Patent Publication Number: US-9418593-B2

Title: Organic light emitting display device and driving method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of Korean Patent Application No. 10-2013-0070206, filed on Jun. 19, 2013, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     1. Field 
     Exemplary embodiments of the present invention relate to an organic light emitting display device and a driving method thereof. 
     2. Discussion of the Background 
     Flat panel display devices currently in use include a liquid crystal display, a field emission display, a plasma display panel, an organic light emitting display, and the like. 
     Among these flat panel displays, the organic light emitting display displays images using organic light emitting diodes that emit light through recombination of electrons and holes. The organic light emitting display has high speed and a fast response time, and is driven with low power consumption. However, the amount of current flowing through the organic light emitting diode depends on a variation in threshold voltage of the driving transistor included in each pixel, which may result in undesirable variations in display quality. That is, the characteristic of the driving transistor may be changed depending on manufacturing process variables of the driving transistor included in each pixel. The manufacturing of the organic light emitting display so that all the transistors have the same characteristics is practically impossible in the current displays. Accordingly, variations in threshold voltages of the driving transistor may occur. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     Exemplary embodiments of the present invention provide an organic light emitting display and a driving method thereof, which can display an image with a desired luminance, regardless of the threshold voltage of a driving transistor. 
     Exemplary embodiments of the present invention also provide an organic light emitting display and a driving method thereof, which can extract threshold voltage information of a driving transistor, which reflect characteristics of an organic light emitting diode. 
     Additional features of the invention will be set forth in the description which follows, and in part will become apparent from the description, or may be learned from practice of the invention. 
     An exemplary embodiment of the present invention discloses an organic light emitting display, including: pixels positioned in an area defined by data lines, scan lines, and control lines, the pixels each including an organic light emitting diode; a scan driver configured to drive the scan lines; a data driver configured to drive the data lines; a control line driver configured to drive the control lines; and a compensation unit configured to extract threshold voltage information of a driving transistor included in each pixel during a sensing period. The compensation unit supplies a preset voltage to a gate electrode of the driving transistor so that a first current flows during the sensing period, and supplies a reference voltage to a drain electrode of the driving transistor during the period in which the first current flows. 
     An exemplary embodiment of the present invention also discloses a method of driving an organic light emitting display, the method including: applying a reference voltage to a drain electrode of a driving transistor so that a first current flows through an organic light emitting diode coupled to the drain electrode of the driving transistor; applying a preset voltage to a gate electrode of the driving transistor; and applying the reference voltage to the drain electrode of the driving transistor while measuring a second current flowing through the driving transistor and corresponding to the preset voltage. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a diagram illustrating an organic light emitting display according to an exemplary embodiment of the present invention. 
         FIG. 2  is a circuit diagram illustrating a pixel according to an exemplary embodiment of the present invention. 
         FIG. 3  is an exemplary embodiment of the present invention illustrating a channel of a compensation unit. 
         FIG. 4A ,  FIG. 4B , and  FIG. 4C  are diagrams illustrating driving waveforms supplied during a sensing period according to an exemplary embodiment of the present invention. 
         FIG. 5  is a graph illustrating an operation point corresponding to characteristics of a driving transistor and an organic light emitting diode, according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of elements may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ, ZZ) 
       FIG. 1  is a diagram illustrating an organic light emitting display according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , the organic light emitting display includes a pixel unit  130  including pixels  140  positioned at intersection points of scan lines S 1  to Sn and data lines D 1  to Dm, a scan driver  110  configured to drive the scan lines S 1  to Sn and emission control lines E 1  to En, and a control line driver  160  configured to drive control lines CL 1  to CLn. 
     The organic light emitting display further includes a data driver  120  configured to supply a data signal to the data lines D 1  to Dm, a compensation unit  170  configured to extract, from each pixel  140 , degradation information of an organic light emitting diode and/or threshold voltage information of a driving transistor, and a timing controller  150  configured to control the drivers  110 ,  120  and  160  and the compensation unit  170 . 
     The pixel unit  130  includes the pixels  140  positioned in an area defined by the scan lines S 1  to Sn, the data lines D 1  to Dm, and the control lines CL 1  to CLn. The pixels  140  receive power from first and second power sources ELVDD and ELVSS supplied from outside the organic light emitting display. Each pixel  140  controls the amount of current supplied from the first power source ELVDD to the second power source ELVSS via an organic light emitting diode (to be described later), the amount of current corresponding to a data signal. 
     The scan driver  110  supplies a scan signal to the scan lines S 1  to Sn, and supplies an emission control signal to the emission control lines E 1  to En, under the control of the timing controller  150 . For example, the scan driver  110  progressively supplies a scan signal to the scan lines S 1  to Sn, and progressively supplies an emission control signal to the emission control lines E 1  to En, under the control of the timing controller  150 . Here, the scan signal is set to a voltage at which transistors included in the pixels  140  can be turned on, and the emission control signal is set to a voltage at which the transistors included in the pixels  140  can be turned off. 
     The control line driver  160  supplies a control signal to the control lines CL 1  to CLn under the control of the timing controller  150 . For example, the control line driver  160  may progressively supply a control signal to the control lines CL 1  to CLn during a period in which threshold voltage information is extracted from each pixel  140 . 
     The data driver  120  generates data signals, using second data Data 2  supplied from the timing controller  150 , and supplies the generated data signals to the data lines D 1  to Dm. 
     The compensation unit  170  extracts at least one of degradation information and threshold voltage information from each pixel  140 . Subsequently, it is assumed that, for convenience of illustration, the threshold voltage information is extracted from the compensation unit  170 . During a sensing period, the compensation unit  170  is coupled to pixels  140  selected by a control signal or scan signal transmitted via the data lines D 1  to Dm, and extracts threshold voltage information of the driving transistor from each pixel  140 . Here, the compensation unit is  170  extracts the threshold voltage information in consideration of characteristics of the organic light emitting diode (to be described later). Additionally, the compensation unit  170  controls the data lines D 1  to Dm to be coupled to the data driver  120  during a driving period in which an image is displayed in the pixel unit  130 . 
     The timing controller  150  controls the scan driver  110 , the data driver  120 , the control line driver  160 , and the compensation unit  170 . The timing controller  150  generates a second data Data 2  by converting the bit value of a first data Data 1  that is input from outside the organic light emitting display, so that the threshold voltage of the driving transistor can be compensated by an amount corresponding to the threshold voltage information supplied from the compensation unit  170 . Here, the first data Data 1  is set to i (i is a natural number) bit(s), and the second data Data 2  is set to a j (j is a natural number greater than i) bits. 
       FIG. 2  is a circuit diagram illustrating a pixel according to an exemplary embodiment of the present invention. For convenience of illustration, an exemplary pixel coupled to an n-th scan line Sn and an m-th data line Dm is shown in  FIG. 2 . 
     Referring to  FIG. 2 , the pixel  140  includes an organic light emitting diode OLED and a pixel circuit  142  configured to supply current to the organic light emitting diode OLED. 
     An anode electrode of the organic light emitting diode OLED is connected to the pixel circuit  142 , and a cathode electrode of the organic light emitting diode OLED is connected to the second power source ELVSS. The organic light emitting diode OLED generates light with a luminance corresponding to current supplied from the pixel circuit  142 . 
     The pixel circuit  142  supplies a current to the organic light emitting diode OLED corresponding to a data signal during a driving period. The pixel circuit  142  provides threshold voltage information of a driving transistor M 2  during a sensing period, where the pixel circuit is  142  includes four transistors M 1 , M 2 , M 3 , and M 4 , and a storage capacitor Cst. 
     A gate electrode of the first transistor M 1  is connected to the scan line Sn, and a first electrode of the first transistor M 1  is connected to the data line Dm. A second electrode of the first transistor M 1  is connected to a gate electrode of the second transistor M 2 . The first transistor M 1  is turned on when a scan signal is supplied to the scan line Sn. 
     The gate electrode of the second transistor M 2  (i.e., the driving transistor) is connected to the second electrode of the first transistor M 1 , and a first electrode of the second transistor M 2  is connected to the first power source ELVDD. A second electrode of the second transistor M 2  is connected to a first node N 1 . The second transistor M 2  controls the amount of current flowing from the first power source ELVDD to the first node N 1 , the amount of current corresponding to a voltage applied to the gate electrode of the second transistor M 2 , i.e., a voltage stored in the storage capacitor Cst. 
     A first electrode of the third transistor M 3  is connected to the first node N 1 , and a second electrode of the third transistor M 3  is connected to the anode electrode of the organic light emitting diode OLED. A gate electrode of the third transistor M 3  is connected to an emission control line En. The third transistor M 3  is turned off when an emission control signal is supplied to the emission control line En, and is turned on when the emission control signal is not supplied. 
     A gate electrode of the fourth transistor M 4  is connected to a control line CLn, and a first electrode of the fourth transistor M 4  is connected to the first node N 1 . A second electrode of the fourth transistor M 4  is connected to the data line Dm. The fourth transistor M 4  is turned on when a control signal is supplied to the control line CLn, and is turned off when the control signal is not supplied. 
     The structure of the pixel  140  of the present invention is not limited to what is illustrated in  FIG. 2 . The pixel  140  may be used in other configurations including the fourth transistor M 4  so that the threshold voltage information can be extracted. For example, the pixel  140  may be configured as any one of circuits currently known in the art. 
       FIG. 3  illustrates an exemplary embodiment of a channel of the compensation unit  170 . 
     Referring to  FIG. 3 , a voltage control unit  172  and a current measurement unit  174  are provided in each channel of the compensation unit  170 . The voltage control unit  172  applies a preset voltage Vp, which may equal a reference voltage Vref, to the data line Dm during the sensing period. The current measurement unit  174  measures the amount of current flowing in the pixel  140  corresponding to the preset voltage supplied from the voltage control unit  172 . 
     For convenience of illustration, only a component configured to extract threshold voltage information has been illustrated in  FIG. 3 , but the present invention is not limited thereto. For example, the compensation unit  170  may additionally include a component configured to extract degradation information of an organic light emitting diode, a component configured to selectively couple the data lines D 1  to Dm to the data driver  120 , etc. 
       FIGS. 4A to 4C  are diagrams illustrating an exemplary embodiment of driving waveforms supplied during the sensing period. 
     Referring to  FIGS. 4A to 4C , a control signal is first supplied to the control line CLn, as shown in  FIG. 4A , so that the fourth transistor M 4  is turned on. If the fourth transistor M 4  is turned on, the data line Dm and the first node N 1  are electrically coupled to each other. Then, the reference voltage Vref supplied from the voltage control unit  172  via the data line Dm is supplied to the first node N 1 . 
     The emission control signal is supplied to the emission control line En during the period in which the reference voltage Vref is supplied and, hence, the third transistor M 3  is set in a turn-on state. Then, a first current Ioled flows through the organic light emitting diode OLED, corresponding to the reference voltage Vref supplied to the first node N 1 . In this case, the current measurement unit  174  measures the current value of the first current Ioled, and temporarily stores the measured value. 
     Subsequently, as shown in  FIG. 4B , a scan signal is supplied to the scan line Sn, and simultaneously, a preset voltage Vp is supplied to the data line Dm. If the scan signal is supplied to the scan line Sn, the first transistor M 1  is turned on. If the first transistor M 1  is turned on, the preset voltage Vp from the data line Dm is supplied to the gate electrode of the second transistor M 2 . Then, a voltage corresponding to the preset voltage Vp is stored in the storage capacitor Cst. For example, the preset voltage Vp may be set as a voltage equal to the reference voltage Vref. 
     After the voltage corresponding to the preset voltage Vp is stored in the storage capacitor Cst, as shown in  FIG. 4C , the control signal is supplied to the control line CLn and, simultaneously, the reference voltage Vref is supplied to the data line Dm. Then, the fourth transistor M 4  is turned on, and the reference voltage Vref is supplied to the first node N 1 . 
     If the fourth transistor M 4  is turned on, a second current Itft from the second transistor M 2  is supplied to the current measurement unit  174  via the data line Dm, corresponding to the preset voltage Vp. Then, the current measurement unit  174  compares the amount of the first current Ioled with that of the second current Itft, and supplies, to the voltage control unit  172 , a control signal corresponding to the compared amount of current. 
     The voltage control unit  172  receiving the control signal controls the preset voltage Vp so that the second current Itft can be equal to the first current Ioled. The voltage control unit  172  and the current measurement unit  174  repeat the procedure of  FIGS. 4B and 4C  so that the first current Ioled and the second current Itft can be equal to each other. 
     Subsequently, if it is determined that the first current Ioled and the second current Itft have the same current value, the voltage control unit  172  transmits the preset voltage Vp as threshold voltage information Vtft to the timing controller  150 . Here, the threshold voltage information Vtft is a voltage set so that the second current Itft has the same current value as the first current Ioled. The threshold voltage information Vtft includes threshold voltage information of the driving transistor M 2 . 
     The timing controller  150  receiving the threshold voltage information Vtft transmitted from the voltage control unit  172  converts the threshold voltage information Vtft into a digital value, and stores the converted digital value. Subsequently, the timing controller  150  generates the second data Data 2  by changing bits of the first data Data 1  so that the threshold voltage information Vtft of the driving transistor M 2  can be compensated corresponding to the digital value. 
     In the present invention, the threshold voltage information is extracted from each pixel  140  while repeating the procedure described above during the sensing period. Here, the threshold voltage information Vtft of the pixels  140  may be differently set corresponding to the threshold voltage of the driving transistor M 2  included in each pixel  140 . 
     The second electrode (i.e., the drain electrode) of the second transistor M 2  maintains the same voltage, i.e., the reference voltage Vref, during the period in which the first current Ioled flows through the organic light emitting diode OLED and the period in which the second current Itft is extracted. Then, the operation points during the sensing and driving periods correspond to each other, thereby extracting exact threshold voltage information. 
     Specifically, the current supplied from the driving transistor M 2  during the period in which the pixel  140  emits light is supplied to the organic light emitting diode OLED via the third transistor M 3 . In this case, the current Itft supplied from the driving transistor M 2  and the current Ioled flowing through the organic light emitting diode OLED are set equal to each other. 
     In this case, the first node N 1  is set to the voltage of a first operation point Va, as shown in  FIG. 5 , corresponding to the current Itft supplied to the driving transistor M 2  and the current Ioled flowing through the organic light emitting diode OLED. That is, if the first node N 1  is set to the same voltage when the current Itft is supplied from the driving transistor M 2  and when the current Ioled is supplied to the organic light emitting diode OLED, the operation points correspond to each other, thereby extracting exact threshold voltage information without any difference in current. 
     For example, in a case where the voltage at the first node N 1  when the current Itft is supplied from the driving transistor M 2  is different from that at the first node N 1  when the current Ioled is supplied to the organic light emitting diode OLED, the operation points do not correspond to each other and, therefore, a current difference occurs. In other words, if the voltage of a second operation point Vb is applied when the current Itft is supplied from the driving transistor M 2 , a sensing error occurs. Accordingly, the reliability of the threshold voltage information may be lowered. 
     Meanwhile, although it has been described in this exemplary embodiment that the threshold voltage information is extracted for each pixel  140 , the present invention is not limited thereto. For example, the threshold voltage information may be extracted for each block including at least one pixel. 
     Additionally, although it has been described in the present invention that the transistors are shown as PMOS transistors for convenience of illustration, the present invention is not limited thereto. For example, the transistors may be formed as NMOS transistors. 
     In the present invention, the organic light emitting diode OLED generates light of red, green, or blue, corresponding to the amount of current supplied from the driving transistor. However, the present invention is not limited thereto. For example, the organic light emitting diode OLED may generate white light, corresponding to the amount of the current supplied from the driving transistor. In this case, a color image is implemented using a separate color filter or the like. 
     By way of summation and review, an organic light emitting display includes pixels arranged in a matrix form at intersection points of data lines, scan lines, and power lines. Each pixel generally includes an organic light emitting diode, two or more transistors including a driving transistor, and one or more capacitors. 
     In the organic light emitting display and the driving method thereof according to the exemplary embodiments, threshold voltage information of the driving transistor is extracted during the sensing period, and the bits of data are controlled using the extracted threshold voltage information, thereby displaying an image with uniform luminance regardless of the threshold voltage of the driving transistor. Further, according to various embodiments, the operation point when the threshold voltage is extracted corresponds to when the pixel is driven to emit light. Accordingly, it is possible to extract exact threshold voltage information. 
     Exemplary embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit or scope of the present invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.