Patent Publication Number: US-2007115209-A1

Title: Luminance control apparatus for light emitting device

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based on and incorporates herein by reference Japanese Patent Application No. 2005-338757 filed on Nov. 24, 2005.  
     FIELD OF THE INVENTION  
      The present invention relates to a luminance control apparatus for a light emitting device and, more particularly, to an apparatus that improves luminance resolution of a light emitting device.  
     BACKGROUND OF THE INVENTION  
      As one method for controlling a luminance of a light emitting device such as a light emitting diode, a current supplied to the light emitting device is increased or decreased as disclosed in JP 2004-281922A. This current control is used for light emitting diodes used as a backlight of a liquid crystal display. In this current control, a brightness detecting device is provided to detect brightness of a surrounding area, and the backlight (light emitting diodes) is driven to emit more light as the detected brightness increases. The amount of light emission is adjustable manually by a user.  
      As another method for a luminance control, a current supplied to the light emitting device is controlled by pulse-width modulation (PWM). This PWM control is, as disclosed in JP 2004-281349A for instance, combined with a cycle period control. If the luminance is controlled by only the PWM control in high resolution, the pulse width must be modulated finely or in fine steps. That is, the amount of each pulse width change must be small as much as possible. If the cycle period control is combined to the PWM control and the cycle period of PWM control is lengthened, the luminance control can be attained with high resolution without modulating the pulse width in fine steps.  
      If a light emitting device is used as a backlight of a display in a vehicle, the luminance of the backlight must be differentiated between daytime and nighttime. That is, the luminance at nighttime must be decreased to be lower than that of the daytime. The luminance is finely adjustable by a user at both daytime and nighttime.  
      A light emitting device such as light emitting diodes changes hue in correspondence to the current supplied thereto. Therefore, it is difficult to control the luminance without changing the hue of the emitted light in the case of the current control. On the other hand, no hue change is caused in the case of the PWM control. Therefore, it is proposed to differentiate the luminance of the light emitting device at daytime and nighttime by the PWM control while maintaining the current unchanged.  
       FIG. 3  shows luminance controllable ranges in the case of controlling the luminance between the daytime and the nighttime by only the PWM control and without the current control. So far as the current is maintained unchanged, the luminance increases proportionately as the duty ratio increases. Since the luminance at daytime must be higher than that at the nighttime, the duty ratio at the daytime is set to be in a relatively high duty ratio range and the duty ratio at the night time is set to be in a relatively low duty ratio range. These two ranges may or may not overlap each other.  
      Human eyes are more sensitive to luminance changes in the darker surrounding. Therefore it is preferred to set the luminance more finely, that is, to set the luminance in higher resolution, in the nighttime than in the daytime.  
      In the case that the luminance is controlled by only the PWM control, it is necessary to allow the duty ratio to vary in more steps within the nighttime duty ratio range. As such each change amount of the pulse width must be set small. To attain such a small change, a higher performance CPU is necessitated. This results in increased costs.  
      If the cycle period control is combined to the PWM control, the luminance control in the low luminance range can be attained with high resolution. If a user is allowed to vary the setting of the luminance, the luminance control cycle period is varied in the end. This variation in the luminance control cycle period may cause imbalance between the image display cycle period of a display screen and the luminance control cycle period of the backlight. As a result, noises may appear on the display screen.  
      If the current is decreased, the luminance change range is narrowed. As a result, the amount of each luminance change width corresponding to one pulse width step is also decreased. Thus, it becomes possible to set the luminance with high resolution. In this instance, however, a maximum luminance is also decreased and hence sufficient luminance in the daytime may not be provided.  
      The above disadvantages arise not only in the backlight using the light emitting device but also in an apparatus, which is required to control the luminance with high resolution in both the high luminance range and the low luminance range.  
     SUMMARY OF THE INVENTION  
      The present invention therefore has an object to provide a luminance control apparatus for a light emitting device, the apparatus being capable of setting a luminance in high resolution in a low luminance range while maintaining a high luminance.  
      According to one aspect of the present invention, a luminance control apparatus is provided for a light emitting device. The apparatus has a switching device for supplying a current to the light emitting device when turned on, a pulse-width modulation circuit for turning on and off the switching device with a variable duty ratio determined by a pulse-width modulation, and a current control circuit for changing the current supplied to the light emitting device to a low level and a high level that are different from each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
       FIG. 1  is a circuit diagram showing a luminance control apparatus according to an embodiment of the present invention;  
       FIG. 2  is a graph showing a luminance control range of a light emitting diode used in the first embodiment; and  
       FIG. 3  is a graph showing a luminance control range, in which luminance between a daytime and a nighttime is differentiated by only a PWM control while maintaining the current unchanged. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT  
      Referring to  FIG. 1 , a luminance control apparatus  10  is constructed to control a luminance of series-connected light emitting diodes (LEDs) as a light emitting device  50 . The LEDs may be connected in parallel in part. The device  50  is used as a backlight of a liquid crystal display device for a vehicle.  
      The apparatus  10  is constructed with a first transistor T 1 , a second transistor T 2 , a microcomputer  12  and a driver circuit  20 . The first transistor T 1  is a normally-off type P-MOSFET and operates as a switching device. The source and the drain of the FET are connected to a vehicle battery  52  and the light emitting device  50 , respectively. The second transistor T 2  is a bipolar type. The collector, the emitter and the base are connected to the light emitting device  50 , the ground and the microcomputer  12  through a resistor R 1 , respectively.  
      The driver circuit T 3  includes a third transistor T 3 , which is a bipolar type, a resistor R 2  and a resistor R 3 . The resistor R 2  is connected to the base of the third transistor T 3  and the microcomputer  12 . The resistor R 3  is connected at one end to the resistor R 2  and the base of the third transistor T 3 . The resistor R 3  is connected at the other end to the emitter of the third transistor T 3  and the ground. The collector of the third transistor T 3  is connected to the gate of the first transistor T 1  through a resistor R 4 . A resistor R 5  is connected to the resistor R 4  and the gate of the first transistor T 1 . The resistor  5  is also connected to the source of the first transistor T 1  and the battery  52 .  
      The microcomputer  12  includes a CPU, a ROM, a RAM, and I/O port, which are not shown, and A/D conversion circuits  14 . The microcomputer  12  is applied with a light signal SL indicating ON/OFF of vehicle lights and a luminance setting signal SB indicating a user&#39;s setting of luminance. The microcomputer  12  executes a control program stored in the ROM while utilizing a temporary storage function of the RAM. The microcomputer  10  thus produces output signals to the driver circuit  20  and the second transistor T 2  through the A/D conversion circuits  14 , respectively, thereby to control the luminance of the light emitting device  50 .  
      In the luminance control apparatus  10  constructed as above, the microcomputer  12  checks based on the light signal SL whether the vehicle lights are ON indicating that it is the nighttime. If the lights are ON, the microcomputer  12  produces the output signal to the base of the second transistor T 2  so that the current supplied to the light emitting device  50  is at a low current level CL under a condition that the first transistor T 1  is ON. If the lights are OFF indicating that it is the daytime, the microcomputer  12  produces the output signal to the base of the second transistor T 2  so that the current supplied to the light emitting device  50  is at a high current level CH (&gt;CL) under a condition that the first transistor T 1  is ON. Thus, the microcomputer  12  and the second transistor T 2  form a current control circuit, which controls the current level to either high or low.  
      Since the light emitting device  50  has a characteristic of changing the hue in correspondence to the current level, the hue of the light emitted from the device  50  differs between the cases of high current level CH (daytime) and low current level CL (nighttime). This hue change due to difference in the current level does not become critical, if color of the image display is set to differ between the daytime and the night time.  
      The microcomputer  12  also determines the duty ratio based on the luminance setting signal SB and produces the output signal to the driver circuit  20 . This output signal is in the pulse signal form having the determined duty ratio. The driver circuit  20  amplifies this pulse signal to a level sufficient to drive the first transistor T 1  and applies the amplified signal to the base of the first transistor T 1 . The first transistor T 1  turns on and off when the pulse signal is high and low, respectively. The microcomputer  12  and the driver circuit  20  thus form a pulse-width modulation circuit.  
      As the current supplied to the second transistor T 2  is higher at daytime than at nighttime (CH&gt;CL), a maximum luminance LDmax of the light device  50  at the daytime is higher than a maximum luminance LNmax at the nighttime as shown in  FIG. 2 . The maximum luminances LDmax and LNmax in this embodiment ( FIG. 2 ) are set to be equal to those in the prior art ( FIG. 3 ).  
      Further, the duty ratio of the pulse signal is set to vary from 0% to 100% at nighttime in this embodiment as shown in  FIG. 2 . Therefore, even if the amounts of each change in the pulse widths are limited to be the same between the embodiment and the prior art due to the use of the same microcomputer, the pulse width can be modulated more finely, that is, in higher resolution, at the nighttime than in the prior art, in which the duty ratio control range is limited to be from 0% to about 60%. It is noted that the duty ratio control for the daytime in the embodiment is the same as in the prior art.  
      In the embodiment, a high luminance of the light emitting device  50  is attained by driving the first transistor T 1  with 100% duty ratio while supplying the current of high level CH through the second transistor T 2 . Further, the luminance of the light emitting device  50  is linearly varied between a small luminance range from 0 to LNmax by driving the first transistor T 1  with the varying duty ratio (0% to 100%) while supplying the current of low level CL through the second transistor T 2 . Thus, the luminance resolution of the light emitting device  50  at the night time is enhanced. Further, the amount of each change in the PWM control is increased in the embodiment relative to the amount of each change in the case of controlling the luminance only by the pulse-width modulation without changing the current supplied by the second transistor T 2 . Therefore, the microcomputer  12  need not be a high performance type, thus reducing costs of the luminance control apparatus  10 .  
      The above embodiment may be modified in many ways. For instance, the luminance of the light emitting device  50  may be automatically adjusted by detecting the brightness of the surrounding area.