Patent Publication Number: US-8531358-B2

Title: Organic light emitting display device having improved brightness

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0025841, filed on Mar. 26, 2009, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The following description relates to organic light emitting display devices. 
     2. Discussion of Related Art 
     Recently, various flat panel display devices capable of reducing weight and volume, which are disadvantages of cathode ray tubes, have been developed. Among the flat panel display devices, there are liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices, etc. 
     Among the above discussed flat panel display devices, the organic light emitting display devices display images using organic light emitting diodes that generate light by the recombination of electrons and holes. Organic light emitting display devices are driven at low power consumption, with rapid response speed. 
       FIG. 1  is a schematic circuit diagram showing a conventional pixel of an organic light emitting display device. In  FIG. 1 , the transistors included in the pixel are NMOS transistors. 
     Referring to  FIG. 1 , the pixel  4  of the conventional organic light emitting display device includes a pixel circuit  2  that is coupled to an organic light emitting diode OLED, a data line Dm, and a scan line Sn to control the organic light emitting diode OLED. 
     The anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit  2 , and the cathode electrode of the organic light emitting diode OLED is coupled to a second power supply ELVSS. The organic light emitting diode OLED generates light having a brightness (e.g., a predetermined brightness) corresponding to the current supplied from the pixel circuit  2 . 
     The pixel circuit  2  controls the amount of current supplied to the organic light emitting diode OLED according to the data signal supplied to the data line Dm and a scan signal supplied to the scan line Sn. To this end, the pixel circuit  2  includes a second transistor M 2  (i.e., a driving transistor) coupled between a first power supply ELVDD and the organic light emitting diode OLED, a first transistor M 1  coupled between the second transistor M 2 , the data line Dm, and the scan line Sn, and a storage capacitor Cst that is coupled between the gate electrode and a first electrode of the second transistor M 2 . 
     The gate electrode of the first transistor M 1  is coupled to the scan line Sn, and a first electrode of the first transistor M 1  is coupled to the data line Dm. A second electrode of the first transistor M 1  is coupled to one terminal of the storage capacitor Cst. Here, the first electrode of the first transistor M 1  is either a source electrode or a drain electrode, and the second electrode of the first transistor M 1  is an electrode other than the electrode of the first electrode. For example, if the first electrode is the source electrode, the second electrode is the drain electrode. When the scan signal is supplied to the scan line Sn, the first transistor M 1  coupled between the scan line Sn and the data line Dm is turned on to supply the data signal supplied from the data line Dm to the storage capacitor Cst. Thus, the storage capacitor Cst is charged with a voltage corresponding to the data signal. 
     The gate electrode of the second transistor M 2  is coupled to one terminal of the storage capacitor Cst, and the first electrode is coupled to the first power supply ELVDD. The second electrode of the second transistor M 2  is coupled to the other terminal of the storage capacitor Cst and is also coupled to the anode electrode of the organic light emitting diode OLED. The second transistor M 2  controls the amount of current flowing from the first power supply ELVDD to the second power supply ELVSS via the organic light emitting diode OLED in accordance with the voltage stored in the storage capacitor Cst. 
     One terminal of the storage capacitor Cst is coupled to the gate electrode of the second transistor M 2 , and the other terminal of the storage capacitor Cst is coupled to the anode electrode of the organic light emitting diode OLED. The storage capacitor Cst is charged with the voltage corresponding to the data signal. 
     A conventional pixel  4  as described above supplies a current corresponding to the voltage charged in the storage capacitor Cst to the organic light emitting diode OLED, thereby displaying an image having a brightness (e.g., a predetermined brightness). However, an issue with this conventional organic light emitting display device is that it cannot display an image having a uniform brightness due to the deviation of the threshold voltage of the second transistor M 2 . 
     Actually, when the threshold voltage of the second transistors M 2  are different in the respective pixels  4 , the respective pixels  4  generate light having different brightness corresponding to the same data signal, and the conventional organic light emitting display device cannot display an image having a uniform brightness. 
     SUMMARY 
     An aspect of an embodiment of the present invention provides an organic light emitting display device that compensates for variations of the threshold voltage of driving transistors. 
     According to an embodiment of the present invention, an organic light emitting display device includes a scan driver for sequentially supplying scan signals to a plurality of scan lines; a data driver for supplying an initial power during a first portion of a period when the scan signals are supplied to the scan lines, and for supplying data signals during a second portion of the period other than the first portion of the period; and pixels at respective crossings of the scan lines and the data lines. A pixel of the pixels at an i th  (i is a natural number) horizontal line includes an organic light emitting diode having a cathode electrode coupled to a second power supply; a first transistor for controlling a current flowing from a first power supply to the second power supply via the organic light emitting diode; a second transistor coupled between a data line of the data lines and a second node, and is configured to be turned on when a scan signal of the scan signals is supplied to an i th  scan line; a third transistor coupled between a first node coupled to the gate electrode of the first transistor and the second node, and is configured to maintain a turn-off state when the second transistor is turned on; a fourth transistor coupled between the first node and a reference power supply, and is configured to be turned on when the scan signal is supplied to the i th  scan line; a first capacitor coupled between the second node and an anode electrode of the organic light emitting diode; and a second capacitor coupled between the first node and the anode electrode of the organic light emitting diode. 
     In some embodiments, the initial power is adapted to have a higher voltage than a voltage of the data signal. The reference power may have a voltage adapted to turn off the first transistor. The third transistor may be configured to be turned on when the scan signal is supplied to an i+1 th  scan line. The scan driver may be configured sequentially to supply emission control signals to emission control lines substantially parallel to the scan lines. The emission control signal supplied to an i th  emission control line may overlap the scan signal supplied to the i th  scan line, and may have a voltage adapted to turn off the third transistor. The gate electrode of the third transistor may be coupled to the i th  emission control line. 
     With the organic light emitting display device according to various embodiments of the present invention, the threshold voltage of the driving transistor is substantially compensated, thus displaying an image having a substantially uniform brightness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a schematic circuit diagram showing a conventional pixel of an organic light emitting display device; 
         FIG. 2  is a schematic block diagram showing an organic light emitting display device according to an embodiment of the present invention; 
         FIG. 3  is a schematic circuit diagram showing another embodiment of the exemplary embodiment of  FIG. 2 ; 
         FIG. 4  is a waveform timing diagram showing a method for driving the pixel of  FIG. 3 ; 
         FIG. 5  is a schematic circuit diagram showing another embodiment of the exemplary embodiment of  FIG. 2 ; and 
         FIG. 6  is a waveform timing diagram showing a method for driving the pixel of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Also, in the context of the present application, when an element is referred to as being coupled to another element, it can be directly coupled to the another element or be indirectly coupled to the another element with one or more intervening elements interposed therebetween. Like reference numerals designate like elements throughout the specification. 
     Hereinafter, exemplary embodiments of the present invention, proposed so that a person having ordinary skill in the art can easily carry out the present invention, will be described in more detailed with reference to the accompanying  FIG. 2  to  FIG. 6 . 
       FIG. 2  is a schematic block diagram showing an organic light emitting display device according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the organic light emitting display device according to the exemplary embodiment of the present invention includes pixels  140  positioned to be coupled to scan lines S 1  to Sn+1 and data lines D 1  to Dm, a scan driver  110  that drives the scan lines S 1  to Sn+1, a data driver  120  that drives the data lines D 1  to Dm, and a timing controller  150  that controls the scan driver  110  and the data driver  120 . 
     The scan driver  110  receives a scan driving control signal SCS from the timing controller  150 . The scan driver  110  supplied with the scan driving control signal SCS generates scan signals, and sequentially supplies the generated scan signals to the scan lines S 1  to Sn+1. 
     The data driver  120  receives a data driving control signal DCS from the timing controller  150 . The data driver  120  supplied with the data driving control signal DCS supplies an initial power during a first period of the period when the scan signals are supplied and supplies the data signals during a second period other than the first period. Here, the initial power is set to have a higher voltage than the data signals. 
     The timing controller  150  generates the data driving control signal DCS and the scan driving control signal SCS corresponding to synchronization signals supplied from an external source. The data driving control signal DCS generated by the timing controller  150  is supplied to the data driver  120 , and the scan driving control signal SCS generated by the timing controller  150  is supplied to the scan driver  110 . The timing controller  150  supplies data Data, which is supplied from the external source, to the data driver  120 . 
     The pixel unit  130  receives a first power ELVDD, a second power ELVSS, and a reference power Vref from the external source, and supplies them to the respective pixels  140 . The respective pixels  140  supplied with the first power ELVDD, the second power ELVSS, and the reference power Vref generate light in accordance with the data signal. 
     Here, the first power supply ELVDD is set to have a higher voltage than the second power supply ELVSS to supply a current (e.g., a predetermined current) to the organic light emitting diode OLED. The reference power Vref has a voltage adapted to turn off the driving transistor. 
     In addition, the pixel  140  positioned at an i th  (i is a natural number) horizontal line is coupled to an i th  scan line and an i+1 th  scan line. The pixel  140  includes a plurality of NMOS-type transistors and supplies the current, which is compensated for variations of the threshold voltage of the driving transistor, to the organic light emitting diode OLED. 
       FIG. 3  is a schematic circuit diagram showing a pixel according to an embodiment of the present invention. For convenience of explanation,  FIG. 3  shows the pixel  140  positioned on a n th  horizontal line and coupled to an m th  data line Dm. 
     Referring to  FIG. 3 , the pixel  140  according to the exemplary embodiment of the present invention includes a pixel circuit  142  that is coupled to an organic light emitting diode OLED, the m th  data line Dm, n th  scan line Sn, and n+1 th  scan line Sn+1 to control the organic light emitting diode OLED. 
     An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit  142 , and a cathode electrode of the organic light emitting diode OLED is coupled to the second power supply ELVSS. The organic light emitting diode OLED generates light having a brightness (e.g., a predetermined brightness) corresponding to the current supplied from the pixel circuit  142 . 
     The pixel circuit  142  is charged with a voltage corresponding to a data signal supplied to the m th  data line Dm when the scan signal is supplied to the n th  scan line Sn, and corresponding to the threshold voltage of a first transistor M 1  (that is, a driving transistor), and supplies the current corresponding to the charged voltage when the scan signal is supplied to the n+1 th  scan line Sn+1 to the organic light emitting diode OLED. To this end, the pixel circuit  142  includes first to fourth transistors M 1  to M 4 , a first capacitor C 1 , and a second capacitor C 2 . 
     A gate electrode of the first transistor M 1  is coupled to a first node N 1 , and a first electrode of the first transistor M 1  is coupled to a first power supply ELVDD. A second electrode of the first transistor M 1  is coupled to the anode electrode of the organic light emitting diode OLED (i.e., to a third node N 3 ). The first transistor M 1  controls the amount of current supplied from the first power supply ELVDD to the second power supply ELVSS via the organic light emitting diode OLED in accordance with the voltage applied to the first node N 1 . 
     A gate electrode of the second transistor M 2  is coupled to the n th  scan line Sn, and a first electrode of the second transistor M 2  is coupled to the m th  data line Dm. A second electrode of the second transistor M 2  is coupled to a second node N 2 . The second transistor M 2  is turned on when the scan signal is supplied to the n th  scan line Sn to couple (e.g., to conductively couple) the data line Dm to the second node N 2 . 
     A gate electrode of the third transistor M 3  is coupled to the n+1 th  scan line Sn+1, and a first electrode of the third transistor M 3  is coupled to the second node N 2 . A second electrode of the third transistor M 3  is coupled to the first node N 1  (that is, the gate electrode of the first transistor M 1 ). The third transistor M 3  is turned on when the scan signal is supplied to the n+1 th  scan line Sn+1 to couple (e.g., to conductively couple) the first node N 1  to the second node N 2 . Meanwhile, the third transistor M 3  maintains a turn-off state when the second transistor M 2  is turned on. 
     A gate electrode of the fourth transistor M 4  is coupled to the n th  scan line Sn, and a first electrode of the fourth transistor M 4  is coupled to the reference power Vref. A second electrode of the fourth transistor M 4  is coupled to the first node N 1 . The fourth transistor M 4  is turned on when the scan signal is supplied to the n th  scan line Sn to supply the voltage of the reference power Vref to the first node N 1 . 
     The first capacitor C 1  is coupled between the second node N 2  and a third node N 3  (that is, the anode electrode of the organic light emitting diode OLED). Thus, the first capacitor C 1  is charged with the voltage corresponding to the data signal when the second transistor M 2  is in a turn-on state. 
     The second capacitor C 2  is coupled between the first node N 1  and the third node N 3  (that is, the anode electrode of the organic light emitting diode OLED). Thus, the second capacitor C 2  is charged with the voltage corresponding to the threshold voltage of the first transistor M 1 . 
       FIG. 4  is a waveform timing diagram showing a method for driving the pixel of  FIG. 3 . 
     Describing the operation process of the pixel  140  in detail by combining  FIGS. 3 and 4 , the scan signal is first supplied to the n th  scan line Sn, and an initial power Vint is supplied to the m th  data line Dm during a first period of the period when the scan signal is supplied. 
     When the scan signal is supplied to the scan line Sn, the second transistor M 2  and the fourth transistor M 4  are turned on. When the fourth transistor M 4  is turned on, the voltage of the reference power supply Vref is supplied to the first node N 1 . Here, the voltage of the reference power supply Vref has a low voltage, which maintains the first transistor M 1  in a turn-off state. When the first transistor M 1  is turned off, the current is not supplied to the organic light emitting diode OLED, and accordingly, the organic light emitting diode OLED is in a turn-off state. 
     When the second transistor M 2  is turned on, the initial power Vint from the m th  data line Dm is supplied to the second node N 2 . In this case, both terminals of the first capacitor C 1  are set to the initial power Vint and the voltage applied to the anode electrode of the organic light emitting diode OLED at the time of turn-off. 
     Thereafter, the data signal is supplied to the m th  data line Dm during a second period, and accordingly, the voltage of the second node N 2  falls from the initial voltage Vint to the voltage of the data signal Vdata. If the voltage of the second node N 2  falls, the voltage of the third node N 3  also falls by a coupling phenomenon of the first capacitor C 1 . Here, the first transistor M 1  is turned on, and the voltage of the third node N 3  rises to the voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the voltage of the reference power supply Vref. To this end, the voltage of the reference power supply Vref is set so that the voltage of the third node N 3  falls to a lower voltage than the voltage of the reference power supply Vref when the data signal is supplied. 
     When the voltage of the third node N 3  rises to the voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the voltage of the reference power supply Vref, the second capacitor C 2  is charged with the threshold voltage of the first transistor M 1 . Here, the first capacitor C 1  is charged with the voltage obtained by the equation Vdata−Vref+Vth(M 1 ). Here, Vdata represents the voltage of the data signal. 
     Thereafter, the supply of the scan signal to the n th  scan line Sn stops, and the second transistor M 2  and the fourth transistor M 4  are turned off. The scan signal is supplied to the n+1 th  scan line Sn+1, so the third transistor M 3  is turned on. When the third transistor M 3  is turned on, the first node N 1  and the second node N 2  are coupled (e.g., conductively coupled) to each other. Then, the voltage stored in the first capacitor C 1  and the second capacitor C 2  are shared and averaged. In this case, the voltage finally applied to the first node and the second node N 2  are shown in equation 1:
 
 V   N1,N2 =( C 1 ×V data+ C 2× Vref )/( C 1+ C 2)  [Equation 1]
 
     The voltage of the third node N 3  is set as shown in equation 2:
 
 V   N3   =Vref−Vth ( M 1)  [Equation 2]
 
     When the voltages of the nodes N 1 , N 2 , and N 3  are set as shown in equations 1 and 2, a gate-source voltage Vgs of the first transistor M 1  is shown in equation 3:
 
 Vgs =( C 1 ×V data+ C 2 ×Vref )/( C 1 +C 2)− Vref+Vth ( M 1)  [Equation 3]
 
     When the gate-source voltage Vgs of the first transistor M 1  is as shown in equation 3, the current flowing through the organic light emitting diode OLED is as shown in equation 4: 
     
       
         
           
             
               
                 
                   
                     
                       
                         loled 
                         = 
                         
                           β 
                           ( 
                           
                             Vgs 
                             - 
                             
                               
                                 Vth 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     M 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                   ) 
                                 
                               
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           β 
                           ⁢ 
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       C 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                       × 
                                       Vdata 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                       × 
                                       Vref 
                                     
                                   
                                   ) 
                                 
                                 / 
                                 
                                   ( 
                                   
                                     
                                       C 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                     + 
                                     
                                       C 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               Vref 
                               + 
                               
                                 Vth 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     M 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   Vth 
                                   ( 
                                   
                                     M 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                   } 
                                 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           β 
                           ⁢ 
                           
                             
                               { 
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         C 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                         × 
                                         Vdata 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                         × 
                                         Vref 
                                       
                                     
                                     ) 
                                   
                                   / 
                                   
                                     ( 
                                     
                                       
                                         C 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                       + 
                                       
                                         C 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                                 - 
                                 Vref 
                               
                               } 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Referring to equation 4, the current flowing through the organic light emitting diode OLED is determined irrespective (or substantially independent) of the threshold voltage of the first transistor M 1 . Therefore, in an embodiment of the present invention, an image having a substantially uniform brightness can be displayed. 
       FIG. 5  is a schematic circuit diagram showing a pixel according to another embodiment of the present invention. When describing  FIG. 5 , portions having the same structure and/or function as  FIG. 3  will be given with the same reference numerals and the detailed description thereof will be omitted. 
     Referring to  FIG. 5 , the pixel  140 ′ is coupled to an emission control line En. Here, the emission control lines are formed for each horizontal line to be substantially parallel to the scan lines S 1  to Sn. An emission control signal supplied to an i th  (i is a natural number) emission control line Ei is supplied to overlap in time with the scan signal supplied to an i th  scan line Si, as shown in  FIG. 6 . 
     Meanwhile, the scan signals sequentially supplied to the scan lines S 1  to Sn have a voltage (for example, having a high polarity) that turns on the corresponding transistors, and the emission control signals supplied to the emission control lines E 1  to En have a voltage (for example, having a low polarity) that turns off the corresponding transistors. 
     A gate electrode of the third transistor M 3 ′ included in the pixel circuit  142 ′ is coupled to the emission control line En, and a first electrode of the third transistor M 3 ′ is coupled to the second node N 2 . A second electrode of the third transistor M 3  is coupled to the first node N 1 . 
     The operation process of the pixel  140 ′ as described above is substantially the same as that of the pixel shown in  FIG. 3 , except that the third transistor M 3 ′ is controlled by the emission control signal. Therefore, the detailed operation process thereof will not be provided again. 
     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.