Abstract:
A LED controller and a control method thereof are disclosed. The LED controller can generate a control signal to drive a LED, measure a luminous intensity value of the LED, and selectively adjust the control signal according to the measured value to dynamically adjust the luminous intensity. Besides, a simple design of driving circuit is applied to achieve an effect of dynamically adjusting the current of the LED.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates in general to light-emitting diodes (LED), and more particularly to a LED controller and a control method thereof.  
         [0003]     2. Description of the Prior Art  
         [0004]     In recent years, the LED technology develops rapidly and the performance of LED gets better with a lower manufacturing cost. Thus, the application range of LED is also extended wider gradually. However, the LED may be aging and its luminous intensity is lowered after operating for a long time. Further, in some applications such as the projector using the LED as a light source, the aging LED would cause bad color saturation.  
         [0005]     Besides, in the application of color-mixing with various colors of LED, e.g. using red, green and blue LEDs to mix a white light, it is possible to cause an unbalanced result and deviate from the expected white color, due to the aging LED or other variation factors. At this time, if we can determine the degree of deviation according to the actual color-mixing result and change the mix ratio for each color of LED, then a more ideal color-mixing result would be achieved.  
       SUMMARY OF INVENTION  
       [0006]     In view of this, an object of the present invention is to provide a LED controller and a control method thereof, which can dynamically adjust the current of a LED according to the luminous intensity of the LED, thereby changing the subsequent luminous intensity of the LED.  
         [0007]     Another object of the present invention is to provide a LED controller and a control method thereof, which can adjust the mix ratio of various colors of LED by dynamically adjusting the current flowing through each color of LED, thereby achieving a desired color-mixing effect.  
         [0008]     Another object of the present invention is to provide a LED driving device which can achieve the effect of dynamically adjusting the LED current by a simple circuit design.  
         [0009]     Accordingly, in attainment of the aforementioned objects, the LED controller of the present invention comprises a sensing unit, a control unit, and a driving unit. The sensing unit can sense a luminous intensity of a LED and output a corresponding sensing signal to the control unit. According to the sensing signal, the control unit can output a control signal to the driving unit. The driving unit can drive the LED according to the control signal.  
         [0010]     In another aspect, the LED control method of the present invention comprises: generating a control signal to drive a LED; sensing a luminous intensity value of the LED; and selectively adjusting the control signal according to the luminous intensity value.  
         [0011]     In another aspect, the LED driving device of the present invention comprises: a LED unit for emitting a luminous intensity according to a corresponding driving signal, and a first adjusting unit for generating an adjusting signal to the LED unit according to a pulse width modulation (PWM) signal and adjusting the luminous intensity according to the adjusting signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram of a preferred embodiment of the LED controller according to the present invention.  
         [0013]      FIG. 2  is a block diagram showing a preferred embodiment of the control unit of  FIG. 1 .  
         [0014]      FIG. 3  is a block diagram showing a preferred embodiment of the driving unit of  FIG. 1 .  
         [0015]      FIG. 4  is a detailed circuit diagram of the driving unit of  FIG. 3 .  
         [0016]      FIG. 5  is a flow chart of a preferred embodiment of the LED control method according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  is a block diagram of a preferred embodiment of the LED controller according to the present invention. As shown in  FIG. 1 , the LED controller  10  comprises a sensing unit  11 , a control unit  12 , and a driving unit  13 . The sensing unit  11  can detect the luminous intensity of a LED (within the driving unit  13  and not shown in  FIG. 1 ), and output a corresponding sensing signal. In one embodiment, the sensing unit  11  employs a photodiode (not shown) to detect the luminous intensity.  
         [0018]     The control unit  12 , coupled to the sensing unit  11 , can determine if the luminous intensity of the LED reaches a predetermined value according to the sensing signal. Then, the control unit  12  outputs a control signal to the driving unit  13 . When the intensity reaches the predetermined value, the control unit  12  remains to output the original control signal such that the driving unit  13  can keep the luminous intensity at the predetermined value. However, when the intensity deviates from the predetermined value, the control unit  12  would adjust the control signal such that the driving unit  13  can change the luminous intensity (described later). In one embodiment, the color of the LED is one of red, green and blue, and the LED is used to mix white light. In the colorimetry suggested by the Commission International de L&#39;Eclairage (CIE), white can be represented as a linear combination of red, green and blue. Thus, the predetermined value can be generated according to the CIE colorimetry. For example, if the color of the LED is blue, the proportion of blue in the above linear combination can be used as the predetermined value.  
         [0019]     After adjusting the luminous intensity to the predetermined value, the control unit  12  can further determine if the LED is aging by comparing the control signal and the subsequent sensing signal. That is, the control unit  12  can record the values of the control signal and the corresponding ideal values of the sensing signal in a table. When the “actual” value of the sensing signal is lower than the ideal value over a default degree, it means that the LED intensity does not reach the expected value, and then the LED can be judged as aging. If the LED is aging, its intensity is subject to deviate from the predetermined value. Thus, the control unit  12  would require the sensing unit  11  to perform detection again after a shorter time. On the other hand, if the LED is not aging, its intensity is not subject to deviate from the predetermined value. Thus, the control unit  12  would require the sensing unit  11  to perform detection again after a longer time.  
         [0020]      FIG. 2  is a block diagram showing a preferred embodiment of the control unit  12  of  FIG. 1 . In  FIG. 2 , the control unit  12  comprises an analog-to-digital converter (ADC)  121 , a microprocessor  122  and a memory  123 . The ADC  121  can convert the above analog sensing signal into a digital response value of the luminous intensity. The memory  123  can record the correspondence between the value of the control signal and the ideal response value, and the correspondence can be used to judge if the LED is aging. The microprocessor  122 , coupled to the ADC  121  and the memory  123 , can execute related operations to determine whether the LED intensity reaches the predetermined value according to the actual response value provided by the ADC  122 . Besides, the microprocessor  122  accesses the memory  123  and calculates the difference between the actual and ideal response values, thereby judging if the LED is aging.  
         [0021]     The driving unit  13  is coupled to the control unit  12 , and drives the LED according to the control signal provided by the control unit  12 .  FIG. 3  is a block diagram showing a preferred embodiment of the driving unit  13  of  FIG. 1 . In  FIG. 3 , the driving unit  13  comprises a LED unit  131 , a first adjusting unit  132  and a second adjusting unit  133 . In this embodiment, the control signal includes a driving signal, a PWM signal and a switch signal. The LED unit 1   131  includes the LED and associated control circuit, and switches between a “light-on” state and a “light-off” state according to the switch signal. Also, the LED unit  131  can receive the driving signal and drive the LED to emit a corresponding luminous intensity.  
         [0022]     The first adjusting unit  132  receives the PWM signal and generates a corresponding adjusting signal to the LED unit  131 , thereby adjusting the luminous intensity of the LED. By changing the pulse width of the PWM signal, various adjusting signals can be generated to adjust the intensity by different degrees. The second adjusting unit  133  generates a feedback signal to the control unit  12 , and then the control unit  12  generates the corresponding driving signal according to the feedback signal. Thus, by adjusting the feedback signal, the driving signal can be changed, and the LED intensity can further be adjusted. Besides, the second adjusting unit  133  can accelerate discharge for the LED when the LED unit  131  switches from the light-on state to the light-off state, thereby enabling a more rapid and precise switch.  
         [0023]      FIG. 4  is a detailed circuit diagram of the driving unit  13  of  FIG. 3 . In  FIG. 4 , the LED unit  131  comprises: a LED for receiving the driving signal, a N-channel metal oxide semiconductor (NMOS) Q 1  coupled to the LED and used as a switch, and a resistor R 1  with one end coupled to ground and the other to the drain of Q 1 . The first adjusting unit  132  comprises: an operational amplifier OP 1  with a non-inverting input, an inverting input and an output, wherein the inverting input receives the PWM signal and the non-inverting input is coupled to ground; a resistor Rd coupled between the non-inverting input and the output; and a resistor Rs coupled between the output and the LED unit  131 . The adjusting unit  133  comprises: a power source Vcc, a resistor R 2  coupled to Vcc, a PMOS Q 2  used as a switch, a variable resistor R 3  coupled to Vcc and the source of Q 2 , and a resistor R 4  coupled between the sources of Q 1  and Q 2 .  
         [0024]     As shown in  FIG. 4 , the switch signal is applied to the gates of Q 1  and Q 2  such that Q 1  and Q 2  are not connected simultaneously. When Q 1  is connected, Q 2  is disconnected. At this time, a current is generated by the driving voltage V i  (i.e. the driving signal) to flow through the LED, and the LED is in the light-on state. The first adjusting unit  132  uses OP 1  to convert the PWM signal provided by the control unit  12  into a corresponding current and sends it to the LED unit  131 , thereby generating a fine-tuning effect on the current flowing through the LED. In addition, the second adjusting unit  133  feedbacks a voltage value V f  (i.e. the feedback signal) to the control unit  12  so as to generate the corresponding driving voltage V i . Compared to the first adjusting unit  132 , the second adjusting unit  133  performs a rough tuning on the LED current. On the other hand, when Q 2  is connected, Q 1  is disconnected. At this time, the connected Q 2  provides a discharge path to accelerate the discharge of the LED, thereby achieving the effect of rapid switch mentioned above.  
         [0025]      FIG. 5  is a flow chart of a preferred embodiment of the LED control method according to the present invention. As shown in  FIG. 5 , the flow comprises the steps of: 
         51  generating a control signal to drive a LED;      52  sensing a luminous intensity value of the LED;      53  determining whether the luminous intensity value reaches a predetermined value, if yes then jumping to step  55 , otherwise proceeding to step  53 ;      54  adjusting the control signal and jumping to step  52 ;      55  determining whether the LED is aging, if no then jumping to step  57 , otherwise proceeding to step  56 ;      56  waiting a first time and jumping to step  52 ; and      57  waiting a second time and jumping to step  52 .          
         [0033]     If the step  53  determines that the luminous intensity value does not reach the predetermined value, the steps  52  to  54  are executed repeatedly until the intensity value reaches the predetermined value. In one embodiment, the color of the LED is one of red, green and blue, and in the step  53 , the predetermined value is generated according to the CIE colorimetry.  
         [0034]     Besides, if the luminous intensity value reaches the predetermined value, then the step  55  is executed to determine whether the LED is aging. This determination is performed by comparing the control signal and the subsequent luminous intensity value. If the LED is aging, a shorter first time is waited (step  56 ) and then the step  52  is executed again to perform detection. If the LED is not aging, a longer second time is waited before the step  52  is executed again (step  57 ).  
         [0035]     While the present invention has been shown and described with reference to the preferred embodiments thereof and in terms of the illustrative drawings, it should not be considered as limited thereby. Various possible modifications and alterations could be conceived of by one skilled in the art to the form and the content of any particular embodiment, without departing from the scope and the spirit of the present invention.