Abstract:
Provided is a light emitting device and a method of controlling the same are disclosed. The light emitting unit includes a power supply unit for supplying a drive voltage to the light emitting unit, and a control unit for comparing a first current level previously applied to the light emitting unit with a second current level to be applied to the light emitting unit in accordance with image information to be displayed using the light emitting unit, and controlling a voltage level applied to the light emitting unit based on a result of comparison.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2006-049678, filed Jun. 2, 2006, in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light emitting device and a method of controlling the same. More particularly, the present invention relates to a light emitting device and a method of controlling the same, which can optimize a drive voltage to prevent a stress to peripheral devices, without distorting an optical output of a light emitting unit, and thus improve the efficiency of a system. 
     2. Description of the Related Art 
     Conventional display devices include direct view cathode ray tubes (CRT), a flat panel displays (FPD) and front and rear projectors. Exemplary FPDs include a liquid crystal display (LCD) panel and a plasma display panel (PDP). New display technologies, such as an organic electroluminescent (EL), liquid crystal on silicon (LCOS)and a digital light processing (DLP), are continuing to be developed for use in one or more types of display devices. 
     A display device using LCD, LCOS or a DLP technologies employs a light emitting device, such as a light emitting diode (LED), as a light source. An LED is a point light source, and has a high luminance and good color reproducibility. An LED driven by an electric current minimizes a ripple component of an output electric current so as to improve the quality of the displayed image. Further, an LED driven by an electric current requires a drive unit having a quick response characteristic in view of the characteristics of the display device. To achieve this, a linear current source may be used. 
       FIG. 1  is a circuit diagram illustrating a conventional LED driving device. 
     A conventional LED driving device  10  includes a variable voltage source  12 , a control logic unit  14 , a low-pass filter  16 , a transistor  18 , a current control unit  20 , and a light emitting unit  22 . 
     The variable voltage source  12  generates an optimum voltage so as to improve the efficiency of the LED driving device  10  when the light emitting unit  22  is driven. The control logic unit  14  monitors the voltage of V d  (i.e., V o -V AK ) so as to control the output of the variable voltage source  12 , and generates a PWM signal so as to generate a reference voltage to be applied to the light emitting unit  22  using the monitored voltage. The low-pass filter  16  performs smoothing of the PWM signal generated by the control logic unit  14 . The transistor  18  is connected in series with the light emitting unit  22 , and generates the constant current required in the LED driving device  10  using the voltage provided from the variable voltage source  12 . The current control unit  20  adjusts the amount of the current generated by the transistor  18 . The light emitting unit  22  includes at least one LED which receives the constant current from the transistor  18  to emit light. 
       FIG. 2  is a graph depicting a variable output voltage outputted from the variable voltage source and a waveform of an electric current applied to the light emitting unit, according to the conventional LED driving device. 
     The LED driving device  10  must generate the optimum voltage so that the light emitting unit  22  emits light. In the LED driving device  10 , the control logic unit  14  generates the PWM signal so as to output a constant voltage during an early driving stage. The control logic unit  14  controls the optimum value of the output voltage in such a manner that it waits until a time point t 1  at which time the variable voltage source  12  has generated a stable initial voltage, and it progressively decreases the pulse width of the PWM signal after the time point t 1  to reduce the output voltage. 
     The time point where the light emitting unit  22  emits light is a point after the time point t 1  where the initial voltage is set. From this time point on the control logic unit  14  generates a current command value, so that the current control unit  20  operates. As such, the LED driving device monitors the voltage V d  applied to the transistor at regular intervals during the emission time of the light emitting unit  22 , and reduces the pulse width of the PWM signal if the voltage V d  is higher than a predetermined threshold value V th , while the LED driving device increases the pulse width of the PWM signal if the voltage V d  is lower than the predetermined threshold value V th , thereby minimizing a thermal loss of the transistor  18  and adjusting the voltage so that the voltage does not affect the light emitting unit  22 . 
     The display device using the above LED light source varies the command value of the output current depending upon brightness information of the image signal to be displayed. Under this condition, it is necessary to vary the voltage, which is applied to the light emitting unit  22  in accordance with the variation of the output current, depending upon the brightness change of the image signal, so that the optical output is not distorted. That is, the output voltage must be quickly varied from a low value to a high value when a dark image is switched over to a bright image. In this case, if the switching speed is low, the light emitting unit  22  may not produce a sufficient amount of luminance. By contrast, the output voltage must be varied from a high value to a low value when a bright image is switched over to a dark image. In this case, if the switching speed is low, the corresponding high voltage is applied to the peripheral devices, and this causes the occurrence of a thermal loss. Consequently, the efficiency of the display device is reduced, and thus a heat radiating structure must be designed correspondingly. 
     Accordingly, there is a need for an improved a light emitting device and a method of controlling the same, which can optimize a drive voltage to prevent a stress to peripheral devices and thus improve an efficiency of a system, without distorting an optical output of a light emitting unit. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention address at least the above problems and/or disadvantages and provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a light emitting device and a method of controlling the same, which can optimize a drive voltage to prevent a stress to peripheral devices and thus improve an efficiency of a system, without distorting an optical output of a light emitting unit. 
     The foregoing and other objects and advantages are substantially realized by providing a light emitting device including a light emitting unit for emitting light, according to exemplary embodiments of the present invention, which includes a power supply unit for supplying a drive voltage to the light emitting unit, and a control unit comparing a first current level previously applied to the light emitting unit with a second current level to be applied to the light emitting unit in accordance with image information to be displayed using the light emitting unit, and controlling a voltage level applied to the light emitting unit based on a result of comparison. 
     The control unit may control the power supply unit such that if the second current level is higher than the first current level and a difference between the first current level and the second current level is higher than a current threshold value, the control unit applies the drive voltage to the light emitting unit by increasing the voltage level corresponding to the first current level by a specified offset voltage, while if the second current level is lower than the first current level and the difference between the first current level and the second current level is higher than the current threshold value, the control unit applies the drive voltage to the light emitting unit by reducing the voltage level corresponding to the first current level by a specified offset voltage. 
     The control unit may further control the power supply unit such that if a difference between the first current level and the second current level is less than the threshold current value and the drive voltage is greater than a threshold voltage level, the control unit applies the drive voltage to the light emitting unit by decreasing the voltage level by a corrected value, wherein if a difference between the first current level and the second current level is less than the threshold current value and the drive voltage is less than a threshold voltage level, the control unit applies the drive voltage to the light emitting unit by increasing the voltage level by a corrected value. The light emitting device may further comprise a memory comprising a lookup table for storing corrected values for the voltage level applied to the light emitting unit corresponding to the result of comparing the first and second current levels, and the control unit may adjust the voltage level of the drive voltage to be output from the power supply unit with reference to the lookup table. 
     The control unit may further control the power supply unit such that if the second current level is higher than the first current level, the control unit applies the drive voltage to the light emitting unit by increasing the voltage level corresponding to the first current level by a specified level, while if the second current level is lower than the first current level, the control unit applies the drive voltage to the light emitting unit by reducing the voltage level corresponding to the first current level by a specified level. 
     The control unit may further control the power supply unit such that if the second current level is higher than a minimum current level the control unit compares the first current level with the second current level, while if the second current level is lower than the minimum current level the control unit applies the drive voltage to the light emitting unit corresponding to the first current level. 
     In another aspect of an exemplary embodiment of the present invention, there is provided a method of controlling a light emitting device including a light emitting unit for emitting light, which includes supplying a drive voltage to the light emitting unit, and comparing a first current level previously applied to the light emitting unit with a second current level to be applied to the light emitting unit in accordance with image information to be displayed using the light emitting unit, and controlling a voltage level to be applied to the light emitting unit based on a result of comparison. 
     The control step may control the light emitting device such that if the second current level is higher than the first current level and a difference between the first current level and the second current level is higher than a threshold current value, the drive voltage is applied to the light emitting unit by increasing the voltage level corresponding to the first current level by a specified offset voltage, while if the second current level is lower than the first current level and the difference between the first current level and the second current level is higher than the threshold current value, the drive voltage is applied to the light emitting unit by reducing the voltage level corresponding to the first current level by a specified offset voltage. 
     The control step may control the light emitting device such that if a difference between the first current level and the second current level is less than the threshold current value and the drive voltage is greater than a threshold voltage level, the drive voltage is applied to the light emitting unit by decreasing the voltage level by a corrected value, wherein if a difference between the first current level and the second current level is less than the threshold current value and the drive voltage is less than a threshold voltage level, the drive voltage is applied to the light emitting unit by increasing the voltage level by a corrected value. 
     The control step may control the light emitting device such that the drive voltage level to be applied to the light emitting unit is adjusted with reference to a lookup table, wherein the lookup table stores corrected values for the drive voltage level to be applied to the light emitting unit corresponding to the result of comparing the first and second current levels. The control step may control the light emitting device such that if the second current level is higher than the first current level, the drive voltage applied to the light emitting unit by increasing the voltage level corresponding to the first current level by a specified level, while if the second current level is lower than the first current level, the drive voltage is applied to the light emitting unit by reducing the voltage level corresponding to the first current level by a specified level. 
     The control step may control the light emitting device such that if the second current level is higher than a minimum current level the first current level and the second current level are compared, while if the second current level is lower than the minimum current level the drive voltage is applied to the light emitting unit corresponding to the first current level. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram illustrating a conventional light emitting device; 
         FIG. 2  is a graph depicting a variable output voltage outputted from a variable voltage source and a waveform of an electric current applied to a light emitting unit, according to a conventional light emitting device; 
         FIG. 3  is a circuit diagram illustrating the construction of a light emitting device according to an exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating a process of controlling a light emitting device according an exemplary embodiment of the present invention; and 
         FIG. 5  is a flowchart illustrating a process of controlling a light emitting device according another exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention and are merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
       FIG. 3  is a circuit diagram illustrating the construction of a light emitting device according to an exemplary embodiment of the present invention. 
     A light emitting device  100  according to this exemplary embodiment includes a light emitting unit  110 , a power supply unit  120 , a low-pass filter  130 , a transistor  140 , a control unit  150 , and a memory  170 . 
     The light emitting unit  110  emits light to a screen (not shown) for displaying an image. The light emitting unit  110  of this exemplary embodiment may have a plurality of light emitting diodes (LED) as a light source. Further, the light emitting unit  110  may have light emitting diodes of various colors such as red (R), green (G), and blue (B), or a laser diode. 
     The power supply unit  120  is a power source for supplying a constant electric voltage to the light emitting unit  110 . The power supply unit  120  outputs a variable voltage to the light emitting unit  110  so as to maintain a voltage V d , which is applied to the transistor  140  by the control unit  150 , at a constant level. Preferably, the power supply unit  120  is capable of varying the voltage from a level higher than a maximum voltage which can be applied to the light emitting unit  110  to a level lower than a minimum voltage which can be applied to the light emitting unit  110 . 
     The low-pass filter  130  filters a pulse width modulation (PWM) signal received from the control unit  150  to generate an analog reference voltage. 
     Although the low-pass filter  130  is used in this exemplary embodiment, a digital to analog converter (DAC) for converting a digital signal into an analog signal may be used depending upon the application. Alternatively, an analog to digital converter (ADC) for converting a signal to be input to the control unit  150  into a digital signal may be used, if necessary. 
     The transistor  140  is connected in series with the light emitting unit  110 , and generates a constant current required for the light emitting device  100  using the voltage provided from the power supply unit  120 . 
     The transistor  140  may include a switching element (not shown) such as a field effect transistor (FET) or a bipolar junction transistor (BJT). The transistor  140  adjusts a signal applied to a gate electrode of the FET or a base terminal of the BJT, thereby controlling a current flowing through a collector-emitter or drain-source. Therefore, if a circuit having the FET and the BJT is used, a current can be precisely supplied to the light emitting unit  110  of the light emitting device  100  in a rapid switching speed, without generating a noise. For example, since the current flowing in the drain-source of the FET in a saturated region is maintained at a constant value, irrespective of the voltage applied to the drain-source, the constant current to be applied to the light emitting unit  110  can be generated using the above property. 
     The control unit  150  may generate a reference voltage to be input to the power supply unit  120  so as to control the output voltage of the power supply unit  120 . The control unit  150  may include a control logic unit. For example, the control unit  150  can generate the PWM signal using a digital logic unit such as a microcomputer or a filed programmable gate array (FPGA). 
     Further, the control unit  150  of this exemplary embodiment obtains a value of an amount of current (hereinafter referred to as a “present current level”) for the voltage to be applied to the light emitting unit  110  based on image information to be input, and compares the present current level with a current level previously applied to the light emitting unit  110  (hereinafter referred to as a “previous current level”). The previous current level may be obtained from previous image information that was input to the light emitting device  100  prior to the present image information being input. 
     The control unit  150  adjusts a level of the voltage supplied to the light emitting unit  110  from the power supply unit  120  according to the compared result. If the previous current level is higher than the present current level, the voltage level of the driving voltage to be output from the power supply unit  110  is reduced by a select level. If the previous current level is lower than the present current level, the voltage level to be output from the power supply unit  110  is increased by a select level. The control unit  150  compares the previous current level with the present current level, and adjusts the drive voltage to be applied to the light emitting unit  110  with respect to the difference between the current levels, thereby quickly optimizing the output of the drive voltage without distorting the optical output of the light emitting unit  110 . 
     The control unit  150  according to this exemplary embodiment continuously adjusts the level of the drive voltage supplied from the power supply unit  120  at the time of normal drive, as well as the time of initial drive. The normal time of the light emitting unit  110  means the point when a predetermined time elapses after the initial drive. That is, the normal time of the light emitting unit  110  means the period from the time when the current and voltage applied to the light emitting unit  110  are stabilized to the time when the operation of the light emitting unit  110  is completed. 
     Further, the control unit  150  can adjust the level of the drive voltage supplied to the light emitting unit  110  from the power supply unit  120  with reference to a lookup table  160  that is stored in the memory  170 . 
     The control unit  150  outputs the PWM signal to the power supply unit  120 , and the level of the drive voltage to be output from the power supply unit  120  is varied depending upon the pulse width of the PWM signal. 
     The lookup table  160  stores corrected values for the voltage levels applied to the light emitting unit  110  corresponding to the compared results of the previous current level and the present current level. Table 1 is one example of the lookup table. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Absolute Value of (Present Current 
                   
               
               
                   
                 Level − Previous Current Level) 
                 Corrected Value 
               
               
                   
                   
               
             
             
               
                   
                 1~5 
                 2 
               
               
                   
                  6~10 
                 4 
               
               
                   
                 11~15 
                 6 
               
               
                   
                 16~20 
                 8 
               
               
                   
                   
               
             
          
         
       
     
     The control unit  150  can adjust the voltage level of the drive voltage to be applied to the light emitting unit  110  based on the lookup table  160 . For example, if the previous current level is 3 higher than the present current level. The control unit  150  adjusts the drive voltage of the power supply unit  120  by reducing the pulse width of the PWM signal, with reference to the lookup table  160 , to decrease the drive voltage to be applied to the light emitting unit  110  by the corrected value  2 . Similarly, when the control unit  150  of the light emitting device  100  adjusts the drive voltage to be applied to the light emitting unit  110  with reference to the lookup table  160 , the control unit  150  can adjust the drive voltage to be applied to the light emitting unit  110  more quickly. 
     The control unit  150  according to an exemplary embodiment of the present invention can store the information on the maximum value and minimum value of the voltage level of the drive voltage. As such, while the control unit  150  adjusts the voltage level of the drive voltage according to the above method, the control unit  150  determines that the light emitting unit  110  operates incorrectly if the voltage level of the drive voltage deviates from the maximum value or minimum value or if it is necessary to adjust the voltage level of the drive voltage even though it reaches the maximum value or minimum value. 
     Specifically, the control unit  150  according an exemplary embodiment of the present invention determines that the light emitting unit  120  is in an open state, if the voltage V d  applied to the transistor  140  remains lower than the threshold value V th  despite the drive voltage having reached the maximum value. Further, the control unit  150  determines that the light emitting unit  120  is in a short state, if the voltage V d  applied to the transistor  140  is higher than the threshold value V th  despite the drive voltage having reached the minimum value. In this case, a specified range may be a difference between the maximum value and the minimum value of the current level to the voltage level of the drive voltage. 
       FIG. 4  is a flowchart illustrating a process of controlling the light emitting device according an exemplary embodiment of the present invention. 
     The control unit  150  controls the power supply unit  120  to apply the drive voltage to the light emitting unit  110  (S 210 ), so that light emitting unit  120  emits light (S 220 ). The control unit  150  analyzes the image information continuously received, and obtains the value of the present current level for the drive voltage to be applied to the light emitting unit  110  (S 230 ). 
     The control unit  150  determines whether the present current level obtained in step S 230  is higher than the previous current level for the drive voltage applied to the light emitting unit  110  (S 240 ). As a result, if the present current level is higher than the previous current level previously applied (“Yes” in S 240 ), the control unit  150  increases the drive voltage to be applied to the light emitting unit  110  from the power supply unit  120  by a specified level. 
     However, if the present current level is lower than the previous current level previously applied (“No” in S 240 ), the control unit  150  reduces the drive voltage to be applied to the light emitting unit  110  from the power supply unit  120  by a specified level (S 245 ). 
       FIG. 5  is a flowchart illustrating a process of controlling the light emitting device according another exemplary embodiment of the present invention. 
     The control unit  150  obtains the value of the present current level for the drive voltage to be applied to the light emitting unit  110  based on the input image information (S 310 ). 
     The control unit  150  determines whether the obtained present current level is higher or equal to a minimum current level (S 320 ). If the present current level is higher than or equal to the minimum current level (“Yes” in S 320 ), the control unit  150  determines whether the previous current level is higher than or equal to the present current level (S 330 ). But, if the present current level is not higher than or equal to the minimum current level (“No” in S 320 ), the control unit  150  maintains the previous voltage level to be applied to the light emitting unit  110  (S 325 ). 
     If the previous current level is higher than or equal to the present current level (“Yes” in S 330 ), the control unit  150  determines whether the difference between the previous current level and the present current level is higher than or equal to the threshold value (S 340 ). If it is determined that the difference between the previous current level and the present current level is higher than or equal to the threshold value, it can determine whether the value of the drive voltage to be applied to the light emitting unit  110  is abruptly varied. In this embodiment, if the difference between the previous current level and the present current level is higher than or equal to the threshold value, the value of the drive voltage to be applied to the light emitting unit  110  is abruptly varied. However, if the difference is not higher than or equal to the threshold value, then the value of the drive voltage to be applied to the light emitting unit  110  is not abruptly varied. 
     If the difference between the previous current level and the present current level is not higher than or equal to the threshold level (“No” in S 340 ), the control unit  150  determines whether the voltage level of the voltage V d  to be applied to the transistor  140  is higher than or equal to a threshold voltage level (S 350 ). If the difference between the previous current level and the present current level is higher than or equal to the threshold level (“Yes” in S 340 ), the control unit  150  controls the power supply unit  120  by applying a drive voltage, which is determined by subtracting an offset value of the PWM signal from the previous voltage level, to the light emitting unit  110  (S 355 ). 
     If the previous current level is not higher than or equal to the present current level (“No” in S 330 ), the control unit  150  determines whether the difference between the previous current level and the present current level is higher than or equal to the threshold value (S 345 ). If it is determined that the difference between the previous current level and the present current level is not higher than or equal to the threshold value (“No” in S 345 ), the control unit  150  determines whether the voltage level of the voltage V d  to be applied to the transistor  140  is higher than or equal to a threshold voltage level (S 350 ). 
     If the voltage level of the voltage V d  to be applied to the transistor  140  is higher than or equal to the threshold voltage level (“Yes” in S 350 ), the control unit  150  applies a voltage level, which is determined by subtracting a corrected value from the previous voltage level with reference to the lookup table  160 , to the light emitting unit  110  (S 362 ). If the voltage level of the voltage V d  to be applied to the transistor  140  is not higher than or equal to the threshold voltage level (“No” in S 350 ), the control unit  150  applies a voltage level, which is determined by adding a corrected value to the previous voltage level with reference to the lookup table  160 , to the light emitting unit  110  (S 364 ). 
     If the difference between the previous current level and the present current level is higher than or equal to the threshold level (“Yes” in S 345 ), the control unit  150  controls the power supply unit  120  by applying a drive voltage, which is determined by adding an offset value of the PWM signal to the previous voltage level, to the light emitting unit  110  (S 366 ). 
     As described above, according to exemplary embodiments of the present invention, the light emitting device and the method of controlling the same can optimize the drive voltage without distorting the optical output of the light emitting unit, and thus improve the efficiency of the system. 
     While certain exemplary embodiments of the invention has have been shown and described hereinwith reference to a certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.