Abstract:
An image display device comprises a pixel circuit including a display element; a first transistor for controlling a current output to a third electrode according to a voltage applied between the first and second electrodes; a first switch for diode-connecting the first transistor in response to a select signal; a first capacitor; a second switch for coupling a first electrode of the first capacitor to a power in response to the select signal; a second capacitor; a third switch for transmitting the data voltage to a second electrode of the second capacitor in response to the select signal; and a fourth switch for intercepting the first electrode of the first capacitor and the second electrode of the second capacitor in response to the select signal.

Description:
[0001]     This application claims priority to and the benefit of Korea Patent Application No. 10-2003-0083581, filed on Nov. 24, 2003, which is hereby incorporated by reference for all purposes as if fully set forth herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an image display device and driving method thereof. More specifically, the present invention relates to an organic EL (electroluminescent) display device.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Generally, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-programs N×M emitting cells to display images. As shown in  FIG. 1 , a typical organic emitting cell comprises an anode (made of indium tin oxide (ITO)), an organic thin film, and a cathode layer (metal). The organic thin film may have a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL),an electron injecting layer (EIL) and a hole injecting layer (HIL).  
         [0006]     Methods for driving organic emitting cells include the passive matrix method, and the active matrix method, which uses thin film transistors (TFTs)or metal-oxide-semiconductor field-effect transistors (MOSFETs). The passive matrix method forms crossing cathodes and anodes and selectively drives data and scan lines. The active matrix method couples a TFT and a capacitor to each ITO pixel electrode to maintain the voltage by utilizing the capacitor. The active matrix method includes a voltage programming method or a current programming method, depending upon signal forms supplied for programming a voltage at a capacitor.  
         [0007]      FIG. 2  shows a conventional voltage programming pixel circuit for driving an organic EL element.  
         [0008]     As shown, the conventional voltage programming pixel circuit comprises transistors M 1 , M 2 , M 3 , and M 4 , capacitors C 1  and C 2 , and an organic EL element OLED. The data line Dm transmits data voltages for displaying image signals to the pixel circuit, the capacitor C 2  is coupled to the power V DD , and a cathode of the organic EL element OLED is coupled to a power V SS . A threshold voltage of V TH  at the driving transistor M 1  is compensated by select signals provided from three scan lines S n , AZ, and AZB, and a current corresponding to a data voltage V DATA  is controlled to flow to the organic EL element OLED.  
         [0009]     The conventional pixel circuit compensates for deviation of the threshold voltage V TH  of the driving transistor M 1 , but requires three additional scan lines for such compensation. This many scan lines may degrade the display device&#39;s aperture ratio and provide a complicated driving circuit.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides a pixel circuit of an image display device with less signal lines.  
         [0011]     The present invention also provides an image display device with an improved aperture ratio by simplifying a driving circuit and a pixel circuit.  
         [0012]     The present invention also provides an image display device with accurately compensated deviation of a threshold voltage at a driving transistor.  
         [0013]     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention.  
         [0014]     The present invention discloses an image display device including a plurality of data lines for transmitting a data voltage corresponding to an image signal, a plurality of scan lines for transmitting a select signal, and a pixel circuit coupled to a data line and a scan line. The pixel circuit comprises a display element for displaying an image corresponding to an applied current, and a first transistor, including a first electrode, a second electrode coupled to a power, and a third electrode coupled to the display element, for outputting a current corresponding to a voltage applied between the first and second electrodes to the third electrode. A first switch diode-connects the first transistor in response to the select signal provided from the scan line, and a second switch couples a first electrode of a first capacitor to the power in response to the select signal provided from the scan line. A second electrode of the first capacitor is coupled to the first electrode of the first transistor, and a second capacitor has a first electrode coupled to the power. A third switch transmits the data voltage to a second electrode of the second capacitor in response to the select signal provided from the scan line. A fourth switch, coupled between the first electrode of the first capacitor and the second electrode of the second capacitor, intercepts the first electrode of the first capacitor and the second electrode of the second capacitor in response to the select signal provided from the scan line.  
         [0015]     The present invention also discloses an image display device including a plurality of data lines for transmitting a data voltage corresponding to an image signal, a plurality of first scan lines for transmitting a select signal, a plurality of second scan lines for transmitting a control signal, and a pixel circuit coupled to a data line, a first scan line, and a second scan line. The pixel circuit comprises a display element for displaying an image corresponding to an applied current, and a first transistor, including a first electrode, a second electrode coupled to a power, and a third electrode coupled to the display element, for outputting a current corresponding to a voltage applied between the first and second electrodes to the third electrode. A first switch diode-connects the first transistor in response to a first control signal, and a second switch couples a first electrode of the first capacitor to the power in response to a second control signal. A second electrode of the first capacitor is coupled to the first electrode of the first transistor, and a second capacitor has a first electrode coupled to the power. A third switch transmits the data voltage to a second electrode of the second capacitor in response to the select signal provided from the scan line. A fourth switch, coupled between the first electrode of the first capacitor and the second electrode of the second capacitor, intercepts the second electrode of the first capacitor and the second electrode of the second capacitor in response to a third control signal.  
         [0016]     The present invention also discloses a method for an image display device. The image display device includes a plurality of data lines for transmitting a data voltage corresponding to an image signal, a plurality of scan lines for transmitting a select signal, and a pixel circuit coupled to a data line and a scan line. The pixel circuit comprises a driving transistor having a first electrode, a second electrode coupled to a power, and a third electrode, and it outputs a current corresponding to a voltage applied between the first and second electrodes to the third electrode. A display element is coupled to the third electrode of the driving transistor and displays an image in correspondence to an amount of the applied current. A first capacitor has a second electrode coupled to the first electrode of the driving transistor, and a second capacitor has a first electrode coupled to the power. A method for driving such an image display device comprises diode-connecting the driving transistor, coupling a first electrode of the first capacitor to the power, and coupling a second electrode of the second capacitor to the data line during a first period. The first electrode of the first capacitor and the second electrode of the second capacitor are coupled during a second period.  
         [0017]     The present invention also discloses a driving method of an image display device. The image display device includes a plurality of data lines for transmitting a data voltage corresponding to an image signal, a plurality of first scan lines for transmitting a select signal, a plurality of second scan lines for transmitting a control signal, and a pixel circuit coupled to a data line, a first scan line, and a second scan line. The pixel circuit comprises a driving transistor having a first electrode, a second electrode coupled to a power, and a third electrode, and it outputs a current corresponding to a voltage applied between the first and second electrodes to the third electrode. A display element is coupled to the third electrode of the driving transistor and displays an image in correspondence to an amount of the applied current. A first capacitor has a second electrode coupled to the first electrode of the driving transistor, and a second capacitor has a first electrode coupled to the power. A method for driving such an image display device comprises diode-connecting the driving transistor, and coupling a first electrode of the first capacitor to the power during a first period. A second electrode of the second capacitor is coupled to the data line during a second period, and the first electrode of the first capacitor is coupled to the second electrode of the second capacitor during a third period.  
         [0018]     The present invention also discloses a driving method of an image display device. The image display device includes a plurality of data lines for transmitting a data voltage corresponding to an image signal, a plurality of scan lines for transmitting a select signal, and a pixel circuit coupled to a data line and a scan line. The pixel circuit comprises a driving transistor having a first electrode, a second electrode coupled to a power, and a third electrode. A display element is coupled to the third electrode of the driving transistor. A first capacitor has a second electrode coupled to the first electrode of the driving transistor, and a second capacitor has a first electrode coupled to the power. The method for driving such an image display device comprises storing a threshold voltage at the driving transistor in the first capacitor and storing a data voltage in the second capacitor during a first period. The first capacitor and the second capacitor are coupled in series so that the voltage stored in the first capacitor and the voltage stored in the second capacitor may be applied to the first electrode of the driving transistor during a second period.  
         [0019]     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  
       [0020]     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 embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0021]      FIG. 1  shows a conceptual diagram of an organic EL display element.  
         [0022]      FIG. 2  shows a conventional voltage programming pixel circuit for driving an organic EL element.  
         [0023]      FIG. 3  shows an image display device according to an exemplary embodiment of the present invention.  
         [0024]      FIG. 4  shows a pixel circuit according to a first exemplary embodiment of the present invention.  
         [0025]      FIG. 5  shows a driving waveform for driving the pixel circuit of  FIG. 4 .  
         [0026]      FIG. 6  shows an equivalent circuit of the pixel circuit shown in  FIG. 4  during a period t 1  of  FIG. 5 .  
         [0027]      FIG. 7  shows an equivalent circuit of the pixel circuit shown in  FIG. 4  during a period t 2  of  FIG. 5 .  
         [0028]      FIG. 8  shows a pixel circuit according to a second exemplary embodiment of the present invention.  
         [0029]      FIG. 9  shows a driving waveform for driving the pixel circuit shown in  FIG. 8 .  
         [0030]      FIG. 10  shows a pixel circuit according to a third exemplary embodiment of the present invention.  
         [0031]      FIG. 11  shows a pixel circuit according to a fourth exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.  
         [0033]     To couple one thing to another includes to directly couple the first one to the second one and to couple the first one to the second one with others provided therebetween. To clarify the present invention, parts which are not described in the specification are omitted, and parts for which similar descriptions are provided have the same reference numerals.  
         [0034]      FIG. 3  shows an organic EL display device according to an exemplary embodiment of the present invention.  
         [0035]     As shown, the organic EL display device comprises an organic EL display panel  100 , a scan driver  200 , and a data driver  300 .  
         [0036]     The organic EL display panel  100  comprises a plurality of data lines D I  to D M  in the column direction, a plurality of scan lines S 1  to S N  in the row direction, and a plurality of pixel circuits  10 . The data lines D 1  to D M  transmit data voltages for displaying image signals to the pixel circuit  10 , and the scan lines S 1  to S N  transmit select signals to the pixel circuit  10 . The pixel circuit  10  is formed at a pixel area defined by two adjacent data lines D 1  to D M , and two adjacent scan lines S 1  to S N .  
         [0037]     The scan driver  200  sequentially applies select signals to the scan lines S 1  to S N , and the data driver  300  applies the data voltage for displaying image signals to the data lines D 1  to D M .  
         [0038]     The scan driver  200  and/or the data driver  300  may be coupled to the display panel  100 , or they may be installed, in a chip format, or in a tape carrier package (TCP), coupled to the display panel  100 . They may also be attached to the display panel  100 , and installed, in a chip format, on a flexible printed circuit (FPC) or a film coupled to the display panel  100 . On the other hand, the scan driver  200  and/or the data driver  300  may be installed on the glass substrate of the display panel. Specifically, they may be substituted for the driving circuit formed in the same layers of the scan lines, the data lines, and TFTs on the glass substrate, or they may be directly installed on the glass substrate.  
         [0039]     Referring to  FIG. 4 ,  FIG. 5 ,  FIG. 6  and  FIG. 7 , the pixel circuit  10  of the organic EL display device according to the first exemplary embodiment will be described.  
         [0040]      FIG. 4  shows an equivalent circuit diagram of a pixel circuit according to a first exemplary embodiment of the present invention, and  FIG. 5  shows a driving waveform for driving the driving circuit of  FIG. 4 . For ease of description, the pixel circuit coupled to the m-th data line Dm and the n-th scan line Sn will be described.  
         [0041]     As shown, the pixel circuit  10  according to the first exemplary embodiment of the present invention comprises an organic EL element OLED, transistors M 1  to M 6 , and capacitors C 1  and C 2 .  
         [0042]     The transistor M 1 , coupled between a power V DD  and the organic EL element OLED, controls the current flowing to the organic EL element OLED. The source electrode of the transistor M 1  is coupled to the power V DD , and its drain electrode is coupled to an anode of the organic EL element OLED through the transistor M 4 . A cathode of the organic EL element OLED is coupled to a power V SS . Since the transistor M 1  is realized with a P-type transistor, the power V SS  supplies a lesser voltage than the power V DD , such as a ground voltage.  
         [0043]     The transistor M 2  diode-connects the transistor M 1  in response to a select signal provided from the scan line S n .  
         [0044]     The transistor M 5  couples a first electrode of the capacitor C 1  and the power VDD in response to the select signal applied to the scan line S n , and a second electrode of the capacitor C 1  is coupled to a gate electrode of the transistor M 1 .  
         [0045]     A first electrode of the capacitor C 2  is coupled to the power V DD , and the transistor M 6  couples a second electrode of the capacitor C 2  to the first electrode of the capacitor C 1  in response to a select signal applied to the scan line S n .  
         [0046]     The transistor M 3  transmits the data current provided from the data line Dm to the second electrode of the capacitor C 2  in response to a select signal provided from the scan line S n .  
         [0047]     The transistors M 2 , M 3 , and M 5  may be formed with a first channel type, and the transistors M 4  and M 6  may be formed with a second channel type in the first exemplary embodiment.  
         [0048]     Therefore, the transistors M 4  and M 6  are turned off when the transistors M 2 , M 3 , and M 5  are turned on, and vice versa. In other words, with p-type transistors M 2 , M 3 , and M 5  and n-type transistors M 4  and M 6 , when a low level select signal is applied to the scan line Sn, the p-type transistors M 2 , M 3 , and M 5  are turned on, and the n-type transistors M 4  and M 6  are turned off. Consequently, one select signal may control the five switching transistors M 2 -M 6 .  
         [0049]     An operation of the pixel circuit according to the first exemplary embodiment will now be described with reference to  FIG. 5 , FIG. 6  and  FIG. 7 .  
         [0050]     Referring to  FIG. 5 , the transistors M 2 , M 3 , and M 5  are turned on and the transistors M 4  and M 6  are turned off when a low level select signal is applied in the period of t 1 .  
         [0051]     Therefore, as shown in  FIG. 6 , the first electrode of the capacitor C 1  is coupled to the power VDD through the transistor M 5 , and the driving transistor M 1  is diode-connected by the transistor M 2 . Hence, the capacitor C 1  is charged with a voltage corresponding to the threshold voltage V TH  at the transistor M 1 . Also, the second electrode of the capacitor C 2  is coupled to the data line D m , thereby charging the capacitor C 2  with the data voltage. When a high level select signal is applied in the period t 2 , the transistors M 4  and M 6  are turned on, and the transistors M 2 , M 3 , and M 5  are turned off.  
         [0052]     As shown in  FIG. 7 , the second electrode of the capacitor C 2  is coupled to the first electrode of the capacitor C 1  by the transistor M 6 , and the first electrode of the capacitor C 2  is coupled to the power V DD . Hence, since the capacitors C 1  and C 2  are coupled in series, the voltage applied to the gate of the transistor M 1  substantially corresponds to the total of the voltage stored in the capacitor C 1  plus the voltage stored in the capacitor C 2 .  
         [0053]     In this instance, with the transistor M 4  turned on, the current flowing to the driving transistor M 1  is transmitted to the organic EL element OLED, and the organic EL element OLED displays an image corresponding to the applied current.  
         [0054]     The current I OLED  flowing to the organic EL element OLED is given in Equation 1. 
 
 I   OLED =β/2( V   GS   −V   TH ) 2 =β/2( V   DD   −V   TH   −V   DATA   −|V   TH |) 2    Equation 1 
 
         [0055]     where I OLED  is a current flowing to the organic EL element OLED, V GS  is a voltage between the source electrode and the gate electrode of the transistor M 1 , V TH  is a threshold voltage at the transistor M 1 , V DATA  is a data voltage, and β is a constant.  
         [0056]     Equation 1 may be expressed as Equation 2, where it is shown that the current I OLED  flowing to the organic EL element OLED is not influenced by the deviation of the threshold voltage of the driving transistor M 1 . 
 
 I   OLED =β/2( V   DD   −V   DATA ) 2    Equation 2 
 
         [0057]     Therefore, the threshold voltage deviation may be compensated and the pixel circuit may be driven by a single select signal according to the first embodiment, thereby reducing the complexity of the pixel circuit and the driving circuit, and obtaining the desired aperture ratio.  
         [0058]     A pixel circuit according to a second exemplary embodiment of the present invention will now be described with reference to  FIG. 8  and  FIG. 9 .  
         [0059]      FIG. 8  shows a pixel circuit according to a second exemplary embodiment of the present invention, and  FIG. 9  shows a driving waveform for driving the pixel circuit shown in  FIG. 8 .  
         [0060]     The pixel circuit according to the second exemplary embodiment differs from the first exemplary embodiment in that separate select signals are applied to the transistor M 3  and the transistors M 2 , M 4 , M 5 , and M 6 .  
         [0061]     Specifically, a select signal from the scan line Sn is applied to the transistor M 3 , and a select signal from an additional scan line En is applied to the transistors M 2 , M 4 , M 5 , and M 6 . Accordingly, the threshold voltage of V TH  at the driving transistor M 1  is more precisely compensated by allowing different periods of the select signals from the scan line S n  and the scan line E n .  
         [0062]     A driving method of the pixel circuit according to the second exemplary embodiment will now be described referring to  FIG. 9 .  
         [0063]     When the select signal provided from the scan line En becomes low level in the period t 1 , the transistors M 2  and M 5  are turned on, the driving transistor M 1  is diode-connected, and the first electrode of the capacitor C 1  is coupled to the power V DD . Therefore, the capacitor C 1  is charged with the threshold voltage of V TH  of the driving transistor M 1 , and the charging operation is consecutively performed during the period t 2 .  
         [0064]     When the select signal from the scan line Sn becomes low level in the period t 2 , the transistor M 3  turns on, and the data voltage from the data line D m  is charged in the capacitor C 2 .  
         [0065]     When the select signals become high level during the period t 3 , the capacitor C 1  and the capacitor C 2  are coupled in series in a manner like that of  FIG. 7 , and a current corresponding to the data voltage V DATA  flows to the organic EL element OLED.  
         [0066]     Separating the scan line Sn and the scan line En, and differentiating the periods of their respective select signals, may allow the capacitor C 1  to be accurately charged with the threshold voltage of the driving transistor M 1 .  
         [0067]     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the 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.  
         [0068]     For example, in  FIG. 4  and  FIG. 8 , the transistor M 1  may be realized with active elements that include a first electrode, a second electrode, and a third electrode, where a difference of the voltages between the first and second electrodes controls the current output to the third electrode. Also, the transistors M 2 , M 3 , M 4 , and M 5  are elements for switching both coupled terminals according to applied control signals, and they are not restricted to the specific elements shown in  FIG. 4  and  FIG. 8 .  
         [0069]     Further,  FIG. 4  and  FIG. 8  show the transistor M 3  having one gate electrode, however, the transistor M 3  may be replaced with dual gate transistor (M 7 ) as shown in  FIG. 10  and  FIG. 11  to reduce leakage current.