Patent Publication Number: US-9418611-B2

Title: LED backlight controller

Description:
This application claims benefit from Provisional Application No. 61/433,465 filed on Jan. 17, 2011 for Tuomas Tapani Tuikkanen et al. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to LED backlights and, more particularly, to an LED backlight controller. 
     2. Description of the Related Art 
     A liquid crystal display (LCD) panel is a type of display panel commonly used in electronic devices, such as lap top computers, cell phones, and televisions. The image displayed on an LCD panel is comprised of an array of dots or picture elements (pixels). In a conventional color image, each dot or pixel includes a number of colored dots or sub-pixels, such as a red dot or sub-pixel, a green dot or sub-pixel, and a blue dot or sub-pixel. 
     LCD panels include a light source, a pair of polarizers, and an array of liquid crystal regions. Color LCD panels have a liquid crystal region with a color filter for each colored dot or sub-pixel, and a number of liquid crystal regions (e.g., one with a red filter, one with a green filter, and one with a blue filter) for each dot or pixel. 
     In operation, light from the light source passes through a first polarizer of the pair of polarizers, and then into the array of liquid crystal regions. The liquid crystal regions are individually controlled by thin-film transistors that vary the voltages across the liquid crystal regions which, in turn, varies the amount of light from the light source that can pass through the liquid crystal regions. 
     For example, when a first voltage lies across a liquid crystal region, the liquid crystal region rotates the polarization of the light, which then passes out of a second polarizer of the pair of polarizers with a maximum light intensity. On the other hand, when a second voltage lies across the liquid crystal region, the liquid crystal region rotates the polarization of the light so that substantially none of the light passes out of the second polarizer. Voltages that lie between the first and second voltages, in turn, allow varying amounts of light to pass out of the second polarizer. 
     Thus, when a liquid crystal region is covered with a red filter, which represents a red dot or sub-pixel, red light with a maximum intensity passes out of the second polarizer when the first voltage lies across the liquid crystal region, no light passes out of the second polarizer when the second voltage lies across the liquid crystal region, and one of a number of shades of red passes out of the second polarizer when one of a number of voltages between the first and second voltages lies across the liquid crystal region. For example, 256 shades of red require 256 voltage steps between the first voltage (maximum intensity) and the second voltage (no light) which, in turn, can be represented with an eight-bit word. 
       FIG. 1  shows a block diagram that illustrates an example of a conventional LCD device  100 . As shown in  FIG. 1 , LCD device  100  includes an LCD panel  110  that has a number of liquid crystal regions  112  that are arranged in an array with m columns and n lines. Each line of liquid crystal regions  112  represents a number of pixels  114 , while each pixel  114  includes a number of sub-pixels  116 , such as a red sub-pixel  116 R, a green sub-pixel  116 G, and a blue sub-pixel  116 B. For example, a conventional LCD panel with 1024 pixels per line and three sub-pixels per pixel has 3,072 (1024×3) liquid crystal regions  112  per line. 
     As further shown in  FIG. 1 , LCD device  100  also includes a source (or column) driver circuit  120  that is electrically connected to the thin-film transistors that are associated with the liquid crystal regions  112  within LCD panel  110 . Source driver circuit  120  includes a number of latches  122  that are arranged in a row so that each column of liquid crystal regions  112  has a corresponding latch  122 . The row of latches  122 , in turn, is connected to receive a stream of image data DBS, a pixel clock signal PCLK, and an enable signal (not shown). 
     In operation, sub-pixel image data from the stream of image data DBS is sequentially loaded into the latches  122  on the rising edges of the pixel clock signal PCLK. For example, a first latch  122 - 1  can be enabled to latch a first eight-bit word (which identifies one of 256 voltage steps) from the stream of image data DBS on a first rising edge of the pixel clock signal PCLK, while a second latch  122 - 2  can be enabled to latch a second eight-bit word from the stream of image data DBS on a second rising edge of the pixel clock signal PCLK. 
     Further, a 3,072 nd  latch  122 - 3072  can be enabled to latch a 3,072 nd  eight-bit word from the stream of image data DBS on a 3,072 nd  rising edge of the pixel clock signal PCLK. After sub-pixel image data has been loaded into each latch  122  in the row, the rising edge of a local line clock signal LLCLK (which coincides with the 3,073 rd  rising edge of the pixel clock signal PCLK) causes the sub-pixel image data stored in the row of latches  122  to be latched and output by a row of secondary latches. A row of digital-to-analog (D/A) converter driver circuits then converts the sub-pixel image data output by the row of secondary latches to analog values, and drives out the analog values. 
     As additionally shown in  FIG. 1 , LCD device  100  also includes a gate (or row) driver circuit  130  that is electrically connected to the thin-film transistors that are associated with the liquid crystal regions  112  within LCD panel  110 . Gate driver circuit  130  can be implemented with a shift register that has one output for each line of the array. 
     In operation, gate driver circuit  130  drives a gate voltage to sequential rows of the thin-film transistors that are associated with sequential rows of liquid crystal regions  112  in response to the rising edges of the local line clock signal LLCLK. For example, after the 3,072 nd  rising edge of the pixel clock signal PCLK has loaded sub-pixel image data into latch  122 - 3072 , a first rising edge of the local line clock signal LLCLK causes gate driver circuit  130  to drive the gate voltage to the thin-film transistors associated with the liquid crystal regions  122  in the first row. 
     At the same time, the first rising edge of the local line clock signal LLCLK also causes source driver circuit  120  to output analog voltages that correspond to the digital values stored in the row of secondary latches. Since the thin-film transistors associated with the liquid crystal regions  122  in the first row are the only transistors to receive the gate voltage, only the thin-film transistors associated with the liquid crystal regions  122  in the first row respond to the analog voltages output by source driver circuit  120 . 
     During the next 3,072 nd  rising edges of the pixel clock signal PCLK, the sub-pixel image data stored in the latches  122  are overwritten with new sub-pixel image data from the stream of image data DBS. After the next 3,072 nd  rising edge of the pixel clock signal PCLK has loaded sub-pixel image data into latch  122 - 3072 , a second rising edge of the local line clock signal LLCLK causes gate driver circuit  130  to drive the gate voltage to the thin-film transistors associated with the liquid crystal regions  122  in the second row. 
     At the same time, the second rising edge of the local line clock signal LLCLK also causes source driver circuit  120  to output analog voltages that correspond to the new digital values that are now stored in the row of secondary latches. Since the thin-film transistors associated with the liquid crystal regions  122  in the second row are the only transistors to receive the gate voltage, only the thin-film transistors associated with the liquid crystal regions  122  in the second row respond to the analog voltages output by source driver circuit  120 . 
     As also shown in  FIG. 1 , LCD device  100  includes a timing controller  140  that receives the stream of image data DBS and a number of timing signals, including the pixel clock signal PCLK, a line clock signal LCLK, and a frame clock signal FCLK from a graphics processor unit (GPU)  142 . In addition, timing controller  140  outputs the stream of image data DBS and a number of timing signals, including the pixel clock signal PCLK, the local line clock signal LLCLK, and a local frame clock signal FLCLK. 
     The frequency of the pixel clock signal PCLK is approximately equal to the number of pixels in a line multiplied by the number of lines in a frame multiplied by the frame rate. For example, an LCD display having an image size of 1280 pixels by 800 lines and a frame rate of 60 Hz has a pixel clock frequency of approximately 61.44 MHz (ignoring the blanking times to simplify the example). 
     In operation, timing controller  140  divides down the frequency of the pixel clock signal PCLK to generate the local line clock signal LLCLK and the local frame clock signal FLCLK. For example, timing controller  140  can divide down the 61.44 MHz pixel clock signal PCLK to generate a 48.00 KHz local line clock signal LLCLK and a 60 Hz local frame clock signal FLCLK. 
     Timing controller  140  generates the local frame clock signal FLCLK because the frame clock signal FCLK output by GPU  142  is subject to jitter relative to the pixel clock signal PCLK, thereby making the frame clock signal FCLK less accurate than the local frame clock signal FLCLK. The line clock signal LCLK, which is over two orders of magnitude greater than the frame clock signal FCLK, is sufficiently accurate to be used, thereby making the decision on whether to locally generate the line clock signal optional. 
       FIGS. 2A-2D  show a series of timing diagrams that illustrate the operation of LED device  100 .  FIG. 2A  shows the local line clock signal LLCLK.  FIG. 2B  shows a representative voltage V 1  for a first row of sub-pixels.  FIG. 2C  shows a representative voltage V 2  for a second row of sub-pixels.  FIG. 2D  shows a representative voltage V 3  for a third row of sub-pixels. 
     During each pulse of the local line clock signal LCLK, the voltages across all of the liquid crystal regions in a row of liquid crystal regions are individually charged up. However, after being charged up, the voltages decay until charged up again. Thus, there is a jump in voltage that results from the increased charge, followed by a slow decay period. 
     As shown in  FIGS. 2A-2D , the voltages across all of the liquid crystal regions in the first line of liquid crystal regions are individually charged up during period T 1  of the local line clock signal LLCLK. In addition, the voltages across all of the liquid crystal regions in the second line of liquid crystal regions are individually charged up during period T 2 , and the voltages across all of the liquid crystal regions in the third line of liquid crystal regions are individually charged up during period T 3 . 
     As further shown in  FIGS. 2A-2D , the progression from line to line of the jump in voltage (resulting from the increase in charge) causes an image defect known as LCD ripple. However, since the LCD ripple occurs at the local line clock frequency (one jump per local line clock period), the defect can not be seen by the human eye. 
     The source of light in an LCD panel is commonly provided by a number of lamps that are miniature versions of fluorescent tubes, but is increasingly being provided by strings of light emitting diodes (LED). For example, rather than using lamps, a number of strings of LEDs (e.g., two strings, three strings, or six strings) can alternately be used. LED strings have a number of advantages over conventional lamps, including lower power requirements and a longer service life. 
     In the  FIG. 1  example, LCD device  100  also includes an LED backlight source  150  that includes three LED strings  152 , and an LED backlight controller  154  that controls the operation of the LED strings  152 . In the present example, LED backlight controller  154  includes a synchronizer  160  that synchronizes a control clock signal CCLK from a system host controller  156  and the frame clock signal FCLK from GPU  142  to generate a synchronized control clock signal SCLK. 
     LED backlight controller  154  also includes a pulse width modulator  162  that pulse width modulates the synchronized clock signal SCLK in response to a duty cycle bias voltage BS from system host controller  156  to generate a pulse width modulated control clock signal MCLK that drives the LED strings  152 . The brightness of the light produced by the LED strings  152  is controlled by the duty cycle of the pulse width modulated control clock signal MCLK. 
     One of the problems with using LED strings in place of lamps as the source of light is that the frequency difference between the local line clock signal LLCLK and the modulated control clock signal MCLK causes visible line banding artifacts to appear having alternating groups of brighter and darker lines. In addition, if synchronizer  160  is omitted so that the modulated control clock signal MCLK is not synchronized to the frame clock signal FCLK (by way of the synchronized clock signal SCLK), then the line banding artifacts scroll up or down. 
       FIG. 3  shows a timing diagram of the modulated control clock signal MCLK that illustrates the formation of line banding artifacts. As shown in  FIG. 3 , the modulated control clock control signal MCLK is conventionally much slower than the line clock signal LCLK, typically having a frequency of approximately 1 KHz as compared to the 48 KHz line clock signal LCLK. 
     As further shown in  FIG. 3 , if the modulated control clock signal MCLK is used to simultaneously drive the three LED strings  152 , then the LED strings turn on and turn off, thereby generating an image defect known as LED flicker. LED flicker occurs at the frequency of the control clock signal CCLK (the LED strings are on and off once per control clock period), and as a result can not be seen by the human eye. However, as additionally shown in  FIG. 3 , the interaction between the LCD ripple and the LED flicker produces banding artifacts that generate bands of brighter lines and bands of darker lines that are visible to the human eye. 
     One approach to reducing the line banding artifacts is to modify LED backlight controller  154  to drive the LED strings  152  individually with the modulated control clock signal MCLK and a number of phase delayed versions of the modulated control clock signal MCLK. For example, the modulated control clock signal MCLK and two delayed versions of the modulated clock signal MCLK, each delayed 120° from the previous control signal, can be utilized with three LED strings  152 . 
     In the  FIG. 1  example, the pulse width modulator  162  of LED backlight controller  154  includes delay circuitry that generates a delayed modulated control clock signal DCLK 1  which is delayed 120° from the modulated control clock signal MCLK, and a delayed modulated control clock signal DCLK 2  which is delayed 120° from the delayed modulated control clock signal DCLK 1 . 
       FIGS. 4A-4C  show a series of timing diagrams that illustrate the operation of LED device  100  with phase delayed LED strings.  FIG. 4A  shows a delayed modulated control clock signal DCLK 1  which is delayed 120° from the modulated control clock signal MCLK, while  FIG. 4B  shows a delayed modulated control clock signal DCLK 2  which is delayed 120° from the delayed modulated control clock signal DCLK 1 .  FIG. 4C  shows a composite total LED on time. 
     As shown in  FIGS. 3 and 4A-4C , while the line banding artifacts have not been eliminated, the intensity of the banding artifacts has been substantially reduced by individually driving the LED strings  152  with the modulated control clock signal MCLK and the delayed modulated control clock signals DCLK 1  and DCLK 2 . However, the improvement shown in  FIG. 4C , which is based on a 50% duty cycle, fades as the duty cycle of the modulated clock signal MCLK is changed. 
     Many LCD devices sense the ambient light, and adjust the duty cycle of the modulated control clock signal MCLK to adjust the brightness of the light produced by the LED strings in response to the intensity of the ambient light. The change in duty cycle then significantly worsens the line banding artifacts. Thus, there is a need for an LCD device that reduces line banding artifacts when the duty cycle of the modulated clock signal MCLK is varied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of a conventional LCD device  100 . 
         FIGS. 2A-2D  are a series of timing diagrams illustrating the operation of LED device  100 .  FIG. 2A  illustrates the local line clock signal LLCLK.  FIG. 2B  illustrates a representative voltage V 1  for a first row of sub-pixels.  FIG. 2C  illustrates a representative voltage V 2  for a second row of sub-pixels.  FIG. 2D  illustrates a representative voltage V 3  for a third row of sub-pixels. 
         FIG. 3  is a timing diagram of the modulated control clock signal MCLK illustrating the formation of line banding artifacts. 
         FIGS. 4A-4C  are a series of timing diagrams illustrating the operation of LED device  100  with phase delayed LED strings.  FIG. 4A  illustrates a delayed modulated control clock signal DCLK 1  which is delayed 120° from the modulated control clock signal MCLK.  FIG. 4B  illustrates a delayed modulated control clock signal DCLK 2  which is delayed 120° from the delayed modulated control clock signal DCLK 1 .  FIG. 4C  illustrates a composite total LED on time. 
         FIG. 5  is a block diagram illustrating an example of an LCD device  500  in accordance with the present invention. 
         FIGS. 6A-6N  are a series of timing diagrams illustrating the operation of LED device  500  in accordance with the present invention.  FIG. 6A  illustrates the line clock signal LCLK.  FIG. 6B  illustrates a representative voltage V 1  for a first row of sub-pixels.  FIG. 6C  illustrates a representative voltage V 2  for a second row of sub-pixels.  FIG. 6D  illustrates a representative voltage V 3  for a third row of sub-pixels.  FIG. 6E  illustrates a representative voltage V 4  for a fourth row of sub-pixels.  FIG. 6F  illustrates a representative voltage V 5  for a fifth row of sub-pixels.  FIG. 6G  illustrates a representative voltage V 6  for a sixth row of sub-pixels. 
         FIG. 6H  illustrates the modulated control clock signal MCLK.  FIG. 6I  illustrates a delayed modulated control clock signal DCLK 1  that is delayed 60° from the modulated control clock signal MCLK.  FIG. 6J  illustrates a delayed modulated control clock signal DCLK 2  that is delayed 60° from the LED control signal DCLK 1 . 
         FIG. 6K  illustrates a delayed modulated control clock signal DCLK 3  that is delayed 60° from the delayed modulated control clock signal DCLK 2 ,  FIG. 6L  illustrates a delayed modulated control clock signal DCLK 4  that is delayed 60° from the delayed modulated control clock signal DCLK 3 ,  FIG. 6M  illustrates a delayed modulated control clock signal DCLK 5  that is delayed 60° from the delayed modulated control clock signal DCLK 4 , and  FIG. 6N  illustrates a composite total LED on time. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 5  shows a block diagram that illustrates an example of an LCD device  500  in accordance with the present invention. LCD device  500  is similar to LCD device  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both LCD devices. 
     As shown in  FIG. 5 , LCD device  500  differs from LCD device  100  in that LCD device  500  can utilize a different number of LED strings  152 . In addition, LED device  500  utilizes an LED backlight controller  510  in lieu of LED backlight controller  154 . Further, LCD device  500  also routes a backlight clock signal BCLK to LED backlight controller  510  in lieu of the frame clock signal FCLK, and eliminates the control clock signal CCLK. 
     In accordance with the present invention, the frequency of the modulated control clock signal MCLK and the number of LED strings  152  are selected so that the product of the frequency of the modulated control clock signal MCLK multiplied by the number of LED strings  152  is equal to the line clock frequency LCLK. 
     The line clock frequency LCLK is equal to the product of the frame rate multiplied by the number of lines. For example, an LCD display having an image size of 1280 pixels by 800 lines and a frame rate of 60 Hz has a line clock frequency of 48.00 KHz (60*800). Thus, eight LED strings  152  require a modulated control clock frequency of 6 KHz, while six LED strings  152  require a modulated control clock frequency of 8 KHz and four LED strings  152  require a modulated control clock frequency of 12 KHz. In the present example, LED device  500  utilizes six LED strings  152 . 
     As further shown in  FIG. 5 , LED backlight controller  510  includes a clock divider  512  that divides down the frequency of the backlight clock signal BCLK to form an intermediate clock signal NCLK. The frequency of the intermediate clock signal NCLK determines the frequency of the modulated clock signal MCLK. 
     For example, as additionally shown in  FIG. 5 , a 160-count counter C 1  can be used to divide down a 7.68 MHz backlight clock signal BCLK to form a 48 KHz local line clock signal LOCLK, which is then divided down by a 6-count counter C 2  to form an 8 KHz intermediate clock signal NCLK. In this case, the local line clock signal LOCLK and the line clock signal LCLK are substantially identical and in phase. Alternately, a 960-count counter can be used to divide down a 7.68 MHz backlight clock signal BCLK to form an 8 KHz intermediate clock signal NCLK. 
     LED backlight controller  510  also includes a pulse width modulator  514  that pulse width modulates the intermediate clock signal NCLK in response to the duty cycle bias voltage BS that is received from system host controller  156  to generate a pulse width modulated control clock signal MCLK. 
     In addition, pulse width modulator  514  also includes delay circuitry that generates a number of delayed modulated control clock signals DCLK that are equal to one less than the number of LED strings  152 . Since LED device  500  utilizes six LED strings  152  in the present example, pulse width modulator  514  generates delayed modulated control clock signals DCLK 1 , DCLK 2 , DCLK 3 , DCLK 4 , and DCLK 5 . 
     The backlight clock signal BCLK, in turn, is synchronized to the pixel clock signal PCLK, and has a frequency that lies in a range of frequencies. The range of frequencies is defined at an upper end by the frequency of the pixel clock signal PCLK and at the lower end by a divided down frequency. The divided down frequency, which is a critical point, is the lowest frequency which is sufficient to maintain a low jitter margin and thereby an accurate clock signal. 
     For example, a backlight clock signal BCLK that has a frequency which is approximately two orders of magnitude greater than the frequency of the frame clock signal FCLK, e.g., approximately greater than 6 KHz for a 60 Hz frame clock signal FCLK, can be the lowest frequency which is sufficient to maintain a low jitter margin and thereby an accurate clock signal. 
     Thus, in the present example, the range of frequencies extends from 61.44 MHz (the pixel clock frequency) to 6 KHz (two orders of magnitude greater than the frame clock signal of 60 Hz). Further, in the present example, GPU  142  divides down the frequency of the 61.44 MHz pixel clock signal PCLK by eight to output a 7.68 MHz backlight clock signal BCLK. A 7.68 MHz clock backlight clock signal BCLK remains highly synchronized to the pixel clock signal (little effect from jitter), but requires less power and generates less electromagnetic interference (EMI). 
       FIGS. 6A-6N  show a series of timing diagrams that illustrate the operation of LCD device  500  in accordance with the present invention.  FIGS. 6A-6G  show that the LCD ripple image defect that results from the progression from line to line of the jump in voltage (resulting from the increase in charge), which was present in LCD device  100 , is also present in LCD device  500 . 
       FIG. 6A  shows the line clock signal LCLK.  FIG. 6B  shows a representative voltage V 1  for a first row of sub-pixels.  FIG. 6C  shows a representative voltage V 2  for a second row of sub-pixels.  FIG. 6D  shows a representative voltage V 3  for a third row of sub-pixels.  FIG. 6E  shows a representative voltage V 4  for a fourth row of sub-pixels.  FIG. 6F  shows a representative voltage V 5  for a fifth row of sub-pixels. 
       FIGS. 6H-6M  show the six equally spaced clock signals MCLK, DCLK 1 , DCLK 2 , DCLK 3 , DCLK 4 , and DCLK 5  that are used to individually drive the six LED strings  152 .  FIG. 6H  shows the modulated control clock signal MCLK.  FIG. 6I  shows a delayed modulated control clock signal DCLK 1  that is delayed 60° from the modulated control clock signal MCLK.  FIG. 6J  shows a delayed modulated control clock signal DCLK 2  that is delayed 60° from the LED control signal DCLK 1 . 
       FIG. 6K  shows a delayed modulated control clock signal DCLK 3  that is delayed 60° from the delayed modulated control clock signal DCLK 2 .  FIG. 6L  shows a delayed modulated control clock signal DCLK 4  that is delayed 60° from the delayed modulated control clock signal DCLK 3 . FIG.  6 M shows a delayed modulated control clock signal DCLK 5  that is delayed 60° from the delayed modulated control clock signal DCLK 4 . 
       FIG. 6N  shows a composite total LED on time. In accordance with the present invention, as shown in  FIG. 6N , by selecting the frequency of the modulated control clock signal MCLK and the number of LED strings  152  so that the product of the frequency of the modulated control clock signal MCLK multiplied by the number of LED strings  152  is equal to the line clock frequency LCLK, the line banding image artifact is substantially eliminated. 
     Thus, an LCD device that substantially eliminates the line banding artifact that results from the interaction of the LCD ripple and the LED flicker has been described. The LCD device utilizes an LED backlight controller that divides down an accurate high-frequency clock signal that is synchronized to the pixel clock signal to a frequency which, when multiplied by the number of LED strings, is equal to the line clock frequency. 
     It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.