Patent Publication Number: US-7224126-B2

Title: Pulse width modulation method for driving an OLED panel

Description:
BACKGROUND 
   1. Field of Invention 
   The present invention relates to a method for driving an organic light emitting display (OLED) panel. More particularly, the present invention relates to a pulse width modulation method for driving an OLED panel. 
   2. Description of Related Art 
   Flat panel displays are generally classified into inorganic devices and organic devices according to the display materials used in the flat panel displays. The inorganic devices include plasma display panels, field emission displays and the like; the organic devices include liquid crystal displays, organic light emitting displays (OLED) and the like. The OLED is in the spotlight because of its operating speed being faster than that of the liquid crystal display by thirty thousand times. In addition, the OLED has advantages of wide viewing angle and high brightness due to emitting light by itself. 
     FIG. 1  is a schematic view of a conventional OLED  100 . An OLED panel  110  has a plurality of organic light emitting diodes  112 , which are driven by a segment driver  120  and a common driver  130  through segment lines  122  and common lines  132 . Particularly, the organic light emitting diodes  112  are electrically connected to the segment lines  122  and common lines  132  in a matrix structure. In the prior art, a pulse width modulation (PWM) manner is provided to supply driving currents to the organic light emitting diodes  112 . The driving currents of the PWM manner may have different pulse widths. The pulse width determines the intensity of the light emitted from the organic light emitting diode  112 . 
     FIG. 2  is a schematic view of waveforms provided by a conventional PWM manner, in which the waveforms GS 1  to GS 4  of 2-bit grayscales are illustrated as an example. In a period, the pulse widths of the waveforms GS 1  to GS 4  are altered in accordance with different grayscales. However, the rising edges of the waveforms corresponding to different grayscales are all positioned at a starting time t 0  of the period T. The coherent rising of the waveforms GS 1  to GS 4  causes a peak current to be generated at the starting time t 0  of the period T. The peak current increases the required Vcc of the segment driver  120  (as illustrated in  FIG. 1 ), and the power consumption of the OLED  100  is thus raised. 
     FIG. 3  is a schematic view of waveforms provided by another conventional PWM manner, in which the waveforms GS 1  to GS 4  of 2-bit grayscales are illustrated as an example. In this PWM manner, the rising edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are changed from the starting time t 0  to other times (e.g. t 1  and t 3 ) of the period T. The peak current caused by the coherent rising of different grayscale waveforms GS 1  to GS 4  is therefore decreased. Nevertheless, if the organic light emitting diodes  112  electrically connected to the same common line  132  (as illustrated in  FIG. 1 ) are simultaneously represented by the same grayscale, the driving currents having the same waveform are supplied at the same time. Consequently, the peak current issue still remains. 
   SUMMARY 
   It is therefore an aspect of the present invention to provide a method for driving an OLED panel that mitigates the peak current issue. 
   According to one preferred embodiment of the present invention, the OLED panel includes a plurality of organic light emitting diodes. The organic light emitting diodes are electrically connected to a plurality of segment lines and a plurality of common lines in a matrix structure. 
   The organic light emitting diodes electrically connected to the same common lines are divided into a first group and a second group. Driving currents are separately supplied to the organic light emitting diodes of the first group and the second group according to a first pulse width modulation (PWM) manner and a second PWM manner. The first PWM manner and the second PWM manner have complementary waveforms in a period. 
   According to another preferred embodiment of the present invention, the OLED panel includes a plurality of organic light emitting diodes. Driving currents are supplied to a first group of the organic light emitting diodes electrically connected to a common line according to a first pulse width modulation (PWM) manner. Driving currents are supplied to a second group of the organic light emitting diodes electrically connected to the common line according to a second PWM manner. The first PWM manner and the second PWM manner have complementary waveforms in a period. 
   It is another aspect of the present invention to provide an OLED, of which the Vcc of its segment driver is decreased and the power consumption is thus lowered. 
   According to one preferred embodiment of the present invention, the OLED comprises a plurality of segment lines, a plurality of common lines, a plurality of organic light emitting diodes and a segment driver. The organic light emitting diodes are electrically connected to the segment lines and the common lines in a matrix structure. The organic light emitting diodes of one common line are divided into a first group and a second group. The segment driver is electrically connected to the segment lines and supplies driving currents to the organic light emitting diodes of the first group and the second group separately according to a first pulse width modulation (PWM) manner and a second PWM manner. The first PWM manner and the second PWM manner have complementary waveforms in a period. 
   In conclusion, the invention can effectively decrease the peak current usually occurring in the conventional PWM manner for driving the OLED panel and further decrease the Vcc of the segment driver, so as to lower the power consumption of the OLED. 
   It is to be understood that both the foregoing general description and the following detailed description are examples and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where: 
       FIG. 1  is a schematic view of a conventional OLED; 
       FIG. 2  is a schematic view of waveforms provided by a conventional PWM manner; 
       FIG. 3  is a schematic view of waveforms provided by another conventional PWM manner; 
       FIG. 4  is a flow chart of one preferred embodiment of the present invention; 
       FIG. 5A  is a schematic view of waveforms provided by the first PWM manner of one preferred embodiment; 
       FIG. 5B  is a schematic view of waveforms provided by the second PWM manner of one preferred embodiment; 
       FIGS. 6A ,  6 B and  6 C are schematic views of waveforms respectively provided by the first, second and third PWM manner of one preferred embodiment; 
       FIGS. 7A ,  7 B and  7 C are schematic views of waveforms respectively provided by the first, second and third PWM manner of another preferred embodiment; and 
       FIG. 8  is a schematic view of an organic light emitting display of one preferred embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
   The present invention divides the organic light emitting diodes of the same common line into two groups and drives the organic light emitting diodes of the two groups according to different PWM manners, which have complementary waveforms in a period. 
   Generally, an OLED panel includes a plurality of organic light emitting diodes. The organic light emitting diodes are electrically connected to a plurality of segment lines and a plurality of common lines in a matrix structure. 
     FIG. 4  is a flow chart of one preferred embodiment of the present invention. The organic light emitting diodes electrically connected to the same common lines are divided into a first group and a second group (step  402 ). Driving currents are separately supplied to the organic light emitting diodes of the first group and the second group according to a first pulse width modulation (PWM) manner and a second PWM manner. The first PWM manner and the second PWM manner have complementary waveforms in a period (step  404 ). 
   Driving currents are supplied to a first group of the organic light emitting diodes electrically connected to a common line according to a first pulse width modulation (PWM) manner. Driving currents are supplied to a second group of the organic light emitting diodes electrically connected to the common line according to a second PWM manner. The second PWM manner is complementary to the first PWM manner with respect to its waveform in a period. 
     FIG. 5A  is a schematic view of waveforms provided by the first PWM manner of one preferred embodiment, and  FIG. 5B  is a schematic view of waveforms provided by the second PWM manner of one preferred embodiment.  FIG. 5A  and  FIG. 5B  use the waveforms GS 1  to GS 4  of 2-bit grayscales as an example to illustrate that the first and the second PWM manners have complementary waveforms in the period T. In  FIG. 5A , the rising edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at a starting time t 0  of the period T. In  FIG. 5B , the falling edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at an ending time t 4  of the period T. 
   More particularly, from the lowest grayscale (e.g. GS 1 ) to the highest grayscale (e.g. GS 4 ), the waveforms GS 1  to GS 4  of the first PWM manner are increased in length by measuring from the starting time t 0  of the period T, and the waveforms GS 1  to GS 4  of the second PWM manner are increased in length by measuring from the ending time t 4  of the period T. As illustrated in  FIG. 5A  and  FIG. 5B , the waveforms of the same grayscale are temporarily complementary, and therefore the peak current occurring for the organic light emitting diodes with the same grayscale is effectively decreased. 
   In other words, except for the highest grayscale (e.g. GS 4 ), the waveforms representing the same grayscale of the first PWM manner and the second PWM manner can fall at different times in the period T. Alternatively, except for the highest grayscale (e.g. GS 4 ), the waveforms representing the same grayscale of the first PWM manner and the second PWM manner can rise at different times in the period T. 
   Furthermore, the period T preferably is a refresh period of the OLED panel. In the OLED panel, the organic light emitting diodes electrically connected to one half of the segment lines are defined as the first group, and the organic light emitting diodes electrically connected to the other half of the segment lines are defined as the second group. 
   In order to further simplify the panel design, the first group can include the organic light emitting diodes located on one half portion (e.g. the left half portion) of the OLED panel, and the second group can include the organic light emitting diodes located on the other half portion (e.g. the right half portion) of the OLED panel. Alternatively, the segment lines, to which the organic light emitting diodes are electrically connected, can be configured randomly or in an interlaced fashion with respect to the group to which they belong. The interlaced configuration provides segment lines as columns of the panel, where the diodes connected to the segment lines are divided into two groups, such as on segment lines  1 ,  3 ,  5  and  7  in the first group and on segment lines  2 ,  4 ,  6  and  8  in the second group. 
   However, the waveforms of the first PWM manner are not limited to rise at the starting time of the period T, and the waveforms of the first PWM manner are to not limited to fall at the ending time of the period T. Persons skilled in the art should understand that the waveforms of the first and the second PWM manners, which represent the same grayscale, might be discrete, or might rise or fall at other times of the period T as long as the waveforms of the two manners are complementary to decrease the peak current. 
   Furthermore, more than two PWM manners can be applied to one OLED panel for driving its organic light emitting diodes. That is, the organic light emitting diodes on the OLED panel can be defined as more than two groups, and the segment lines to which the organic light emitting diodes electrically connected can be configured randomly, or in different portions of the OLED panel, or in an interlaced fashion with respect to the group to which they belong. 
   The followings provide two examples for interpreting how to apply more than two PWM manners (e.g. three PWM manners) to one OLED panel. One of the examples is illustrated in  FIGS. 6A to 6C  and the other is illustrated  FIGS. 7A to 7C , and both of them apply three PWM manners. 
   In the first example,  FIGS. 6A ,  6 B and  6 C are schematic views of waveforms provided by the first, second and third PWM manner, respectively.  FIGS. 6A ,  6 B and  6 C use the waveforms GS 1  to GS 4  of 2-bit grayscales as an example to illustrate that the first, second and third PWM manners have complementary waveforms in the period T. 
   In  FIG. 6A , the rising edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at a starting time t 0  of the period T. In  FIG. 6B , the falling edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at an ending time t 4  of the period T. In  FIG. 6C , the rising edges of the waveforms GS 1  to GS 3  corresponding to different grayscales are positioned at time t 1  of the period T, and the falling edges of the waveforms GS 1  to GS 4  are positioned at times t 2 , t 3 , t 4  and t 4 , respectively. 
   In the second example,  FIGS. 7A ,  7 B and  7 C are schematic views of waveforms provided by the first, second and third PWM manner, respectively.  FIGS. 7A ,  7 B and  7 C use the waveforms GS 1  to GS 4  of 2-bit grayscales as an example to illustrate that the first, second and third PWM manners have complementary waveforms in the period T. 
   In  FIG. 7A , the rising edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at a starting time t 0  of the period T. In  FIG. 6B , the falling edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at an ending time t 4  of the period T. In  FIG. 6C , the rising edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are respectively positioned at time t 3 , t 0 , t 1  and t 0  of the period T, and the falling edges of the waveforms GS 1  to GS 4  are positioned at times t 4 , t 2 , t 4  and t 4 . 
   In other words, except for the highest grayscale (e.g. GS 4 ), the waveforms representing the same grayscale of these PWM manners can be designed to rise or fall at different times in the period T for achieving complementarity. As illustrated in  FIGS. 6A ,  6 B and  6 C and  FIGS. 7A ,  7 B and  7 C, the waveforms of the same grayscale of each example are temporarily complementary, and therefore the peak current occurring for the organic light emitting diodes with the same grayscale is effectively decreased. 
   For instance, in the OLED panel, the organic light emitting diodes electrically connected to one third of the segment lines are defined as the first group, the organic light emitting diodes electrically connected to another one third of the segment lines are defined as the second group, and the organic light emitting diodes electrically connected to the rest of the segment lines are defined as the third group. 
   In order to further simplify the panel design, the first group can include the organic light emitting diodes located on one-third portion (e.g. the left portion) of the OLED panel, the second group can include the organic light emitting diodes located on another one-third portion (e.g. the middle portion) of the OLED panel, and the third group can include the organic light emitting diodes located on the rest portion (e.g. the right portion) of the OLED panel. 
   Alternatively, the segment lines, to which the organic light emitting diodes are electrically connected, can be configured randomly or in an interlaced fashion with respect to the group to which they belong. The interlaced configuration provides segment lines as columns of the panel, where the diodes connected to the segment lines are divided into third groups, such as on segment lines  1 ,  4 ,  7  and  10  in the first group, on segment lines  2 ,  5 ,  8  and  11  in the second group and segment lines  3 ,  6 ,  9  and  12  in the third group. 
     FIG. 8  is a schematic view of an organic light emitting display of one preferred embodiment of the present invention. An OLED  800  comprises a plurality of segment lines  822 , a plurality of common lines  832 , a plurality of organic light emitting diodes  812  and a segment driver  820 . The organic light emitting diodes  812  are positioned on an OLED panel  810  and are electrically connected to the segment lines  822  and the common lines  832  in a matrix structure. 
   The organic light emitting diodes  812  of one common line  832  are divided into a first group  842  and a second group  844 . The segment driver  820  is electrically connected to the segment lines  822  and supplies driving currents to the organic light emitting diodes  812  of the first group  842  and the second group  844  separately according to a first pulse width modulation (PWM) manner and a second PWM manner. The first PWM manner and the second PWM manner have complementary waveforms in a period. 
   Referring to one preferred embodiment of the present invention as illustrated in  FIG. 5A  and  FIG. 5B , in the first PWM manner, the rising edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at a starting time t 0  of the period T. In the second PWM manner, the falling edges of the waveforms GS 1  to GS 4  corresponding to different grayscales are all positioned at an ending time t 4  of the period T. 
   More particularly, from the lowest grayscale (e.g. GS 1 ) to the highest grayscale (e.g. GS 4 ), the waveforms GS 1  to GS 4  of the first PWM manner are increased in length by measuring from the starting time t 0  of the period T, and the waveforms GS 1  to GS 4  of the second PWM manner are increased in length by measuring from the ending time t 4  of the period T. That is, the waveforms of the same grayscale are temporarily complementary, and therefore the peak current occurring for the organic light emitting diodes  812  with the same grayscale is effectively decreased. 
   In other words, except for the highest grayscale (e.g. GS 4 ), the waveforms representing the same grayscale of the first PWM manner and the second PWM manner can fall at different times in the period T. Alternatively, except for the highest grayscale (e.g. GS 4 ), the waveforms representing the same grayscale of the first PWM manner and the second PWM manner can rise at different times in the period T. 
   Furthermore, the period T preferably is a refresh period of the OLED panel  810 . In the OLED panel  810 , the organic light emitting diodes  812  electrically connected to one half of the segment lines  822  are defined as the first group  842 , and the organic light emitting diodes  812  electrically connected to the other half of the segment lines  822  are defined as the second group  844 . 
   In order to further simplify the panel design, the first group  842  can include the organic light emitting diodes  812  located on one half portion (e.g. the left half portion) of the OLED panel  810 , and the second group  844  can include the organic light emitting diodes  812  located on the other half portion (e.g. the right half portion) of the OLED panel  810 . Alternatively, the segment lines  822  can be configured randomly or in an interlaced fashion with respect to the group to which they belong. 
   In conclusion, the preferred embodiments can effectively decrease the peak current usually occurring in the conventional PWM manner of the OLED panel and further decrease the Vcc of the segment driver, so as to lower the power consumption of the OLED panel. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.