Abstract:
A backlight control device for controlling a driving current of an LED is disclosed. By controlling current outputs from current sources of a plurality of current output units, a display will be able to generate desirable backlight. Then by adjusting currents output by the plurality of current output units, brightness of a plurality of pixels can be dynamically adjusted. The brightness of pixels with higher gray levels can be increased while the brightness of pixels with lower gray levels can be decreased, thereby improving the contrast of image and saving power consumption.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/940,298, entitled “BACKLIGHT CONTROL DEVICE AND METHOD FOR CONTROLLING A DRIVING CURRENT OF AN LED,” filed Nov. 14, 2007, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a backlight module of a thin film transistor liquid crystal display, and more particularly, to a backlight control device for controlling a driving current of an LED. 
     2. Description of the Prior Art 
     Ordinarily, the current control integrated circuit of a light-emitting diode (LED) uses fixed resistance to control the driving current of the backlight module to control the brightness of the backlight module. The disadvantage is that once the resistance value of the bias resistor is established, the driving current of backlight module cannot be adjusted, and a long-term use of circuit components can cause an unexpected variation in the driving current of backlight module. Further, the driving current in certain types of LED, such as Organic LED (OLED), may be changed due to a change in the operating temperature of the current regulator. As a result, the brightness of the LED in the backlight module may be adversely affected. Therefore, a method and an apparatus capable of controlling the driving current for backlight modules in LCD systems are needed. 
       FIG. 1  illustrates a prior art programmable control method to regulate the brightness of LEDs in a backlight module  100 . Microcontrollers are an example where users can input the desired current value or voltage value to an LED controller  103  through a user interface  101  of the microcontroller  102 . According to the current value or voltage value stored in registers, the desired current value or voltage value is generated and provided to the current output unit, and the output current  104  generated by the current output unit is used to regulate the brightness of an LED  105 . 
     However, the method described above uses programming interfaces to regulate the brightness of LEDs, so that the backlight can not regulate the brightness in displaying video. Therefore, the related art can not dynamically reduce the brightness of backlight when the lower brightness is desired. Further, the related art can not dynamically enhance the brightness of backlight when the higher brightness is desired. The related art uses brightness predetermined and inputted by a user, and the backlight module can not dynamically regulate the brightness later on. As a result, it causes low quality in the brightness contrast of images. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention discloses a backlight control device for controlling a driving current of a light-emitting diode. The backlight control device comprises at least a pulse width modulation (PWM) control unit, at least a current output unit coupled to the PWM control unit, and a shift register coupled to the PWM control unit for storing control signals for controlling the PWM control unit. 
     Another embodiment of the present invention discloses a method for controlling a driving current of an LED. The method comprises outputting a predetermined current to cause the LED to emit uniform light, and after outputting the predetermined current, outputting a voltage proportional to brightness of an image to be displayed. 
     These and other objectives of the present invention will no doubt become apparent to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art programmable control method to regulate the brightness of LEDs in a backlight module. 
         FIG. 2  illustrates a backlight control device according to a first embodiment of the present invention. 
         FIG. 3  illustrates of a backlight control device according to a second embodiment of the present invention. 
         FIG. 4  illustrates a backlight control device according to a third embodiment of the present invention. 
         FIG. 5  illustrates a backlight control device according to a fourth embodiment of the present invention. 
         FIG. 6  illustrates a backlight control device according to a fifth embodiment of the present invention. 
         FIG. 7  illustrates a basic operating timing diagram according to the present invention. 
         FIG. 8  is a PWM timing diagram of all channels when the backlight control device is in the dynamic PWM mode. 
         FIG. 9  shows an embodiment of controlling red, green and blue primary colors according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  illustrates a backlight control device  200  according to a first embodiment of the present invention. The backlight control device  200  comprises a plurality of PWM control units  202  each including a PWM register  212  and a control unit  214 , a plurality of current output units  204  coupled to the PWM control units  202 , and a shift register  206  coupled to the PWM registers  212  of the PWM control units  202 . The shift register  206  stores brightness control signals for the PWM control units  202 . 
     Each current output unit  204  comprises a current source  208 , and an NMOS  210 . The current source  208  is coupled to a low voltage terminal such as ground. The source of NMOS  210  is coupled to the current source  208 , and the gate is coupled to the PWM control unit  202 . Each PWM control unit  202  comprises a PWM register  212  coupled to the shift register  206  for storing a brightness control signal. The control unit  214  outputs a corresponding PWM signal according to the brightness control signal stored in the PWM register  212 . The PWM signal is a periodic signal which in some periods is at a high voltage level and in other periods is at a low voltage level. The periodic signal is used to regulate the current by opening and closing the gate of the NMOS  210 . When the PWM signal is at the high voltage level, the gate of the NMOS  210  is opened. Thus a higher percentage of high voltage in the PWM signal corresponds to a greater current output by the current output unit  204 , which corresponds to higher backlight brightness. On the other hand, a lower percentage of high voltage in the PWM signal corresponds to a lower current output by the current output unit  204 , which corresponds to lower backlight brightness. Therefore the PWM signal outputted by the control unit  214  can dynamically regulate the backlight brightness according to the display signal. The input of the control unit  214  is coupled to the PWM register  212 . The output of the control unit  214  is coupled to the gate of the NMOS  210  which is used to regulate the output current of the current output unit  204  through opening and closing the gate of the NMOS  210 . 
     In this embodiment, the shift register  206  is coupled to a plurality of PWM control units  202 , and the memory size of the shift register  206  is not greater than the total memory size of all PWM registers  212 . Therefore, a brightness control signal of a first channel is stored in the shift register  206 , and then it is transferred to a PWM register  212  of the first channel. Afterwards a brightness control signal of a second channel is stored in the shift register  206 , and then it is transferred to a PWM register  212  of the second channel. The shift register  206  transfers brightness control signals to PWM registers  212  of different channels sequentially so as to control the output currents of different current output units  204 . 
       FIG. 3  illustrates of a backlight control device  300  according to a second embodiment of the present invention. The difference between the backlight control device  300  and backlight control device  200  is that the current output unit  304  of the backlight control device  300  comprises a current source  308  and a PMOS  310 . The current source  308  is coupled between a low voltage terminal such as ground and the drain of PMOS  310 . Like the backlight control device  200 , the backlight control device  300  regulates the output current of the current output unit  304  through opening and closing of the gate of PMOS  310 . 
     The PWM signal outputted from the control unit  314  is a periodic signal which in some periods is at a high voltage level and in other periods is at a low voltage level. The periodic signal is used to regulate the current by opening and closing the gate of the PMOS  310 . When the PWM signal is at the low voltage level, the gate of the PMOS  310  is opened. Thus a higher percentage of low voltage in the PWM signal corresponds to a greater current output by the current output unit  304 , which corresponds to higher backlight brightness. On the other hand, a lower percentage of low voltage in the PWM signal corresponds to a lower current output by the current output unit  304 , which corresponds to lower backlight brightness. Therefore the PWM signal outputted by the control unit  314  can dynamically regulate the backlight brightness according to the display signal. The input of the control unit  314  is coupled to the PWM register  312 . The output of the control unit  314  is coupled to the gate of the PMOS  310  which is used to regulate the output current of the current output unit  304  through opening and closing the gate of the PMOS  310 . 
       FIG. 4  illustrates a backlight control device  400  according to a third embodiment of the present invention. The difference between the backlight control device  400  and backlight control device  200  is that the backlight control device  400  comprises a plurality of shift registers  406 , and they are coupled to a plurality of PWM control unit  202 . 
     The first shift register is coupled to the PWM control unit of the first channel. The second shift register is coupled to the PWM control unit of the second channel and so on. Each shift register  406  is coupled to the PWM control unit  202  of a corresponding channel. Compared to the first embodiment shown in  FIG. 2 , the one-to-one coupling in  FIG. 4  is a lot simpler than the one-to-many coupling in  FIG. 2 . 
       FIG. 5  illustrates a backlight control device  500  according to a fourth embodiment of the present invention. The backlight control device  500  comprises a plurality of PWM control units  503 , a plurality of current control units  504 , a plurality of current output units  505 , a selector  502 , and a shift register  501  for storing control signals of the PWM control units or control signals of the current control units  504 . 
     Each current output unit  505  comprises a current source  507  and an NMOS  506 . The current source  507  is coupled between a low voltage terminal and the source of the NMOS  506 . The gate of the NMOS  506  is coupled to the PWM control unit  503 . 
     Each PWM control unit  503  comprises a PWM register  508  and a control unit  509 . The first input of the PWM register  508  is coupled to the selector  502 , the second input of the PWM register  508  is coupled to the shift register  501 , and the output of the PWM register  508  is coupled to the input of a corresponding control unit  509 . The output of the control unit  509  is coupled to the gate of the corresponding NMOS  506 . Therefore, The backlight control device  500  regulates the output current of the current output unit  505  through opening and closing the gate of NMOS  506 . 
     Each current control unit  504  comprises a register  510  and a control unit  511 . The first input of the register  510  is coupled to the selector  502 , and the second input of the register  510  is coupled to the shift register  501 . The output of the register  510  is coupled to the input of a corresponding control unit  511 . The output of the control unit  511  is coupled to the source of the corresponding NMOS  506 , and is used to regulate the voltage of the source so as to regulate the output current of output current unit  505 . 
     Further, due to the differences among electronic components of a backlight module, or due to a long time use of certain electronic components, the driving current of the backlight module may have unpredictable changes. The driving current in certain types of LEDs, such as organic LEDs (OLEDs), can change due to a change in the operating temperature of the current regulator. The current control unit  504  allows a user to adjust the luminance of LEDs inside the backlight module through the programmable interface. For instance, a microcontroller can use an I 2 C programmable interface to input a desired current or voltage value to the shift register  501 , and then the desired value will be transferred to the current control unit  504 . According to the value stored in the register  510 , the control unit  511  controls the current output unit  505  to regulate the current output by the current output unit  505  so as to regulate the brightness of LEDs. 
     In this embodiment, the first output of the shift register  501  is coupled to a plurality of PWM control units  503  and the second output of the shift register  501  is coupled to a plurality of current control units  504 . Whether the output of the shift register  510  is to be input to the PWM control units  503  or the current control units  504  depends on the mode of the selector  502 . Therefore the shift register  501  will store the control signals for every channel and transfer those control signals to the PWM control units  503  or the current control units  504  in sequence. 
       FIG. 6  illustrates a backlight control device  600  according to a fifth embodiment of the present invention. The difference between the backlight control device  600  and backlight control device  500  is that the current output unit  605  of the backlight control device  600  comprises a current source  607  and a PMOS  606 . The current source  607  is coupled between a low voltage terminal and the drain of PMOS  606 . Like the backlight control device  500 , the backlight control device  600  regulates the output current of the current output unit  605  through opening and closing the gate of PMOS  606 . 
     In the embodiment of  FIG. 5 , by regulating the currents of the current output units  505  from the current control units  504  through the programmable interface, a user can regulate the brightness of LEDs in the backlight module so that uniform brightness of backlight can be obtained when the grey levels of different channels are the same. The application can be used in a PC monitor because most of its frames are static, and the distance between a user&#39;s eyes and the screen of monitor is relatively short. The user will feel unpleasant if the backlight is not uniform due to the characteristics of different LED electronic components. The application can also be used in a monitor of the medical science field. If the LEDs cannot generate backlight of uniform brightness, the image displayed on the monitor may be distorted and result in misjudgments of physicians. 
     The embodiments in  FIG. 2 ,  FIG. 3  and  FIG. 4  do not have the current control units  504  shown in  FIG. 5 . The reason is that most frames displayed on TV are dynamic. Thus the grey levels change from picture to picture. In NTSC system there are 24 frames per second showing on TV, so it is difficult for a human eye to identify if the backlight is uniform or not. Instead the human eye is more sensitive to brightness contrast. Therefore, when displaying a frame or a section with lower brightness, the brightness of backlight of the frame or section is reduced. And when displaying a frame or a section with higher brightness, the brightness of backlight of the frame or section is enhanced. This will result in a higher visual contrast. Further when using lower brightness for displaying a dimmer image, power can be saved due to a lower current. Besides, the TV can be installed with the current control unit  504  to more accurately reflect the image to be displayed. In this case, the TV can be switched between the dynamic PWM mode or current mode according to the user&#39;s need to improve the quality of image display. 
     In the related art, a desired brightness value is input by a user. It can not dynamically regulate the brightness of backlight according the grey levels of frames to be displayed causing a low quality of image contrast. In the present invention, the backlight of uniform brightness can be provided and power can be saved by dynamically adjusting the brightness of backlight. Thus the present invention solves the problem of non-uniform backlight, enhances the brightness contrast, and saves power. 
       FIG. 7  illustrates a basic operating timing diagram according to the present invention. A low voltage signal MODE means that the backlight control device is in the current mode, and a high voltage signal MODE means that the backlight control device is in the dynamic PWM mode. The data will be latched according to signal CLK when the signal DIO is triggered. When the backlight control device is in the current mode, the latched data are desired current values. On the other hand, when the backlight control device is in the dynamic PWM mode, the latched data are grey levels of images to be displayed. The rising edge of signal MODE signals that the operation of LED current sink IC is changed from the current mode to dynamic PWM mode. At this moment, the desired current values are latched in the register of the current control unit. 
     In the dynamic PWM mode, when the signal BLANK is at a rising edge, the data are the grey levels of images to be displayed and will be latched by the PWM register. When the signal BLANK is at the low voltage, the outputs of all channels are equal to zero. When the signal BLANK is at the high voltage, the outputs of all channels are dependent on the grey levels latched by the PWM registers. The next frame appears when the signal BLANK is at the falling edge. At this time, the control signal of the PWM control unit is reset to zero so as to calculate the control signals for the next frame. 
     The signal CLK is the timing signal for the PWM control units, the PWM registers and registers of current control units. 
       FIG. 8  is a PWM timing diagram of all channels when the backlight control device is in the dynamic PWM mode. When the signal BLANK is at the rising edge, the latched data (grey levels of video frames) are stored in the PWM registers of PWM control units. When the signal BLANK is at the falling edge, the control signals of PWM control units are reset to zero. 
       FIG. 9  shows an embodiment of controlling red, green and blue primary colors according to the present invention. The red, green and blue backlight devices can use any of the backlight control devices shown in  FIG. 2 ,  3 ,  4 ,  5  or  6  for controlling the red, green and blue backlight. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.