Abstract:
An organic light emitting display and a driving method thereof, capable of increasing manufacturing yield by widening a range of gamma correction. An exemplary organic light emitting display includes a display region with pixels at crossings of data and scan lines; a gamma correction unit; a gamma correction circuit; a data driver; and a scan driver. The gamma correction unit includes first and second register units for storing first and second gamma correction signals, the second gamma correction signal comprising a corrected first gamma correction signal; a booster unit for correcting the second gamma correction signal to generate a third gamma correction signal; and a multiplexer for selecting the second gamma correction signal or the third gamma correction signal outputted from the booster unit, and for transferring it to the gamma correction circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0043174, filed on May 9, 2008, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an organic light emitting display and a method of driving the same, and more particularly to an organic light emitting display with improved image quality and a method of driving the same. 
         [0004]    2. Description of Related Art 
         [0005]    Recently, various flat panel display devices having a reduced weight and volume, which are unfavorable aspects of a cathode ray tube, have been developed. Examples of flat panel display devices include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), organic light emitting displays, and others. 
         [0006]    Among these flat panel display devices, an organic light emitting display displays an image using organic light emitting diodes (OLEDs) that generate light utilizing the recombination of electrons and holes. 
         [0007]    An OLED includes an anode electrode, a cathode electrode, and a light emitting layer positioned therebetween. In the OLED, the light emitting layer emits light when current flows in a direction from the anode electrode to the cathode electrode. Such an organic light emitting display displays the image using characteristics of the OLED. The organic light emitting display includes a plurality of pixels having a plurality of thin film transistors and one or more OLEDs, wherein the amount of current flowing to the OLEDs is controlled by the thin film transistors. 
         [0008]    However, variations in manufacturing processes for forming organic light emitting displays may result in varying light emitting layer thicknesses, light emitting layer efficiencies, and characteristics of the thin film transistors. Accordingly, variations in luminance and color across an organic light emitting display may occur. Products with large variations in luminance and color are typically regarded as defective goods and are discarded, thereby causing a reduction in yield. 
       SUMMARY OF THE INVENTION 
       [0009]    Various exemplary embodiments of the present invention include an organic light emitting display and a driving method thereof capable of increasing a yield by widening a range of gamma correction. 
         [0010]    An organic light emitting display according to an exemplary embodiment of the present invention includes a display region with pixels at crossings of data lines and scan lines; a gamma correction unit for generating gamma a correction signal; a gamma correction circuit for controlling a gray level voltage corresponding to the gamma correction signal; a data driver for generating data signals corresponding to image signals and to the gray level voltage, and for transferring the data signals to the data lines; and a scan driver for generating scan signals and for transferring the scan signals to the scan lines. The gamma correction unit includes a first register unit for storing a first gamma correction signal; a second register unit for storing a second gamma correction signal, the second gamma correction signal comprising a corrected first gamma correction signal; a booster unit for receiving and correcting the second gamma correction signal stored in the second register unit to generate a third gamma correction signal; and a multiplexer for selecting one signal from the group consisting of the second gamma correction signal stored in the second register unit and the third gamma correction signal outputted from the booster unit, and for transferring the selected signal to the gamma correction circuit. 
         [0011]    A method for driving an organic light emitting display according to another exemplary embodiment of the present invention includes adding a first correction value to a first gamma correction signal to correct the first gamma correction signal and to generate a second gamma correction signal; storing the second gamma correction signal; generating a third gamma correction signal by correcting the second gamma correction signal utilizing a second correction value; and outputting a signal selected from the group consisting of the second gamma correction signal and the third gamma correction signal. 
         [0012]    The organic light emitting display and a driving method thereof according to various embodiments of the present invention are capable of improving image quality by widening a correction range of a gamma. Also, it is capable of lowering manufacturing costs by increasing production yield. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
           [0014]      FIG. 1  is a block diagram of an organic light emitting display according to an exemplary embodiment of the present invention. 
           [0015]      FIG. 2  is a block diagram of a gamma correction unit according to an exemplary embodiment of the present invention. 
           [0016]      FIG. 3  is a block diagram of a gamma correction circuit shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0017]    Hereinafter, certain exemplary embodiments according to the present invention are described with reference to the accompanying drawings. Here, when a first element is described as being coupled to a second element, the first element may be directly coupled to the second element, or may be indirectly coupled to the second element via a third element. Further, some of the elements that are not essential to a complete understanding of the invention are omitted for clarity. Also, like reference numerals refer to like elements throughout. 
         [0018]      FIG. 1  is a block diagram of an organic light emitting display according to an exemplary embodiment of the present invention. With reference to  FIG. 1 , the organic light emitting display according to this embodiment includes a display region  100 , a gamma correction unit  200 , a gamma correction circuit  300 , a data driver  400 , and a scan driver  500 . 
         [0019]    The display region  100  includes a plurality of pixels  101  that each include an OLED (not shown) for emitting light corresponding to the flow of current in each pixel. N scan lines (S 1 , S 2 , . . . Sn−1, and Sn) extend in a row direction for transferring scan signals, and m data lines (D 1 , D 2 , . . . Dm−1, and Dm) extend in a column direction for transferring data signals. 
         [0020]    The display region  100  is driven by receiving first power ELVDD and second power ELVSS from the outside. Accordingly, the display region  100  displays images using the scan signals and the data signals, and generates light by driving a current generated by the first power ELVDD and the second power ELVSS to the organic light emitting diode. 
         [0021]    The gamma correction unit  200  generates gamma correction signals according to gamma characteristics of the organic light emitting display and, through the gamma correction circuit  300 , applies the gamma correction signals to the data signals. The data signal applied with the gamma correction signal has a ratio of gray levels to luminance with a curve of a logarithmic function or an exponential function. 
         [0022]    Since the data signal applied with the gamma correction signal displays an image signal according to the gamma characteristics of the organic light emitting display, it should better correspond to the actually desired color and luminance. 
         [0023]    The gamma correction signal is applied with a correction value according to luminance and color displayed in the display region  100 . In other words, luminance and color displayed in the display region  100  substantially correspond to the actually desired luminance and color by correcting the gamma correction signal. 
         [0024]    The gamma correction unit  200  stores the gamma correction signal applied with the correction value and transfers the stored gamma correction signal to the gamma correction circuit  300 , thereby setting a voltage corresponding to each gray level in the gamma correction circuit  300 . The data driver  400  transforms the image signal (RGB data) into the data signal by using the voltages set by the gamma correction circuit  300 . A detailed description of the gamma correction circuit  300  will be described below with reference to  FIG. 3 . 
         [0025]    Even when the data signal generated by the data driver  400  is corrected by the gamma correction signal, the displayed image may not have the actually desired luminance and color. More specifically, when manufacturing the organic light emitting display, although the same gamma correction signal is applied, the image may appear different from the actually desired luminance and color due to process variations, etc. That is, in the organic light emitting display, because variations may occur in a thickness of the light emitting layer, an efficiency of the light emitting layer, operational characteristics of the thin film transistor, etc., a sufficient correction may not be made by utilizing the same correction value on all displays. Thus, during production, a correction value may be applied to the corrected gamma correction signal once more to enable implementation of a wide correction range. 
         [0026]    The data driver  400  generates data signals using the image signal (RGB data) and the gamma correction signal from the gamma correction circuit  300 . The data driver  400  is coupled to the data lines D 1 , D 2 , . . . Dm−1, and Dm of the display region  100  to apply the generated data signals to the display region  100 . 
         [0027]    The scan driver  500 , which generates scan signals, is coupled to the scan lines S 1 , S 2 , . . . Sn−1, and Sn to transfer the scan signals to respective rows of the display region  100 . The data signal output from the data driver  400  is transferred to the pixel  101  receiving the scan signal so that a driving current is generated, wherein the generated driving current flows to the OLED. 
         [0028]      FIG. 2  is a block diagram of a gamma correction unit  200 ′ according to an exemplary embodiment of the present invention. In certain embodiments, the gamma correction unit  200 ′ may be utilized in the place of the gamma correction unit  200  of  FIG. 1 . Referring to  FIG. 2 , the gamma correction unit  200 ′ includes a first register unit  210 , an offset unit  220 , a first adder  230 , a correction unit  240 , a second register unit  250 , a booster unit  260 , first to fourth storing units  221 ,  222 ,  241 , and  261 , a first multiplexer  223 , and a second multiplexer  270 . The output of the second multiplexer is sent to the gamma correction circuit  300 ′. In some embodiments, the gamma correction circuit  300 ′ may be utilized as the gamma correction circuit  300  of  FIG. 1 . 
         [0029]    The first register unit  210  stores a first gamma correction signal for generating light having a brightness corresponding to each gray level according to the characteristics of the OLEDs and the organic light emitting display. In the case of a display with 64 gray levels, the first gamma correction signal utilizes a voltage of an upper level and voltage of a lower level to generate 64 voltages V 0 , V 1  . . . V 62 , and V 63 . 
         [0030]    The offset unit  220  changes the first gamma correction signal in the first register unit  210 . The offset unit  220  includes the first storing unit  221  and the second storing unit  222 , wherein each of the first storing unit  221  and the second storing unit  222  stores offset values. That is, a first offset value stored in the first storing unit  221  is utilized to correct the first gamma correction signal stored in the first register unit  210 , or a second offset value stored in the second storing unit  222  is utilized to correct the first gamma correction signal stored in the first register unit  210 , as selected by the first multiplexer  223 . 
         [0031]    Therefore, in an exemplary embodiment utilizing 64 gray levels, the 64 voltages generated according to the first offset value stored in the first storing unit  221  have different values than the 64 voltages generated according to the second offset value in the second storing unit  222 . The offset unit  220  further includes the first multiplexer  223 , and outputs one of the offset values stored in the first storing unit  221  and the second storing unit  222  by the operation of the first multiplexer  223 . The offset value is utilized to control an offset of the highest level voltage and the lowest level voltage. Therefore, a difference in a slope occurs between a second gamma correction signal, to which the offset value was applied, and the first gamma correction signal. 
         [0032]    The first adder  230  adds the first gamma correction signal stored in the first register unit  210  to the offset value output from the offset unit  220  to correct the first gamma correction signal, thereby generating a second gamma correction signal. That is, the first gamma correction signal is corrected by the offset value by utilizing the first adder  230  to generate the second gamma correction signal. 
         [0033]    The correction unit  240  performs a role of further correcting the second gamma correction signal corrected by the offset unit  220  to allow the second gamma correction signal to be more accurately corrected. The correction unit  240  includes the third storing unit  241  for storing a correction value and a second adder  242  for adding the second gamma correction signal output from the first adder  230  and the correction value stored in the third storing unit  241 . The correction value is set during production by a designer after assessing the color sense and brightness of an image displayed in the display region. The correction unit  240  adds the second gamma correction signal to the correction value utilizing the second adder  242  to generate a third gamma correction signal. 
         [0034]    The second register unit  250  stores the third gamma correction signal generated in the correction unit  240 . The third gamma correction signal stored in the second register unit  250  is transferred to the booster unit  260  and the second multiplexer  270 . 
         [0035]    The booster unit  260  receives the third gamma correction signal from the second register unit  250 . The booster unit  260  includes the fourth storing unit  261  for storing a second correction value. In some embodiments, the second correction value stored in the fourth storing unit  261  is the same correction value as the correction value set in the correction unit  240 . The booster unit  260  further includes a third adder  262  for adding the third gamma correction signal to the correction value stored in the fourth storing unit  261 . Therefore, the booster unit  260  corrects the third gamma correction signal using the correction value stored in the fourth storing unit  261  to generate a fourth gamma correction signal. That is, the third gamma correction signal stored in the second register unit  250  is corrected once more, so that a correction range of the image is further widened. 
         [0036]    The second multiplexer  270  selects one of either the third gamma correction signal stored in the second register unit  250  or the fourth gamma correction signal from the booster unit  260  to transfer to the gamma correction circuit  300 ′. That is, in the case where a correction result from the correction unit  240  is sufficient, the second multiplexer  270  outputs the third gamma correction signal stored in the second register unit  250 . In the case where the correction result corrected in the correction unit  240  is not sufficient, the second multiplexer  270  outputs the fourth gamma correction signal output from the booster unit  260 . In one embodiment, the decision to transfer the third gamma correction signal or the fourth gamma correction signal to the gamma correction circuit  300 ′ is also made by the designer during production. 
         [0037]      FIG. 3  is a block diagram of a gamma correction circuit  300 ″ according to an exemplary embodiment of the present invention in which 64 gray level voltages are utilized. In some embodiments, the gamma correction circuit  300 ″ may be utilized as the gamma correction circuit  300 ′ shown in  FIG. 2  or as the gamma correction circuit  300  shown in  FIG. 1 . Referring to  FIG. 3 , the gamma correction circuit  300 ″ includes a ladder resistor  61 , an amplitude control register  62 , a curve control register  63 , first through sixth selectors  64  through  69 , and a gray level voltage amplifier  70 . 
         [0038]    The ladder resistor  61  defines the highest level voltage VHI supplied from the outside as a reference voltage and includes a plurality of variable resistors coupled in series between the lowest level voltage VLO and the reference voltage VHI. A plurality of gray level voltages are generated through the ladder resistor  61 . Also, when making a value of the ladder resistor small, an amplitude control range becomes narrow, whereas an amplitude control accuracy increases. On the other hand, when making the value of the ladder resistor large, an amplitude control range becomes wide, whereas an amplitude control accuracy decreases. 
         [0039]    In the described embodiment, the gamma correction signal from the gamma correction unit  200  includes 26 bits, wherein the upper 10 bits are input to the amplitude control register  62  and the lower 16 bits thereof are input to the curve control register  63  and selected as register setting values. 
         [0040]    The amplitude control register  62  receives the signal of the upper 10 bits, outputs 3 bits of the upper 10 bits of the gamma correction signal to the first selector  64 , and outputs the remaining 7 bits of the upper 10 bits of the gamma correction signal to the second selector  65 . At this time, it is possible to increase the number of selectable gray levels by increasing the number of setting bits. 
         [0041]    The curve control register  63  receives the lower 16 bits of the gamma correction signal, and outputs 4 bits of the lower 16 bits of the gamma correction signal to each of the respective third through sixth selectors  66  to  69 . 
         [0042]    The first selector  64  selects a gray level voltage V 0  from among the plurality of gray level voltages distributed through the ladder resistor  61 , the gray level voltage V 0  corresponding to a value of 3 bits of the gamma correction signal stored in the amplitude control register  62 , to output as the highest gray level voltage. 
         [0043]    The second selector  65  selects a gray level voltage V 63  from among the plurality of gray level voltages distributed through the ladder resistor  61 , the gray level voltage V 63  corresponding to a value of 7 bits of the gamma correction signal stored in the amplitude control register  62 , to output as the lowest gray level voltage. 
         [0044]    The third selector  66  distributes the gray level voltage V 0  output from the first selector  64  and the gray level voltage V 63  output from the second selector  65  into a plurality of gray level voltages through a plurality of resistor columns, and selects and outputs a gray level voltage V 31  corresponding to a value of 4 bits of the gamma correction signal. 
         [0045]    The fourth selector  67  selects and outputs a gray level voltage V 15  corresponding to a value of 4 bits of the gamma correction signal from among the gray level voltages between the output of the first selector  64  and the output of the third selector  66 . 
         [0046]    The fifth selector  68  selects and outputs a gray level voltage V 7  corresponding to a value of 4 bits of the gamma correction signal from among the gray level voltages between the output of the first selector  64  and the output of the fourth selector  67 . 
         [0047]    The sixth selector  69  selects and outputs a gray level voltage V 3  corresponding to a value of 4 bits of the gamma correction signal from among a plurality of gray level voltages between the output of the first selector  64  and the output of the fifth selector  68 . 
         [0048]    With operation as described above, it is possible to control gamma characteristics to suit characteristics of each light emitting display by enabling curve control of an intermediate gray level unit according to a register setting value of the curve control register  63 . Also, when it is intended to make the gamma curve characteristics concave, as a lower gray level is displayed, a potential difference between each gray level is set to be larger. On the other hand, when it is intended to make gamma curve characteristics convex, as a lower gray level is displayed, potential difference between each gray level is set to be smaller. 
         [0049]    The gray level voltage amplifier  70  outputs a plurality of gray level voltages V 0 ′, V 1 ′, . . . V 63 ′, corresponding to each of the gray levels to be displayed in the display region  100 . 
         [0050]    In an exemplary embodiment of the present invention, the above-mentioned operation is performed by installing a gamma correction circuit as described above for each of R, G, and B groups so that R, G, and B light emitting devices obtain substantially the same brightness, in consideration of variations in the characteristics of each of the R, G, and B light emitting devices themselves. Thereby, it is possible to differently set the amplitude and curve through the curve control register  63  and the amplitude control register  62  by R, G, and B. 
         [0051]    While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.