Abstract:
This invention provides a drive system of a display device preventing an uneven display caused by output current values of current conversion circuits. A drive system of a display device of the invention has a plurality of pixels disposed in a matrix of m rows and n columns and having current drive elements, n pieces of current conversion circuits converting digital display signals inputted from outside into analog signals corresponding to the digital display signals, a first selector circuit selectively supplying the digital display signals to the n pieces of the current conversion circuits, and a second selector circuit selectively supplying current outputs of n pieces of the current conversion circuits to pixel groups divided in columns.

Description:
CROSS-REFERENCE OF THE INVENTION 
   This invention is based on Japanese Patent Application No. 2003-399941, the content of which is incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a drive system of a display device, particularly to such a system having a drive circuit using a current programming method. 
   2. Description of the Related Art 
   In recent years, organic electroluminescent (hereafter, referred to as EL) display device using organic EL elements have been receiving attention as a display device substituting for a CRT or an LCD. Particularly, an active matrix type organic EL display device having thin film transistors as switching elements for driving the organic EL elements has been developed. Different from LCDs, such organic EL elements are self-light-emitting elements providing luminance corresponding to a current flowing in the EL elements. 
   There are various types of drive systems for such an organic EL display device, and one of these is a current programming method. In this method, for obtaining luminance corresponding to a digital display signal, by utilizing such current and luminance correspondence characteristics of the organic EL element described above, a current value corresponding to the digital display signal is set by a current conversion circuit (also called a current DAC) and the current is supplied from the current conversion circuit to each of the pixels. 
   Particularly, in a high-precision organic EL display device, a plurality of the current conversion circuits are provided for each of pixel groups divided in columns in order to secure time for programming the current to the pixel. Such a drive system is called a multi-channel current DAC method since a channel is provided in each of the pixel groups divided in columns. 
     FIG. 4  is a block diagram showing a drive system of an organic EL display device of a conventional art. A plurality of pixels P 11 , P 12  . . . each having an organic EL element is disposed in a matrix of m rows and n columns. The n pieces of current conversion circuits DAC 1  to DACn are disposed for the pixel groups divided in columns, respectively. These current conversion circuits DAC 1  to DACn convert digital display signals D 1  to Dn inputted therein into currents I 1  to In having current values corresponding to the signals D 1  to Dn, respectively, and supplies the currents I 1  to In to the pixel groups divided in columns, respectively. 
   For example, during the first horizontal scanning period, the currents I 1 , I 2 , . . . and In are supplied to the pixels P 11 , P 12 , . . . and P 1   n , in this order. Then, during the next horizontal scanning period, the currents I 1 , I 2 , . . . and In are supplied to the pixels P 21 , P 22 , . . . and P 2   n , in this order, respectively. Such a horizontal scanning is repeated to the whole remaining lines, thereby completing one field scanning period. 
     FIG. 5  is a table showing a correspondence relationship between the pixel groups divided in columns and the current conversion circuits DAC 1  to DACn for driving these pixel groups in this drive system of the organic EL display device. As seen in  FIG. 5 , the pixels in each of the pixel groups divided in columns are driven by the same current conversion circuit. For example, in an n-th field, the pixels of the pixel group in the first column are driven by the current conversion circuit DAC 1  indicated by “1” in  FIG. 5 , and the pixels of the pixel group in the second column are driven by the current conversion circuit DAC 2  indicated by “2” in  FIG. 5 . The correspondence relationship is the same in an n+1 field and an n+2 field. The relating technology is disclosed in the Japanese Patent Application Publication No. 2003-150118. 
   Generally, n pieces of the current conversion circuits DAC 1  to DACn are formed of LSIs, and there occurs variation in output current values of n pieces of the current conversion circuits DAC 1  to DACn due to manufacture variations. This variation in the output current directly causes variations in luminance of the organic EL elements as current drive elements. 
   In the drive system of the display device of the conventional art shown in  FIG. 4 , the pixels of the pixel group in each of the columns are driven by the same current conversion circuit all the time. Therefore, when the value of the output current of the current conversion circuit provided for a certain column is unusually too high or too low compared with others, an uneven display with bright and dark parts appears in the line corresponding to the pixel group in that column. 
   Generally, human eyes can not recognize such an uneven display if variation of luminance is 1% or less, but it is difficult to keep the variation at 1% or less by current LSI manufacturing technologies. 
   SUMMARY OF THE INVENTION 
   The invention provides a drive system of a display device that includes a plurality of pixels provided in a matrix form comprising rows and columns. The pixels have corresponding current drive elements. The system also includes a plurality of current conversion circuits converting digital display signals that the drive system receives into analog currents corresponding to the digital display signals. The number of the current conversion circuits is equal to the number of the columns. The system further includes a first selector circuit supplying the digital display signals to the respective current conversion circuits, and a second selector circuit receiving outputs of the current conversion circuits and supplying the outputted to the respective pixels. 
   The invention also provides a drive system of a display device that includes a plurality of pixels provided in a matrix form comprising rows and columns. The pixels have corresponding current drive elements. The system also includes a current conversion circuit converting a digital display signal that the drive system receives into an analog current corresponding to the digital display signal. This current conversion circuit is provided for each of the columns. The system further includes a first selector circuit receiving the digital display signals that are directed to corresponding columns and routing the received digital display signals to current conversion circuits corresponding to columns that are not the destinations of the digital display signals, and a second selector circuit receiving the analog currents of the current conversion circuits and rerouting the analog currents to the columns that are the destinations of the digital display signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a drive system of an organic EL display device of an embodiment of the invention. 
       FIG. 2  is a table showing one example of a correspondence relationship between pixel groups divided in columns and current conversion circuits DAC  1  to DACn for driving the pixel groups in the drive system of the organic EL display device of  FIG. 1 . 
       FIG. 3  is a diagram showing one example of a changed state of first and second selector circuits in the drive system of the organic EL display device of the embodiment of the invention. 
       FIG. 4  is a block diagram showing a drive system of an organic EL display device of a conventional art. 
       FIG. 5  is a table showing a correspondence relationship between pixel groups divided in columns and current conversion circuits DAC 1  to DACn for driving the pixel groups in the drive system of the organic EL display device of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the invention will be described with reference to  FIGS. 1-3 .  FIG. 1  is a block diagram showing a drive system of an organic EL display device of this embodiment. 
   A plurality of pixels P 11 , P 12 , . . . each having an organic EL element is disposed in a matrix of m rows and n columns. The n pieces of current conversion circuits DAC 1  to DACn are provided. These current conversion circuits DAC 1  to DACn convert digital display signals D 1  to Dn inputted through a first selector circuit  10  into currents I 1  to In having current values corresponding to the digital signals D 1  to Dn, respectively. The first selector circuit  10  is controlled by a horizontal scanning clock CKH, a vertical scanning clock CKV and an input/output pattern selection signal SEL to select which one among the current conversion circuits DAC 1  to DACn is to be inputted with each of the digital display signals D 1  to Dn in each of horizontal scanning periods or field periods. 
   Each of the currents I 1  to In outputted from the current conversion circuits DAC 1  to DACn is supplied to each of pixel groups divided in columns, which is selected through a second selector circuit  20 . Among the pixel groups divided in columns, the pixel group in the first column is the pixel group (P 11 , P 21 , P 31  . . . , Pm 1 ), the pixel group in the second column is the pixel group (P 12 , P 22 , P 32  . . . , Pm 2 ), and the pixel group in the n-th column is the pixel group (P 1   n , P 2   n , P 3   n  . . . , Pmn). The second selector circuit  20  is controlled by the horizontal scanning clock CKH, the vertical scanning clock CKV and the input/output pattern selection signal SEL to select which one among the pixel groups is to be supplied with each of the currents I 1  to In outputted from the current conversion circuits DAC 1  to DACn in each of horizontal scanning periods or field periods. 
   To specifically describe a changing operation when inputting the signals to and outputting the currents from the current conversion circuits DAC 1  to DACn, it is preferable that the first and second selector circuits  10  and  20  use alternatively the current conversion circuits DAC 1  to DACn to be inputted with the digital display signals D 1  to Dn so as to change the pixel groups divided in columns to be supplied with the currents outputted from the current conversion circuits DAC 1  to DACn, respectively, in each of the horizontal scanning periods, so as to avoid keeping the currents I 1  to In being supplied to the same pixel group all the time during the one field period. Furthermore, it is preferable that the first and second selector circuits  10  and  20  use alternatively the current conversion circuits DAC 1  to DACn to be inputted with the digital display signals D 1  to Dn so as to change the pixel groups divided in columns to be supplied with the currents outputted from the current conversion circuits DAC 1  to DACn in a manner different between two filed period, as shown in  FIG. 2 . 
     FIG. 2  is a diagram showing an example of a correspondence relationship between the pixel groups divided in columns and the current conversion circuits DAC 1  to DACn for driving these pixel groups in the drive system of the organic EL display device.  FIG. 2  shows pixels disposed in m rows and n columns, and the numbers in the matrix correspond to the current conversion circuits (DAC 1 -DACn), which supply currents to the corresponding pixels. For example, the pixel P 11  in the first row and column is supplied with a current from the current conversion circuit DAC 1 , and the pixel P 12  in the first row and the second column is supplied with a current from the current conversion circuit DAC 2 . 
   In this example, the relationship between the pixels and the current conversion circuits DAC 1  to DACn is shifted by 2 channels in each of the horizontal scanning periods. For example, in the n-th field (n), in the line scanning of the first row, the current conversion circuits DAC 1  to DACn are applied in order of 1, 2, 3, 4, . . . n. 
   In the line scanning of the second row, the application of the current conversion circuits DAC 1  to DACn to the pixels is shifted by 2 channels. That is, the current conversion circuit DAC 1  supplies a current to the pixel P 23  in the second row and the third column instead of the pixel P 21  in the second row and the first column. Similarly, the current conversion circuit DAC 2  supplies a current to the pixel P 24  in the second row and the fourth column.  FIG. 3  is a diagram showing a changed state by the first and second selector circuits  10  and  20  in the line scanning of the second row. The current conversion circuit DAC 1  is inputted with a digital display signal D 3 , converts this in a current, and supplies the current to the pixel P 23  of the second row and the third column. The current conversion circuit DAC 2  is inputted with a digital display signal D 4 , converts this into a current, and supplies the current to the pixel P 24  of the second row and the fourth column. 
   As a result, the current corresponding to the digital display signal D 1  is supplied to the pixel group of the first column, the current corresponding to the digital display signal D 2  is supplied to the pixel group of the second column, and the current corresponding to the digital display signal D 3  is supplied to the pixel group of the third column, and so on, as is the case with the conventional device. However, the current conversion circuits for converting the digital display signal into a current are alternated among the horizontal scannings of one field period as well as among individual field periods. 
   In the third line, the application of the current conversion circuits DAC 1  to DACn to the pixels is shifted by 2 more channels. Like this manner, the application of the current conversion circuits DAC 1  to DACn to the pixels is rotated by 2 channels in each of the horizontal scanning periods, but this rotation can stop on the midway to return to the same relationship of the application as in the first row. In this example, in the line scanning of the fifth row, the relationship of the application is returned to the same relationship as in the first row. It is noted that returning to the same relationship as in the first row is made in the fifth row for simplifying the description in this embodiment, but the rotation can be continued without resorting back to the original alignment. 
   Then, the scanning of the field (n) is completed, and in the next n+1 th field, the line scanning of the first row is started from the alignment where the relationship of the current conversion circuits DAC 1  to DACn and the pixels is shifted by 4 channels. That is, in the line of the first row, the current conversion circuit DAC 1  supplies a current to the pixel P 15  of the first row and the fifth column. Similarly, the current conversion circuit DAC 2  supplies a current to the pixel P 16  of the first row and the sixth column. Then, in the line scanning of the second row, the application of the current conversion circuits DAC 1  to DACn to the pixels is shifted by 2 channels, like the manner in the previous field (n). For example, the current conversion circuit DAC 1  supplies a current to the pixel P 27  of the second row and the seventh column. 
   Accordingly, by changing correspondence relationships at the first and second selector circuits  10  and  20  in each of the horizontal scanning periods, the effect of variation in output current characteristics of the current conversion circuits DAC 1  to DACn is dispersed between the pixel groups in each of the columns, thereby reducing a linear-shaped uneven display appearing in columns. Furthermore, since correspondence relationships are changed at the first and second selector circuits  10  and  20  in each of the field scanning periods, respectively, a pattern still remaining even by changing in each of the horizontal scanning periods is normalized so that an uneven display is hardly recognized. 
   Furthermore, the variation in the output current characteristics of the current conversion circuits DAC 1  to DACn occurs randomly, so that it is preferable that changing an input pattern and an output pattern of the first and second selector circuits  10  and  20  is set arbitrarily according to the input/output pattern selection signal SEL. This can reduce the uneven display in the display devices and provide an optimal display.