
                 24 
                 0 
                 0.000059 
               
               
                   
                 8 
                 35 
                 0 
                 0.000085 
               
               
                   
                 9 
                 49 
                 0 
                 0.000120 
               
               
                   
                 10 
                 65 
                 0 
                 0.000159 
               
               
                   
                 11 
                 84 
                 0 
                 0.000205 
               
               
                   
                 12 
                 106 
                 0 
                 0.000259 
               
               
                   
                 13 
                 132 
                 0 
                 0.000322 
               
               
                   
                 14 
                 161 
                 0 
                 0.000393 
               
               
                   
                 15 
                 195 
                 0 
                 0.000476 
               
               
                   
                 16 
                 232 
                 0 
                 0.000567 
               
               
                   
                 17 
                 273 
                 0 
                 0.000667 
               
               
                   
                 18 
                 319 
                 0 
                 0.000779 
               
               
                   
                 19 
                 369 
                 0 
                 0.000901 
               
               
                   
                 20 
                 424 
                 0 
                 0.001035 
               
               
                   
                 21 
                 483 
                 0 
                 0.001179 
               
               
                   
                 22 
                 548 
                 0 
                 0.001338 
               
               
                   
                 23 
                 618 
                 0 
                 0.001509 
               
               
                   
                 24 
                 693 
                 0 
                 0.001692 
               
               
                   
                 25 
                 774 
                 0 
                 0.001890 
               
               
                   
                 26 
                 861 
                 0 
                 0.002103 
               
               
                   
                 27 
                 953 
                 0 
                 0.002327 
               
               
                   
                 28 
                 1052 
                 0 
                 0.002569 
               
               
                   
                 29 
                 1156 
                 0 
                 0.002823 
               
               
                   
                 30 
                 1267 
                 0 
                 0.003094 
               
               
                   
                 31 
                 1384 
                 0 
                 0.003380 
               
               
                   
                 32 
                 1508 
                 0 
                 0.003683 
               
               
                   
                 33 
                 1639 
                 0 
                 0.004002 
               
               
                   
                 34 
                 1777 
                 0 
                 0.004339 
               
               
                   
                 35 
                 1921 
                 0 
                 0.004691 
               
               
                   
                 36 
                 2073 
                 0 
                 0.005062 
               
               
                   
                 37 
                 2232 
                 0 
                 0.005451 
               
               
                   
                 38 
                 2399 
                 0 
                 0.005858 
               
               
                   
                 39 
                 2573 
                 0 
                 0.006283 
               
               
                   
                 40 
                 2755 
                 0 
                 0.006728 
               
               
                   
                 41 
                 2945 
                 0 
                 0.007192 
               
               
                   
                 42 
                 3143 
                 0 
                 0.007675 
               
               
                   
                 43 
                 3350 
                 0 
                 0.008181 
               
               
                   
                 44 
                 3564 
                 0 
                 0.008703 
               
               
                   
                 45 
                 3787 
                 0 
                 0.009248 
               
               
                   
                 46 
                 4019 
                 0 
                 0.009814 
               
               
                   
                 47 
                 4095 
                 1 
                 0.010244 
               
               
                   
                 48 
                 4095 
                 4 
                 0.010977 
               
               
                   
                 49 
                 4095 
                 6 
                 0.011465 
               
               
                   
                 50 
                 4095 
                 9 
                 0.012198 
               
               
                   
                 51 
                 4095 
                 12 
                 0.012930 
               
               
                   
                 52 
                 4095 
                 15 
                 0.013663 
               
               
                   
                 53 
                 4095 
                 17 
                 0.014151 
               
               
                   
                 54 
                 4095 
                 21 
                 0.015128 
               
               
                   
                 55 
                 4095 
                 24 
                 0.015861 
               
               
                   
                 56 
                 4095 
                 27 
                 0.016593 
               
               
                   
                 57 
                 4095 
                 30 
                 0.017326 
               
               
                   
                 58 
                 4095 
                 34 
                 0.018303 
               
               
                   
                 59 
                 4095 
                 37 
                 0.019035 
               
               
                   
                 60 
                 4095 
                 41 
                 0.020012 
               
               
                   
                 61 
                 4095 
                 45 
                 0.020989 
               
               
                   
                 62 
                 4095 
                 49 
                 0.021966 
               
               
                   
                 63 
                 4095 
                 52 
                 0.022698 
               
               
                   
                 64 
                 4095 
                 57 
                 0.023919 
               
               
                   
                 65 
                 4095 
                 61 
                 0.024896 
               
               
                   
                 66 
                 4095 
                 65 
                 0.025873 
               
               
                   
                 67 
                 4095 
                 69 
                 0.026850 
               
               
                   
                 68 
                 4095 
                 74 
                 0.028071 
               
               
                   
                 69 
                 4095 
                 79 
                 0.029292 
               
               
                   
                 70 
                 4095 
                 83 
                 0.030269 
               
               
                   
                 71 
                 4095 
                 88 
                 0.031490 
               
               
                   
                 72 
                 4095 
                 93 
                 0.032711 
               
               
                   
                 73 
                 4095 
                 98 
                 0.033932 
               
               
                   
                 74 
                 4095 
                 104 
                 0.035397 
               
               
                   
                 75 
                 4095 
                 109 
                 0.036618 
               
               
                   
                 76 
                 4095 
                 114 
                 0.037839 
               
               
                   
                 77 
                 4095 
                 120 
                 0.039304 
               
               
                   
                 78 
                 4095 
                 126 
                 0.040769 
               
               
                   
                 79 
                 4095 
                 132 
                 0.042234 
               
               
                   
                 80 
                 4095 
                 138 
                 0.043700 
               
               
                   
                 81 
                 4095 
                 144 
                 0.045165 
               
               
                   
                 82 
                 4095 
                 150 
                 0.046630 
               
               
                   
                 83 
                 4095 
                 156 
                 0.048095 
               
               
                   
                 84 
                 4095 
                 163 
                 0.049805 
               
               
                   
                 85 
                 4095 
                 169 
                 0.051270 
               
               
                   
                 86 
                 4095 
                 176 
                 0.052979 
               
               
                   
                 87 
                 4095 
                 183 
                 0.054689 
               
               
                   
                 88 
                 4095 
                 190 
                 0.056398 
               
               
                   
                 89 
                 4095 
                 197 
                 0.058107 
               
               
                   
                 90 
                 4095 
                 205 
                 0.060061 
               
               
                   
                 91 
                 4095 
                 212 
                 0.061770 
               
               
                   
                 92 
                 4095 
                 220 
                 0.063724 
               
               
                   
                 93 
                 4095 
                 227 
                 0.065433 
               
               
                   
                 94 
                 4095 
                 235 
                 0.067387 
               
               
                   
                 95 
                 4095 
                 243 
                 0.069341 
               
               
                   
                 96 
                 4095 
                 251 
                 0.071294 
               
               
                   
                 97 
                 4095 
                 260 
                 0.073492 
               
               
                   
                 98 
                 4095 
                 268 
                 0.075446 
               
               
                   
                 99 
                 4095 
                 277 
                 0.077643 
               
               
                   
                 100 
                 4095 
                 286 
                 0.079841 
               
               
                   
                 101 
                 4095 
                 294 
                 0.081795 
               
               
                   
                 102 
                 4095 
                 304 
                 0.084237 
               
               
                   
                 103 
                 4095 
                 313 
                 0.086435 
               
               
                   
                 104 
                 4095 
                 322 
                 0.088632 
               
               
                   
                 105 
                 4095 
                 332 
                 0.091074 
               
               
                   
                 106 
                 4095 
                 341 
                 0.093272 
               
               
                   
                 107 
                 4095 
                 351 
                 0.095714 
               
               
                   
                 108 
                 4095 
                 361 
                 0.098156 
               
               
                   
                 109 
                 4095 
                 371 
                 0.100598 
               
               
                   
                 110 
                 4095 
                 382 
                 0.103284 
               
               
                   
                 111 
                 4095 
                 392 
                 0.105726 
               
               
                   
                 112 
                 4095 
                 403 
                 0.108413 
               
               
                   
                 113 
                 4095 
                 413 
                 0.110855 
               
               
                   
                 114 
                 4095 
                 424 
                 0.113541 
               
               
                   
                 115 
                 4095 
                 436 
                 0.116471 
               
               
                   
                 116 
                 4095 
                 447 
                 0.119158 
               
               
                   
                 117 
                 4095 
                 458 
                 0.121844 
               
               
                   
                 118 
                 4095 
                 470 
                 0.124774 
               
               
                   
                 119 
                 4095 
                 482 
                 0.127705 
               
               
                   
                 120 
                 4095 
                 494 
                 0.130635 
               
               
                   
                 121 
                 4095 
                 506 
                 0.133565 
               
               
                   
                 122 
                 4095 
                 518 
                 0.136496 
               
               
                   
                 123 
                 4095 
                 530 
                 0.139426 
               
               
                   
                 124 
                 4095 
                 543 
                 0.142601 
               
               
                   
                 125 
                 4095 
                 556 
                 0.145775 
               
               
                   
                 126 
                 4095 
                 569 
                 0.148950 
               
               
                   
                 127 
                 4095 
                 582 
                 0.152125 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1-2 
               
               
                   
                   
               
               
                   
                 Command Value 
                 PWM_L 
                 PWM_H 
                 Brightness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 128 
                 4095 
                 595 
                 0.155299 
               
               
                   
                 129 
                 4095 
                 609 
                 0.158718 
               
               
                   
                 130 
                 4085 
                 623 
                 0.162137 
               
               
                   
                 131 
                 4095 
                 637 
                 0.165556 
               
               
                   
                 132 
                 4095 
                 651 
                 0.168974 
               
               
                   
                 133 
                 4095 
                 665 
                 0.172393 
               
               
                   
                 134 
                 4095 
                 679 
                 0.175812 
               
               
                   
                 135 
                 4095 
                 694 
                 0.179475 
               
               
                   
                 136 
                 4095 
                 709 
                 0.183138 
               
               
                   
                 137 
                 4095 
                 724 
                 0.186801 
               
               
                   
                 138 
                 4095 
                 739 
                 0.190464 
               
               
                   
                 139 
                 4095 
                 754 
                 0.194127 
               
               
                   
                 140 
                 4095 
                 770 
                 0.198034 
               
               
                   
                 141 
                 4095 
                 786 
                 0.201941 
               
               
                   
                 142 
                 4095 
                 801 
                 0.205604 
               
               
                   
                 143 
                 4095 
                 818 
                 0.209756 
               
               
                   
                 144 
                 4095 
                 834 
                 0.213663 
               
               
                   
                 145 
                 4095 
                 850 
                 0.217570 
               
               
                   
                 146 
                 4095 
                 867 
                 0.221722 
               
               
                   
                 147 
                 4095 
                 884 
                 0.225873 
               
               
                   
                 148 
                 4095 
                 901 
                 0.230024 
               
               
                   
                 149 
                 4095 
                 918 
                 0.234176 
               
               
                   
                 150 
                 4095 
                 936 
                 0.238571 
               
               
                   
                 151 
                 4095 
                 954 
                 0.242967 
               
               
                   
                 152 
                 4095 
                 971 
                 0.247118 
               
               
                   
                 153 
                 4095 
                 990 
                 0.251753 
               
               
                   
                 154 
                 4095 
                 1008 
                 0.256154 
               
               
                   
                 155 
                 4095 
                 1026 
                 0.260549 
               
               
                   
                 156 
                 4095 
                 1045 
                 0.265189 
               
               
                   
                 157 
                 4095 
                 1064 
                 0.269829 
               
               
                   
                 158 
                 4095 
                 1083 
                 0.274469 
               
               
                   
                 159 
                 4095 
                 1102 
                 0.279109 
               
               
                   
                 160 
                 4095 
                 1122 
                 0.283993 
               
               
                   
                 161 
                 4095 
                 1142 
                 0.288877 
               
               
                   
                 162 
                 4095 
                 1162 
                 0.293761 
               
               
                   
                 163 
                 4095 
                 1182 
                 0.298645 
               
               
                   
                 164 
                 4095 
                 1202 
                 0.303529 
               
               
                   
                 165 
                 4095 
                 1223 
                 0.308657 
               
               
                   
                 166 
                 4095 
                 1244 
                 0.313785 
               
               
                   
                 167 
                 4095 
                 1265 
                 0.318913 
               
               
                   
                 168 
                 4095 
                 1286 
                 0.324042 
               
               
                   
                 169 
                 4095 
                 1307 
                 0.329170 
               
               
                   
                 170 
                 4095 
                 1329 
                 0.334542 
               
               
                   
                 171 
                 4095 
                 1351 
                 0.339915 
               
               
                   
                 172 
                 4095 
                 1373 
                 0.345287 
               
               
                   
                 173 
                 4095 
                 1395 
                 0.350659 
               
               
                   
                 174 
                 4095 
                 1418 
                 0.356276 
               
               
                   
                 175 
                 4095 
                 1440 
                 0.361648 
               
               
                   
                 176 
                 4095 
                 1463 
                 0.367265 
               
               
                   
                 177 
                 4095 
                 1487 
                 0.373126 
               
               
                   
                 178 
                 4095 
                 1510 
                 0.378742 
               
               
                   
                 179 
                 4095 
                 1534 
                 0.384603 
               
               
                   
                 180 
                 4095 
                 1557 
                 0.390220 
               
               
                   
                 181 
                 4095 
                 1582 
                 0.396325 
               
               
                   
                 182 
                 4095 
                 1606 
                 0.402186 
               
               
                   
                 183 
                 4095 
                 1630 
                 0.408046 
               
               
                   
                 184 
                 4095 
                 1655 
                 0.414151 
               
               
                   
                 185 
                 4095 
                 1680 
                 0.420256 
               
               
                   
                 186 
                 4095 
                 1705 
                 0.426361 
               
               
                   
                 187 
                 4095 
                 1731 
                 0.432711 
               
               
                   
                 188 
                 4095 
                 1757 
                 0.439060 
               
               
                   
                 189 
                 4095 
                 1783 
                 0.445409 
               
               
                   
                 190 
                 4095 
                 1809 
                 0.451758 
               
               
                   
                 191 
                 4095 
                 1835 
                 0.458107 
               
               
                   
                 192 
                 4095 
                 1862 
                 0.464701 
               
               
                   
                 193 
                 4095 
                 1889 
                 0.471294 
               
               
                   
                 194 
                 4095 
                 1916 
                 0.477888 
               
               
                   
                 195 
                 4095 
                 1943 
                 0.484481 
               
               
                   
                 196 
                 4095 
                 1971 
                 0.491319 
               
               
                   
                 197 
                 4095 
                 1999 
                 0.498156 
               
               
                   
                 198 
                 4095 
                 2027 
                 0.504994 
               
               
                   
                 199 
                 4095 
                 2055 
                 0.511832 
               
               
                   
                 200 
                 4095 
                 2084 
                 0.518913 
               
               
                   
                 201 
                 4095 
                 2112 
                 0.525751 
               
               
                   
                 202 
                 4095 
                 2142 
                 0.533077 
               
               
                   
                 203 
                 4095 
                 2171 
                 0.540159 
               
               
                   
                 204 
                 4095 
                 2200 
                 0.547241 
               
               
                   
                 205 
                 4095 
                 2230 
                 0.554567 
               
               
                   
                 206 
                 4095 
                 2260 
                 0.561893 
               
               
                   
                 207 
                 4095 
                 2290 
                 0.569219 
               
               
                   
                 208 
                 4095 
                 2321 
                 0.576789 
               
               
                   
                 209 
                 4095 
                 2352 
                 0.584359 
               
               
                   
                 210 
                 4095 
                 2383 
                 0.591929 
               
               
                   
                 211 
                 4095 
                 2414 
                 0.599499 
               
               
                   
                 212 
                 4095 
                 2446 
                 0.607314 
               
               
                   
                 213 
                 4095 
                 2478 
                 0.615128 
               
               
                   
                 214 
                 4095 
                 2510 
                 0.622943 
               
               
                   
                 215 
                 4095 
                 2542 
                 0.630757 
               
               
                   
                 216 
                 4095 
                 2574 
                 0.638571 
               
               
                   
                 217 
                 4095 
                 2607 
                 0.646630 
               
               
                   
                 218 
                 4095 
                 2640 
                 0.654689 
               
               
                   
                 219 
                 4095 
                 2674 
                 0.662991 
               
               
                   
                 220 
                 4095 
                 2707 
                 0.671050 
               
               
                   
                 221 
                 4095 
                 2741 
                 0.679353 
               
               
                   
                 222 
                 4095 
                 2775 
                 0.687656 
               
               
                   
                 223 
                 4095 
                 2810 
                 0.696203 
               
               
                   
                 224 
                 4095 
                 2844 
                 0.704505 
               
               
                   
                 225 
                 4095 
                 2879 
                 0.713053 
               
               
                   
                 226 
                 4095 
                 2914 
                 0.721600 
               
               
                   
                 227 
                 4095 
                 2950 
                 0.730391 
               
               
                   
                 228 
                 4095 
                 2986 
                 0.739182 
               
               
                   
                 229 
                 4095 
                 3022 
                 0.747973 
               
               
                   
                 230 
                 4095 
                 3058 
                 0.756764 
               
               
                   
                 231 
                 4095 
                 3094 
                 0.765556 
               
               
                   
                 232 
                 4095 
                 3131 
                 0.774591 
               
               
                   
                 233 
                 4095 
                 3168 
                 0.783626 
               
               
                   
                 234 
                 4085 
                 3206 
                 0.792906 
               
               
                   
                 235 
                 4095 
                 3243 
                 0.801941 
               
               
                   
                 236 
                 4095 
                 3281 
                 0.811221 
               
               
                   
                 237 
                 4095 
                 3319 
                 0.820501 
               
               
                   
                 238 
                 4095 
                 3358 
                 0.830024 
               
               
                   
                 239 
                 4095 
                 3396 
                 0.839304 
               
               
                   
                 240 
                 4095 
                 3435 
                 0.848828 
               
               
                   
                 241 
                 4095 
                 3474 
                 0.858352 
               
               
                   
                 242 
                 4095 
                 3514 
                 0.868120 
               
               
                   
                 243 
                 4095 
                 3554 
                 0.877888 
               
               
                   
                 244 
                 4095 
                 3594 
                 0.887656 
               
               
                   
                 245 
                 4095 
                 3634 
                 0.897424 
               
               
                   
                 246 
                 4095 
                 3675 
                 0.907436 
               
               
                   
                 247 
                 4095 
                 3716 
                 0.917448 
               
               
                   
                 248 
                 4095 
                 3757 
                 0.927460 
               
               
                   
                 249 
                 4095 
                 3799 
                 0.937717 
               
               
                   
                 250 
                 4095 
                 3840 
                 0.947729 
               
               
                   
                 251 
                 4095 
                 3882 
                 0.957985 
               
               
                   
                 252 
                 4095 
                 3925 
                 0.968486 
               
               
                   
                 253 
                 4095 
                 3967 
                 0.978742 
               
               
                   
                 254 
                 4095 
                 4010 
                 0.989243 
               
               
                   
                 255 
                 4095 
                 4054 
                 0.999988 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 5  is obtained by superimposing this value on the value in  FIG. 3 . Dimming is implemented even at a low amount of light much smoother than that denoted by the curve of ordinary 12-bit PWM. In addition, dimming is performed using the pulse width PWM_H at and above a certain amount of light. Because there is already a certain amount of light in that range, the stepped brightness change is unobtrusive even at the dimming resolution of the pulse width PWM_H. An actual dynamic range is calculated. The minimum luminance obtained based on the pulse width PWM_L is 1/4,096/100=2.44×10 −6 , and the dynamic range is 409,600. Because the dynamic range of ordinary 12-bit PWM is 4,096, a resolution that is 100 times as high as that of ordinary 12-bit PWM is achieved. This embodiment has described the example where two PWM circuits are used and a ratio between current values is set to 100, however a configuration using three or more PWM circuits is also possible. In such a case, the dynamic range can be increased further by 100 times, and the constraint is substantially removed. 
       FIG. 6  illustrates an embodiment in which PWM circuits are assigned to different LEDs. An LED LED_H is switched on based on a PWM signal generated by the PWM signal generator H. An LED LED_L is switched on based on a PWM signal generated by the PWM signal generator L. In this case, because the brightness of both the LEDs LED_H and LED_L are changed, the brightness can be changed not only by changing the values of the current-limiting resistors but also by changing the number of lamps or models actually used in the LEDs LED_H and LED_L or the presence or absence of an optical filter. Also, in this embodiment, the brightness is controlled by setting pulse trains A′ and B′ to have a relationship between the dimming command value and the pulse width illustrated in  FIG. 4 . 
       FIG. 7  illustrates an embodiment in which illumination devices produced based on this circuitry are used at a planetarium. The illumination devices each including an LED and a driver circuit are installed at a periphery of a dome screen so as to illuminate the entire dome screen. The brightness of the illumination devices are controlled by manual operation performed by a presenter or automatically controlled by a control system at the planetarium.  FIG. 8  illustrates an example of a relationship of the brightness against a change in the solar altitude used in the case of automatic control. The planetarium control system stores therein a real-time solar altitude. The planetarium control system calculates a brightness based on the graph illustrated in  FIG. 8  by using this altitude and sends the calculated brightness value to the illumination devices as a command input, thereby being able to reproduce a change between daytime and nighttime. The use of the circuit according to the embodiments of the present invention allows a subtle brightness change of the night sky during a very dim time period after sunset to be reproduced naturally. 
     With the embodiments of the present invention, LEDs can be smoothly dimmed further from an extremely low illuminance, without increasing the PWM frequency unnecessarily. In the case where the resolution is increased by increasing the number of bits used for PWM control, current needs to be switched at an extremely high speed in order to implement the small pulse width, causing issues related to stability of the circuit and occurrence of noise. In contrast, the method according to the embodiments of the present invention enables smooth dimming even at low illuminance through PWM control using a small number of bits. 
     The embodiments above have described the case where one LED is used and the case where two LEDs are used respectively for a low illuminance region and a high illuminance region, however the advantageous effects of the embodiments of the present invention are also exerted even in the case where an LED is further inserted in series to circuit portions each including an LED. In addition, the example of using resistors to limit the current has been described, however currents at two PWM circuits on the PWM signal generator H side and the PWM signal generator L side can be limited to be different current values by using constant-current diodes instead of the resistors. Further, the example case of setting the ratio between the two current values to 100 has been described, however the ratio may be less than or greater than 100.