Patent Publication Number: US-10783834-B2

Title: Pixel circuit and organic light emitting display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Korean Patent Application No. 10-2018-0024722, filed on Feb. 28, 2018, in the Korean Intellectual Property Office, and entitled: “Pixel Circuit and Organic Light Emitting Display Device,” is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     One or more embodiments relate to a pixel circuit and an organic light emitting display device. 
     2. Description of the Related Art 
     An organic light emitting display device includes a light emitting element, e.g., an organic light emitting diode, whose luminance varies with an applied current. A pixel in the organic light emitting display device includes an organic light emitting diode, a driving transistor for controlling the amount of a current supplied to the organic light emitting diode according to the voltage between a gate electrode and a source electrode, and a switching transistor for transmitting a data voltage for controlling the luminance of the organic light emitting diode to the driving transistor. 
     Due to manufacturing process errors, the driving transistors in the organic light emitting display device may have different threshold voltages from one other. Thus, even if the same data voltage is applied thereto, the amount of a current output by the driving transistors may be different depending on the respective threshold voltages. Further, the amount of a current to be output by the driving transistors during the current frame period may vary with the amount of a current output during the previous frame period. Further, when the organic light emitting diode emits light during the previous frame period, the organic light emitting diode may slightly emit light even though a full black picture should be displayed during the current frame period. 
     Accordingly, the pixel may further include a plurality of transistors in addition to the driving transistor and the switching transistor. Further, control lines for controlling the added transistors may be additionally required. As such, when transistors and control lines for controlling the transistors are added to the pixel, the area occupied by the pixel increases, making it difficult to increase pixel density resolution of the organic light emitting display device. 
     SUMMARY 
     According to one or more embodiments, a pixel circuit to be connected to a data line and first and second power supply lines includes: a light emitting element connected between the first power supply line and the second power supply line; a driving transistor controlling a current flowing from the first power supply line to the second power supply line through the light emitting element according to a voltage of a first node; a first switching element connected between the first node and a second node; a second switching element connected between the second node and a third node; a first capacitor connected between the first power supply line and the first node; and a second capacitor connected between the second node and the data line. 
     According to one or more embodiments, a display device includes: a first power supply line; a second power supply line; a data line; a pixel including a first switching element connected between a first node and a second node, a second switching element connected between the second node and a third node, a driving transistor controlling a current flowing from the first power supply line to the third node according to a voltage of the first node, a light emitting element connected between the third node and the second power supply line, a first capacitor connected between the first power supply line and the first node, and a second capacitor connected between the second node and the data line; and a controller controlling the first and second switching elements, the first and second power supply lines, and the data line, during one frame period including first to seventh sequential periods. 
     According to one or more embodiments, an organic light emitting display device includes: a pixel connected to a first power supply line, a second power supply line, a scan line, a control line, and a data line; and a driver controlling the first power supply line, the second power supply line, the scan line, the control line, and the data line, during first to seventh sequential periods, wherein the pixel includes: an organic light emitting diode having a first electrode and a second electrode connected to the second power supply line; a first transistor having a gate electrode, a first electrode connected to the first power supply line, and a second electrode connected to the first electrode of the organic light emitting diode; a second transistor having a control electrode connected to the scan line, a first electrode connected to the gate electrode of the first transistor, and a second electrode; a third transistor having a control electrode connected to the control line, a first electrode connected to the second electrode of the second transistor, and a second electrode connected to the second electrode of the first transistor; a first capacitor connected between the first power supply line and the gate electrode of the first transistor; and a second capacitor connected between the second electrode of the second transistor and the data line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates a schematic block diagram of an organic light emitting display device according to an embodiment; 
         FIG. 2  illustrates a circuit diagram of a pixel according to an embodiment; 
         FIG. 3  illustrates a timing chart for driving the pixel of  FIG. 2  according to an embodiment; 
         FIG. 4  illustrates a timing chart for driving the pixel of  FIG. 2  according to another embodiment; 
         FIG. 5  illustrates a perspective view of a head-mounted display, which is an example of a display device according to an embodiment; 
         FIG. 6  illustrates a view showing use of the head-mounted display of  FIG. 5 ; and 
         FIG. 7  illustrates a partial exploded perspective view of the head-mounted display of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the exemplary embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of”, when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     Hereinafter, the present disclosure will be described in detail by explaining exemplary embodiments of the present disclosure with reference to the attached drawings. Like reference numerals in the drawings denote like elements, and thus their description will be omitted. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Further, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be understood that the terms such as “include,” “comprise,” and “have” used herein specify the presence of stated features or components, but do not preclude the presence or addition of one or more other features or components. It will be further understood that when a layer, region, or component is referred to as being “on” another layer, region, or component, it can be directly or indirectly on the other layer, region, or component. That is, for example, intervening layers, regions, or components may be present. 
       FIG. 1  is a schematic block diagram of an organic light emitting display device according to an embodiment. Referring to  FIG. 1 , an organic light emitting display device.  100  may include a display unit  110 , a scan driver  120 , a data driver  130 , a timing controller  140 , a voltage generator  150 , and a control driver  160 . 
     The display unit  110  may include a plurality of pixels PX. Although only one pixel PX is shown in  FIG. 1 , this is for just for ease of understanding. The pixels PX may be arranged, in an embodiment, in a matrix. 
     The pixels PX may be connected to scan lines SL 1  to SLn and data lines DL 1  to DLm. The scan lines SL 1  to SLn may transmit scan signals S 1  to Sn output from the scan driver  120  to the pixels PX in the same row, respectively. The data lines DL 1  to DLm may transmit data signals D 1  to Dm output from the data driver  130  to the pixels PX in the same column, respectively. The pixel PX may be connected to the scanning line SL located in the same row among the scanning lines SL 1  to SLn and may be connected to the data line DL located in the same column among the data lines DL 1  to DLm. 
     The pixels PX may be commonly connected to a control line CL and first and second power supply lines PL 1  and PL 2 . The control line CL and the first and second power supply lines PL 1  and PL 2  may be driven by the control driver  160 . 
     The control line CL may include a plurality of sub control lines connected to the pixels PX in the matrix. The sub control lines may extend in the row direction in parallel with the scan lines SL 1  to SLn. The scan lines SL 1  to SLn may transmit the scan signals S 1  to Sn to the pixels PX at different timings, but all of the sub control lines may transmit a control signal GC to the pixels PX at the same timing. The sub control lines may all be electrically connected to each other. The electrically connected sub control lines may be collectively referred to as a control line CL. 
     The first power supply line PL 1  may include a plurality of sub power supply lines connected to the pixels PX in the matrix. The sub power supply lines may extend in the column direction in parallel with the data lines DL 1  to DLm. According to another embodiment, the sub power supply lines may extend in the row direction in parallel with the scan lines SL 1  to SLn. The sub power supply lines may be all electrically connected to each, and may have the same timing and varying voltage levels. The electrically connected sub power supply lines may be collectively referred to as a first power supply line PL 1 . The voltage applied to the first power supply line PL 1  may be varied within one frame period and is referred to as a first power supply voltage PV 1 . The first power supply voltage PV 1  may have two different levels, that is, a first level and a second level. The first power supply voltage PV 1  of the first level may be referred to as a first level voltage PV 1 _ h , and the first power supply voltage PV 1  of the second level may be referred to as a second level voltage PV 1 _ l . The first level voltage PV 1 _ h  may be greater than the second level voltage PV 1 _ l.    
     The second power supply line PL 2  may be commonly connected to the light emitting elements of the pixels PX in the form of a common electrode. The voltage applied to the second power supply line PL 2  may be varied within one frame period and is referred to as a second power supply voltage PV 2 . The second power supply voltage PV 2  may have two different levels, that is, a third level and a fourth level. The second power supply voltage PV 2  of the third level may be referred to as a third level voltage PV 2 _ h , and the second power supply voltage PV 2  of the fourth level may be referred to as a fourth level voltage PV 2 _ l . The third level voltage PV 2 _ h  may be greater than the fourth level voltage PV 2 _ l.    
     According to an embodiment, the first level voltage PV 1 _ h  applied to the first power supply line PL 1  may be substantially equal to the third level voltage PV 2 _ h  applied to the second power supply line PL 2 . In this case, the first level voltage PV 1 _ h  and the third level voltage PV 2 _ h  may be generated from a high level voltage PVh. Further, the second level voltage PV 1 _ l  applied to the first power supply line PL 1  may be substantially equal to the third level voltage PV 2 _ l  applied to the second power supply line PL 2 . In this case, the second level voltage PV 1 _ l  and the fourth level voltage PV 2 _ l  may be generated from a low level voltage PV 1 . The high level voltage PVh and the low level voltage PV 1  may be referred to as a first driving voltage ELVDD and a second driving voltage ELVSS, respectively. 
     The pixel PX may include a light emitting element and a driving transistor for controlling the amount of a current flowing to the light emitting element based on a data voltage Vdata of the received data signal D. The data signal D may be transmitted from the data driver  130  through the corresponding data line DL, and may include a reference voltage Vref and a data voltage Vdata. The light emitting element may emit light at a luminance determined based on the data voltage Vdata. 
     When a unit pixel includes a plurality of subpixels for displaying a full color, the pixel PX may correspond to a part of the unit pixel, i.e., a subpixel. The light emitting element may be an organic light emitting diode. The pixel PX will be described in more detail below with reference to  FIGS. 2 and 3 . 
     The voltage generator  150  may generate voltages used for the operations of the scan driver  120  and the control driver  160 . In an embodiment, the voltage generator  150  may generate the first level voltage PV 1 _ h  and the second level voltage PV 1 _ l  applied to the first power supply line PL 1  and the third level voltage PV 2 _ h , and the fourth level voltage PV 2 _ l  applied to the second power supply line PL 2 , and may provide these generated voltages to the control driver  160 . The first level voltage PV 1 _ h  and the fourth level voltage PV 2 _ l  may be voltages applied to the first power supply line PL 1  and the second power supply line PL 2  during a light emission period in which the light emitting element emits light. The second level voltage PV 1 _ l  may be a voltage applied to the first power supply line PL 1  during at least a part of a non-light emission period in which the light emitting element does not emit light. The third level voltage PV 2 _ h  may be a voltage applied to the second power supply line PL 2  during the non-light emission period. 
     According to another embodiment, when the first level voltage PV 1 _ h  and the third level voltage PV 2 _ h  are generated from the high level voltage PVh and the second level voltage PV 1 _ l  and the fourth level voltage PV 2 _ l  are generated from the low level voltage PV 1 , the voltage generator  150  may generate the high level voltage PVh and the low level voltage PV 1  and provide these generated voltages to the control driver  160 . 
     The voltage generator  150  may generate a turn-off voltage Voff and a turn-on voltage Von for controlling a switching element e.g., a switching transistor, of the pixel PX, and provide these generated voltages to the scan driver  120  and the control driver  160 . When the turn-off voltage Voff is applied to the gate electrode of the switching transistor, the switching transistor is turned off, and when the turn-on voltage Von is applied to the gate electrode of the switching transistor, the switching transistor is turned on. When the switching transistor is a p-type metal-oxide semiconductor field effect transistor (MOSFET), the level of the turn-off voltage Voff may be higher than the level of the turn-on voltage Von. When the switching transistor is an n-type MOSFET, the level of the turn-off voltage Voff may be lower than the level of the turn-on voltage Von. 
     The voltage generator  150  may generate voltages at other levels in addition to the above-described voltages and provide these generated voltages to the scan driver  120  and the control driver  160 . Further, the voltage generator  150  may generate gamma reference voltages and provide these gamma reference voltages to the data driver  130 . 
     The timing controller  140  may control operation timings of the scan driver  120 , the data driver  130 , and the control driver  160  to control the pixels PX of the display unit  110 . Each of the pixels PX may receive a new data voltage Vdata for each frame and emit light at a luminance corresponding to the received data voltage Vdata, so that the display unit  110  may display an image corresponding to image data RGB of one frame. According to an embodiment, one frame period may include a plurality of periods, e.g., a light-off period, first to third initiation periods, a compensation period, a data writing period, and a light emission period. According to an embodiment, all the pixels PX in the display unit  110  may emit light at the same time. According to another embodiment, when the display unit  110  is divided into a plurality of regions, e.g., a region for displaying an image for a left eye and a region for displaying an image for a right eye, the pixels PX in each region may emit light at the same time. 
     The timing controller  140  may receive a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a data enable signal DE, a clock signal CLK, and image data RGB from the outside. The timing controller  140  may control the operation timings of the scan driver  120 , the data driver  130 , and the control driver  160  by using timing signals such as a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a data enable signal DE, and a clock signal CLK. The timing controller  140  may determine the frame period by counting the data enable signal DE of one horizontal scanning period, and in this case, the vertical synchronization signal Vsync and the horizontal synchronization signal Hsync supplied from the outside may be omitted. The image data (RGB) includes luminance information of the pixels PX. The luminance is a predetermined gray number of, e.g., 1024 (=2 10 ), 256 (=2 8 ), or 64 (=2 6 ). 
     The timing controller  140  may generate control signals including a first gate timing control signal GDC 1  for controlling the operation timing of the scan driver  120 , a data timing control signal DDC for controlling the operation timing of the data driver  130 , and a second gate timing control signal GDC 2  for controlling the operation timing of the control driver  160 . 
     The first gate timing control signal GDC 1  may include a gate start pulse GSP, a gate shift clock GSC, and a gate output enable signal GOE. The gate start pulse GSP is supplied to the scan driver  120  generating the first scan signal at the start of the scan period. The gate shift clock GSC is a clock signal commonly input to the scan driver  120 , and is a clock signal for shifting the gate start pulse GSP. The gate output enable GOE signal controls the output of the scan driver  120 . 
     The data timing control signal DDC may include a source start pulse SSP, a source sampling clock SSC, and a source output enable SOE signal. The source start pulse SSP controls the data sampling start time of the data driver  130  and is provided to the data driver  130  at the start time of the scan period. The source sampling clock SSC is a clock signal for controlling the sampling operation of data in the data driver  130  on the basis of a rising or falling edge. The source output enable signal SOE may control the output of the data driver  130 . The source start pulse SSP supplied to the data driver  130  may be omitted depending on a data transmission method. 
     The second gate timing control signal GDC 2  may be provided to the control driver  160  to distinguish a plurality of periods in each frame period. 
     The scan driver  120  may generate the scan signals S 1  to Sn in response to the first gate timing control signal GDC 1  supplied from the timing controller  140  by using the turn-on voltage Von and the turn-off voltage Voff provided from the voltage generator  150 . The scan driver  120  may provide the scan signals S 1  to Sn to the pixels PX through the scan lines SL 1  to SLn. According to an embodiment, the scan driver  120  may apply the turn-on voltage Von to the scan lines SL 1  to SLn during the third initialization period and the compensation period. The scan driver  120  may sequentially apply the turn-on voltage Von to the scan lines SL 1  to SLn during the data writing period. The scan driver  120  may apply the turn-off voltage Voff to the scan lines SL 1  to SLn during the remaining period. 
     The data driver  130  may sample and latch the digital data signal RGB supplied from the timing controller  140  in response to the data timing control signal DDC supplied from the timing controller  140  to convert the digital data signal RGB into data of a parallel data system. When the data driver  130  converts the digital data signal RGB into the data of the parallel data system, the data driver  130  converts the digital data signal RGB into a gamma reference voltage and converts the gamma reference voltage into an analog data voltage. The data driver  130  provides the data voltage Vdata to the pixels PX of the display unit  110  through the data lines DL 1  to DLm. The pixels PX may receive the data voltage Vdata in response to the scanning signal S. Further, the data driver  130  provides the reference voltage Vref to the pixels PX of the display unit  110  through the data lines DL 1  to DLm. 
     The data driver  130  may output the reference voltage Vref to the data lines DL 1  to DLm according to the data timing control signal DDC during at least a part of the period. The data driver  130  may output different data voltages Vdata to the data lines DL 1  to DLm according to the data signals RGB during the data writing period. The data driver  130  may output the same reference voltage Vref to the data lines DL 1  to DLm during a certain period. 
     The control driver  160  drives the first and second power supply lines PL 1  and PL 2  and the control line CL in response to the second gate timing control signal GDC 2  supplied from the timing controller  140  by using voltages having different levels and provided from the voltage generator  150 . For example, the control driver  160  may drive the first power supply line PL 1  using the first level voltage PV 1 _ h  and the second level voltage PV 1 _ l , may drive the second power supply line PL 2  using the third level voltage PV 2 _ h  and the fourth level voltage PV 2 _ l , and may drive the control line CL using the turn-on voltage Von and the turn-off voltage Voff. 
     According to an embodiment, the control driver  160  may apply the second level voltage PV 1 _ l  to the first power supply line PL 1  during the first to third initialization periods, and may apply the first level voltage PV 1 _ h  to the first power supply line PL 1  during the remaining period. The control driver  160  may apply the third level voltage PV 2 _ h  to the second power supply line PL 2  during the non-light emission period, and may apply the fourth level voltage PV 2 _ l  to the second power supply line PL 2  during the light emission period. However, this is illustrative, and the control driver  160  may apply voltages of different levels to the first and second power supply lines PL 1  and PL 2  in response to the second gate timing control signal GDC 2 . The control driver  160  may apply the turn-on voltage Von to the control line CL during the second and third initialization periods and the compensation period, and may apply the turn-off voltage Voff to the control line CL during the remaining period. 
     Although it is described in the present embodiment that the control driver  160  drives both the first and second power supply lines PL 1  and PL 2  and the control line CL, the control driver  160  may be divided into a first control driver for driving the control line CL and a second control driver for driving the first and second power supply lines PL 1  and PL 2 . According to another embodiment, the first and second power supply lines PL 1  and PL 2  may be directly driven by the voltage generator  150 , and the control line CL may be driven by the scan driver  120 . In the present specification, the control driver  160  integrally refers to a component driving the first and second power supply lines PL 1  and PL 2  and a component driving the control line CL. 
     In the present specification, the component driving or controlling the first and second power supply lines PL 1  and PL 2 , the data line DL, the scanning line SL, and the control line CL is referred to as a controller or a driver. The controller or the driver may include at least one of the scan driver  120 , the data driver  130 , the timing controller  140 , the voltage generator  150 , and the control driver  160 . For example, the controller or the driver may collectively refer to the scan driver  120 , the data driver  130 , and the control driver  160 . 
     The organic light emitting display device  100 , which is a device for displaying an image, may be a portable device including a high-resolution display unit, for example, a smart phone or a head-mounted display. The organic light emitting display device  100  may be a television or a monitor having a large screen. The organic light emitting display device  100  according to the present embodiment may be used to implement an ultra-high resolution display panel having a resolution of about 1200 ppi (pixels per inch), for example, about 1600 ppi. 
       FIG. 2  is a circuit diagram of a pixel according to an embodiment. Referring to  FIG. 2 , a pixel PXij includes a light emitting element OLED, first to third transistors M 1  to M 3 , and first and second capacitors Cst and Cpr. The pixel PXij has first to third nodes N 1  to N 3 . The pixel PXij is connected to a scanning line SLi located in the same row among the scanning lines SL 1  to SLn and receives a scanning signal Si from the scanning driver  120 . The pixel PXij is connected to a data line DLj located in the same column among the data lines DL 1  to DLm and receives a data signal Dj from the data driver  130 . The pixel PXij is connected to the control line CL and the first and second power supply lines PL 1  and PL 2 , and receives the control signal GC and the first and second power supply voltages PV 1  and PV 2  from the control driver  160 . 
     The first transistor M 1  may operate as a driving transistor for controlling a current flowing through the organic light emitting element OLED. The first transistor M 1  may be referred to as a driving transistor. The second transistor M 2  and the third transistor M 3  may be turned on or turned off according to the voltage applied to the gate electrode, i.e., a gate voltage, thereby performing a switching function. The second and third transistors M 2  and M 3  may be referred to as first and second switching elements, or as first and second switching transistors, respectively. 
     Although the first to third transistors M 1  to M 3  are shown as being p-type MOSFETs, this is illustrative, and at least one of the first to third transistors M 1 -M 3  may be a different conductive type (n-type) transistor. According to an embodiment, the first transistor M 1  may be an n-type MOSFET. In this case, the anode of the light emitting element OLED may be connected to the second power supply line PL 2 , and the cathode thereof may be connected to the first transistor M 1 . Further, the voltage level applied to the second power supply line PL 2  when the light emitting element OLED emits light may be higher than the voltage level applied to the first power supply line PL 1 . According to another embodiment, the second and third transistors M 2  and M 3  may be n-type MOSFETs. According to still another embodiment, the first to third transistors M 1  to M 3  may all be n-type MOSFETs. 
     The light emitting element OLED may be connected between the first power supply line PL 1  and the second power supply line PL 2 . The light emitting element OLED may be connected to the first power supply line PL 1  through the first transistor M 1 . The light emitting element OLED may be an organic light emitting diode. The light emitting element OLED may be an organic light emitting diode having an anode connected to the third node N 3  and a cathode connected to the second power supply line PL 2 . 
     The first transistor M 1  may be a driving transistor for controlling the current flowing from the first power supply line PL 1  to the second power supply line PL 2  through the light emitting element OLED according to the voltage of the first node N 1 . The first transistor M 1  may have a gate electrode connected to the first node N 1  and may be connected between the first power supply line PL 1  and the third node N 3 . For example, the first transistor M 1  may have a source electrode connected to the first power supply line PL 1  and a drain electrode connected to the third node N 3 . The current controlled by the first transistor M 1  is supplied to the light emitting element OLED during the light emitting period, and the light emitting element OLED emits light with a luminance corresponding to the intensity of the current. 
     The second transistor M 2  may be a first switching element connected between the first node N 1  and the second node N 2  to connect or disconnect the first node N 1  and the second node N 2 . The second transistor M 2  may be controlled by a scan signal Si provided from the scan line SLi. The second transistor M 2  may have a gate electrode connected to the scan line SLi, a first electrode connected to the first node N 1 , and a second electrode connected to the second node N 2 . When the turn-on voltage Von is applied to the gate electrode of the second transistor M 2  through the scan line SLi, the second transistor M 2  may be turned on to connect the first node N 1  and the second node N 2  to each other. When the turn-off voltage Voff is applied to the gate electrode of the second transistor M 2 , the second transistor M 2  may be turned off to isolate the first node N 1  and the second node N 2  from each other. 
     The third transistor M 3  may be a second switching element connected between the second node N 2  and the third node N 3  to connect or disconnect the second node N 2  and the third node N 3 . The third transistor M 3  may be controlled by a control signal GC provided from the control line CL. The third transistor M 3  may have a gate electrode connected to the control line CL, a first electrode connected to the second node N 2 , and a second electrode connected to the third node N 3 . When the turn-on voltage Von is applied to the gate electrode of the third transistor M 3  through the control line CL, the third transistor M 3  may be turned on to connect the second node N 2  and the third node N 3  to each other. When the turn-off voltage Voff is applied to the gate electrode of the third transistor M 3 , the third transistor M 3  may be turned off to isolate the second node N 2  and the third node N 3  from each other. 
     The first capacitor Cst may be connected between the first power supply line PL 1  and the first node N 1 . The first capacitor Cst may be connected between the gate electrode and the source electrode of the first transistor M 1 . The first capacitor Cst may maintain the gate voltage of the first transistor M 1  during the light emitting period. Since the voltage between the gate electrode and the source electrode of the first transistor M 1  is kept constant by the first capacitor Cst even if the voltage level of the first power supply line PL 1  fluctuates, the current output from the first transistor M 1  may be constant. Although the voltage level of the first power supply line PL 1  may be lowered according to the amount of current consumed by the pixels PX of the display unit  110 , the first capacitor Cst maintains a constant voltage between the gate electrode and the source electrode of the first transistor M 1 , so that the luminance of the light emitted by the light emitting element OLED may be kept constant. Accordingly, the brightness uniformity of the display unit  110  can be increased. 
     The second capacitor Cpr may be connected between the data line DLj and the second node N 2 . The data voltage Vdata of the data signal Dj transmitted through the data line DLj may be transmitted to the first node N 1  through the second capacitor Cpr and the second transistor M 2 . The capacitance of the second capacitor Cpr may be larger than that of the first capacitor Cst. For example, the capacitance of the second capacitor Cpr may be about two to three times the capacitance of the first capacitor Cst. 
       FIG. 3  is a timing chart for driving the pixel of  FIG. 2  during one frame period. Referring to  FIG. 3  together with  FIG. 2 , first and second power supply voltages PV 1  and PV 2 , a control signal GC, first to nth scan signals S 1  to Sn, and a data signal Dj are shown. One frame period includes a plurality of periods. The plurality of periods may include first to seventh periods T 1  to T 7 . The first to seventh periods T 1  to T 7  may be sequential. However, according to some embodiments, some periods (for example, third and fourth periods T 3  and T 4 ) of the first to seventh periods T 1  to T 7  may be repeated a plurality of times. Further, the first to seventh periods T 1  to T 7  do not proceed continuously, and other periods may be further included. For example, a period during which the third transistor M 3  is turned off by the control line CL may be included between the fifth period T 5  and the sixth period T 6 . 
     The first period T 1  may be referred to as a light-off period T 1 . The second to fourth periods T 2  to T 4  may be referred to as first to third initialization periods. The fifth period T 5  may be referred to as a compensation period. The sixth period T 6  may be referred to as a data writing period. The seventh period T 7  may be referred to as a light emission period. The first to sixth periods T 1  to T 6  may be included in the non-light emission period during which the light emitting element OLED does not emit light, and the seventh period T 7  may be included in the light emission period during which the light emitting element OLED emits light. 
     The control driver  160  shown in  FIG. 1  applies the first and second power supply voltages PV 1  and PV 2  to the first and second power supply lines PL 1  and PL 2  as shown in  FIG. 3 . Further, the control driver  160  outputs the control signal GC to the control line CL as shown in  FIG. 3 . The scan driver  120  outputs the first to nth scan signals S 1  to Sn to the first to nth scan lines SL 1  to SLn as shown in  FIG. 3 . The data driver  130  outputs the reference voltage Vref and the data voltage Vdata as the data signal Dj to the data line DLj according to the data timing control signal DDC as shown in  FIG. 3 . 
     The control driver  160  may apply the first level voltage PV 1 _ h  to the first power supply line PL 1  during the first, fifth, sixth, and seventh periods T 1  and T 5  to T 7 , and may apply the second level voltage PV 1 _ l  to the first power supply line PL 1  during the second to fourth periods T 2  to T 4 . The second level voltage PV 1 _ l  may be lower than the first level voltage PV 1 _ h . Alternatively, when the first transistor M 1  is an n-type MOSFET, the second level voltage PV 1 _ l  may be higher than the first level voltage PV 1 _ h.    
     The control driver  160  may apply the third level voltage PV 2 _ h  to the second power supply line PL 2  during the first to sixth periods T 1  to T 6 , and may apply the fourth level voltage PV 2 _ l  to the second power supply line PL 2  during the seventh period T 7 . The fourth level voltage PV 2 _ l  may be lower than the third level voltage PV 2 _ h . Alternatively, when the first transistor M 1  is an n-type MOSFET, the fourth level voltage PV 2 _ l  may be higher than the third level voltage PV 2 _ h.    
     The control driver  160  may output a turn-off voltage Voff for turning off the third transistor M 3  during the first, second, sixth, and seventh periods T 1 , T 2 , T 6 , and T 7  to the control line CL, and may output a turn-on voltage Von for turning on the third transistor M 3  during the third to fifth periods T 3  to T 5  to the control line CL. 
     The scan driver  120  may output a turn-off voltage Voff for turning off the second transistor M 2  during the first, second, third, and seventh periods T 1  to T 3  and T 7  to the scan line SLi, and may output a turn-on voltage Von for turning on the second transistor M 2  during the fourth and fifth periods T 4  and T 5  to the scan line SLi. The scan driver  120  may temporarily output a pulse-like turn-on voltage Von in synchronization with the data voltage Vdata output to the data line DLj during the sixth period T 6 . The scan driver  120  may sequentially apply the pulse-like turn-on voltage Von to the scan lines SL 1  to SLn during the sixth period T 6 . The scan driver  120  may apply a turn-off voltage Voff to the scan lines SL 1  to SLn during the time when the turn-on voltage Von is not applied to the scan lines SL 1  to SLn during the sixth period T 6 . 
     The data driver  130  may output the data voltage Vdata to the data line DLj in synchronization with the pulse-like turn-on voltage Von sequentially applied to the scan lines SL 1  to SLn. For example, when the turn-on voltage Von is applied to the scan lines SL 1  to SLn and the turn-off voltage Voff is applied thereto, e.g., when the scan signals S 1  to Sn have rising edges, the data driver  130  may be in a state of outputting the data voltage Vdata to the data line DLj. Here, the data voltage Vdata refers to a data voltage received by the pixel PXij. 
     The data driver  130  may apply the data voltage Vdata to the data line DLj during the sixth period T 6 , and may apply the reference voltage Vref during at least the fourth and fifth periods T 4  and T 5 . Here, the data voltage Vdata includes the data voltage received by the pixel PXij, and collectively refers to the data voltages respectively received by the plurality of pixels PX connected to the data line DLj. The data line DLj may be in a high-impedance state when the data voltage Vdata or the reference voltage Vref is not applied. According to another embodiment, as shown in  FIG. 3 , the data driver  130  may apply the data voltage Vdata to the data line DLj during the sixth period T 6 , and may apply the reference voltage Vref to the data line DLj during the first to fifth and seventh periods T 1  to T 5  and T 7 . 
     During the seventh period T 7 , i.e, the light emission period, the first level voltage PV 1 _ h  is applied to the first power supply line PL 1 , and the fourth level voltage PV 2 _ l  is applied to the second power supply line PL 2 _ l . Further, the second and third transistors M 2  and M 3  are turned off, so that the first node N 1  and the second node N 2  are electrically isolated from each other and the second node N 2  and the third node N 3  are electrically isolated from each other. When the first transistor M 1  is a p-type MOSFET as shown in  FIG. 2 , the first level voltage PV 1 _ l  may be higher than the fourth level voltage PV 2 _ l . The first transistor M 1  can control the amount of a current flowing from the first power supply line PL 1  to the second power supply line PL 2  through the light emitting element OLED according to the gate voltage, that is, the voltage of the first node N 1 . Here, the current flowing through the light emitting element OLED may be referred to as a driving current output from the first transistor M 1 . 
     Hereinafter, it is assumed that the first transistor M 1  is a p-type MOSFET. However, when the first transistor M 1  is an n-type MOSFET, the timing chart of  FIG. 3  may be modified within the scope of the present disclosure and applied with the same principle. 
     When the first period T 1 , i.e., the light-off period, starts, the third level voltage PV 2 _ h  is applied to the second power supply line PL 2 . The third level voltage PV 2 _ h  may be continuously applied to the second power supply line PL 2  from the start of the first period T 1  until the end of the sixth period T 6 . During the first period T 1  following the seventh period T 7 , the first level voltage PV 1 _ h  is applied to the first power supply line PL 1 , and the second and third transistors M 2  and M 3  are maintained in the turned-off state. The third level voltage PV 2 _ h  applied to the second power supply line PL 2  may be substantially the same level as the first level voltage PV 1 _ h  applied to the first power supply line PL 1 . For example, the difference between the third level voltage PV 2 _ h  and the first level voltage PV 1 _ h  may be smaller than the threshold voltage of the light emitting element OLED. Accordingly, substantially no current may flow between the first power supply line PL 1  and the second power supply line PL 2 , and the light emitting element OLED may not emit light any more. 
     According to another embodiment, the level of the third level voltage PV 2 _ h  may be higher than the level of the first level voltage PV 1 _ h . Further, the voltage level of the third node N 3  is raised by the light emitting element capacitor Coled by a second voltage difference (referred to as “ΔV 2 ”) between the third level voltage PV 2 _ h  and the fourth level voltage PV 2 _ l . The second voltage difference ΔV 2  is defined as an absolute value of a voltage difference between the third level voltage PV 2 _ h  and the fourth level voltage PV 2 _ l . Since the light emitting element OLED functions not only as a light emitting diode, but also as a capacitor having a capacitance, the light emitting element OLED may be modeled as a light emitting diode and a light emitting element capacitor Coled connected in parallel with each other. The light emitting element capacitor Coled indicates a capacitance component of the light emitting element OLED. 
     When the second period T 2 , i.e., the first initiation period, starts the second level voltage PV 1 _ l  is applied to the first power supply line PL 1 . The second level voltage PV 1 _ l  may be continuously applied to the first power supply line PL 1  from the start of the second period T 2  until the end of the fourth period T 4 . During the second period T 2  following the first period T 1 , the third level voltage PV 2 _ h  is applied to the second power supply line PL 2 , and the second and third transistors M 2  and M 3  are maintained in the turned-off state. The level of the second level voltage PV 1 _ l  applied to the first power supply line PL 1  may be lower than the level of the third level voltage PV 2 _ h  applied to the second power supply line PL 2 . 
     As the voltage level of the first power supply line PL 1  is lowered by a first voltage difference (referred to as “ΔV 1 ”) between the first level voltage PV 1 _ h  and the second level voltage PV 1 _ l , the voltage level of the first node N 1  is also lowered by the first voltage difference ΔV 1  by the first capacitor Cst between the first power supply line PL 1  and the first node N 1 . The first voltage difference ΔV 1  is defined as an absolute value of a voltage difference between the first level voltage PV 1 _ h  and the second level voltage PV 1 _ l . Accordingly, the first transistor M 1  is turned on and current flows from the third node N 3  to the first power supply line PL 1 , i.e., in a reverse direction. Since the voltage level of the first node N 1  lowered by the first voltage difference ΔV 1  is sufficiently lower than the voltage level of the third node N 3  raised by the second voltage difference ΔV 2 , the first transistor M 1  is fully turned on. Since the first transistor M 1  is fully turned on in the reverse direction, hysteresis characteristics, in which the intensity of the driving current output from the first transistor M 1  in the previous frame affects the intensity of the driving current output from the first transistor M 1  in the current frame, may be reduced or eliminated. 
     Further, the voltage level of the third node N 3  is lowered to approximately the level of the second level voltage PV 1 _ l . Specifically, if the first transistor M 1  is turned on during the light emission period of the previous frame, a current can flow through the first transistor M 1  until the voltage level of the third node N 3  is lowered to the level of the second level voltage PV 1 _ l , so that the voltage level of the third node N 3  becomes equal to the level of the second level voltage PV 1 _ l . If the first transistor M 1  is turned off and the light emitting element OLED does not emit light during the light emission period of the previous frame, the first transistor M 1  is turned on in the reverse direction due to the voltage level of the third node N 3  which is raised by the second voltage difference ΔV 2 , but the first transistor M 1  is turned off before the voltage level of the third node N 3  is lowered to the level of the second level voltage PV 1 _ l . The voltage level of the third node N 3  may be slightly higher than the level of the second level voltage PV 1 _ l . Therefore, the voltage level of the third node N 3  becomes lower than the third level voltage PV 2 _ h  applied to the second power supply line PL 2  during the second period T 2 , so that the third node N 3  is initialized, and the hysteresis characteristics of the transistor M 1  can be reduced or eliminated. 
     When the third period T 3 , i.e., the second initialization period, starts, the third transistor M 3  is turned on. The third transistor M 3  may be turned on from the start of the third period T 3  to the end of the fifth period T 5 . During the third period T 3  following the second period T 2 , the second level voltage PV 1 _ l  is applied to the first power supply line PL 1 , the third level voltage PV 2 _ h  is applied to the second power supply line PL 2 , and the second transistor M 2  is maintained in the turned-off state. 
     When the third transistor M 3  is turned on, the second node N 2  and the third node N 3  are connected to each other, and the voltage level of the second node N 2  becomes equal to the voltage level of the third node N 3 . The voltage level of the second node N 2  is also lowered to about the level of the second level voltage PV 1 _ l  applied to the first power supply line PL 1  by the first transistor M 1  turned on in the reverse direction. Since the voltage level of the second node N 2  is lowered during the third period T 3 , the second node N 2  may be initialized. 
     When the fourth period T 4 , i.e., the third initialization period, starts, the second transistor M 2  is turned on. The second transistor M 2  may be turned on from the start of the fourth period T 4  to the end of the fifth period T 5 . During the fourth period T 4  following the third period T 3 , the second level voltage PV 1 _ l  is applied to the first power supply line PL 1 , the third level voltage PV 2 _ h  is applied to the second power supply line PL 2 , and the third transistor M 3  is maintained in the turned-on state. 
     When the second transistor M 2  is turned on, the first node N 1  and the second node N 2  are connected to each other, so that charges may be shared between the first capacitor Cst and the second capacitor Cpr. When the voltage of the first node N 1  after the sharing of charges between the first capacitor Cst and the second capacitor Cpr is lower than the voltage (PV 1 _ l −|Vth|) obtained by subtracting a threshold voltage (|Vth|) from the second level voltage PV 1 _ l  of the first power supply line PL 1 , the first transistor M 1  is turned on. Since the gate electrode and the source electrode of the first transistor M 1  are connected by the second and third transistors M 2  and M 3  in the turned-on state, the first transistor M 1  is diode-connected, and the voltage of the first node N 1  becomes equal to the voltage (PV 1 _ l −|Vth|) obtained by subtracting the threshold voltage (|Vth|) from the second level voltage PV 1 _ l . When the voltage of the first node N 1  after the sharing of charges between the first capacitor Cst and the second capacitor Cpr is not lower than the voltage (PV 1 _ l −|Vth|) obtained by subtracting a threshold voltage (|Vth|) from the second level voltage PV 1 _ l  of the first power supply line PL 1 , the first transistor M 1  is not turned on. Even in this case, the voltage of the first node N 1  may be lower than the second level voltage PV 1 _ l  of the first power supply line PL 1 . The threshold voltage (|Vth|) means an absolute value of the threshold voltage of the first transistor M 1 , and the threshold voltages |Vth| of the first transistor M 1  may be different from each other for each of the pixels PX for reasons such as manufacturing tolerances and the like. 
     Since the voltage of the first node N 1  becomes equal to the voltage of the second and third nodes N 2  and N 3 , such that the voltage level of the first node N 1  becomes lower than the voltage of the second level voltage PV 1 _ l  during the fourth period, the first node N 1  may be initialized. 
     The reference voltage Vref may be applied to the data line DLj at least before the end of the fourth period T 4 . The reference voltage Vref may be applied to the data line DLj from the start of the fourth period T 4 . According to another embodiment, the reference voltage Vref may be applied to the data line DLj from the start of the light emission period of the previous frame. 
     The reference voltage Vref may be applied to the data line DLj during the fifth period T 5  until the voltage of the first node N 1  becomes substantially equal to the voltage (PV 1 _ h −|Vth|) obtained by subtracting the threshold voltage (|Vth|) from the first level voltage PV 1 _ h . The reference voltage Vref may be applied to the data line DLj until the end of the fifth period T 5 . 
     When the fifth period T 5 , i.e., the compensation period, starts, the first level voltage PV 1 _ h  is applied to the first power supply line PL 1 . The first level voltage PV 1 _ h  may be continuously applied to the first power supply line PL 1  from the start of the fourth period T 4  to the end of the first period T 1  of the next frame. During the fifth period T 5  following the fourth period T 4 , the third level voltage PV 2 _ h  is applied to the second power supply line PL 2 , and the second and third transistors M 2  and M 3  are maintained in the turned-on state. The first level voltage PV 1 _ h  applied to the first power supply line PL 1  may be substantially equal to the third level voltage PV 2 _ h  applied to the second power supply line PL 2 . The voltage difference between the first level voltage PV 1 _ h  and the third level voltage PV 2 _ h  may be lower than the threshold voltage of the light emitting element OLED. The reference voltage Vref may be applied to the data line DLj during the fifth period T 5 . 
     As the voltage level of the first power supply line PL 1  is increased by the first voltage difference (referred to as “ΔV 1 ”) between the first level voltage PV 1 _ h  and the second level voltage PV 1 _ l , the voltage level of the first node N 1  is also increased by the first capacitor Cst connected between the first power supply line PL 1  and the first node N 1 . However, since the first node N 1  is connected to the second capacitor Cpr through the second node N 2  and is connected to the light emitting element capacitor Coled through the third node N 3 , the voltage level of the first node N 1  becomes lower than the first voltage difference ΔV 1 . For example, the voltage of the first node N 1  may be increased by a value obtained by multiplying the ratio of capacitance of the first capacitor Cst to the sum of capacitances of the first capacitor Cst, the second capacitor Cpr, and the light emitting element capacitor Coled by the first voltage difference ΔV 1 . 
     Since the sum of the capacitances of the second capacitor Cpr and the light emitting element capacitor Coled is greater than the capacitance of the first capacitor Cst, the voltage of the first node N 1  may become significantly lower than the voltage (PV 1 _ h −|Vth|) obtained by subtracting the threshold voltage (|Vth|) from the first level voltage PV 1 _ h . Accordingly, the first transistor M 1  may be fully turned on, and a current may flow from the first power supply line PL 1  to the third node N 3 , i.e., in a forward direction. Since the first transistor M 1  having been fully turned on in the reverse direction during the second period T 2  is fully turned on in the forward direction during the fifth period T 5 , the hysteresis characteristics of the first transistor M 1  can be reduced or eliminated. 
     Since the gate electrode and the source electrode of the first transistor M 1  in the turned-on state are connected by the second and third transistors M 2  and M 3  in the turned-on state, the first transistor M 1  is diode-connected, and the voltage of the first node N 1  becomes equal to the voltage obtained by subtracting the threshold voltage (|Vth|) from the first level voltage PV 1 _ h . Accordingly, charges corresponding to the threshold voltage (|Vth|) may be stored between both electrodes of the first capacitor Cst. Charges corresponding to the threshold voltage (|Vth|) is stored between both electrodes of the first capacitor Cst in order to compensate the threshold voltage (|Vth|) of the first transistor M 1  during the fifth period T 5 . 
     The voltage of the second node N 2  also becomes equal to the voltage (PV 1 _ h −|Vth|) obtained by subtracting the threshold voltage (|Vth|) from the first level voltage PV 1 _ h . Since the reference voltage Vref is applied to the data line DLj, the charges corresponding to Vref−PV 1 _ h +|Vth| may be stored between both electrodes of the second capacitor Cpr. 
     The voltage of the third node N 2  also becomes equal to the voltage (PV 1 _ h −|Vth|) obtained by subtracting the threshold voltage (|Vth|) from the first level voltage PV 1 _ h . At this time, the voltage of the third node N 3  may be lower than the third level voltage PV 2 _ h  of the second power supply line PL 2 . 
     The fifth period T 5  may be finished while the second transistor M 2  is turned off. The third transistor M 3  may be turned off before the sixth section T 6  starts after the second transistor M 2  is turned off. According to another embodiment, the second transistor M 2  and the third transistor M 3  may be turned off at the end of the fifth period T 5 . According to still another embodiment, the third transistor T 3  may be turned off at the end of the fifth period T 5 , only the second transistors T 2  of the pixels PX connected to the second to the nth scanning lines SL 2  to SLn may be turned off, and the second transistor T 2  of the pixel PX connected to the first scanning line SL 1  may be maintained in the turned-on state, e.g., the sixth period T 6  would immediately follow the fifth period T 5 . 
     During the sixth period T 6 , i.e., the data writing period, following the fifth period T 5 , the first level voltage PV 1 _ h  is applied to the first power supply line PL 1 , the third level voltage PV 2 _ h  is applied to the second power supply line PL 2 , and the third transistor M 3  is maintained in the turned-off state. A pulse-like turn on voltage Von may be applied to the scan lines SL 1  to SLn in a preset order over the sixth period T 6 . The data voltage Vdata may be applied to the data line DLj in synchronization with the pulse-like turn on voltage Von applied to the scan lines SL 1  to SLn in the preset order. Here, the data voltage Vdata refers to data voltages respectively received by the plurality of pixels PX connected to the data line DLj. 
     The second transistor M 2  of the pixel PXij is turned on in response to the scan signal Si transmitted through the ith scan line Sli, i.e., when the turn-on voltage Von is applied to the ith scan line SLi. A data voltage Vdata corresponding to the pixel PXij may be applied to the data line DLj. The data voltage Vdata refers to a data voltage received by the pixel PXij among the plurality of pixels PX connected to the data line DLj. 
     The second node N 2  is connected to the first node N 1  through the second transistor M 2  in the turned-on state, and is electrically isolated from the third node N 3  by the third transistor M 3  in the turned-off state. Since the second node N 2  is connected with the first node N 1 , the voltage fluctuation of the data line DLj causes the voltage fluctuation of the first node N 1  through the charge sharing of the first and second capacitors Cst and Cpr. 
     When the reference voltage Vref is applied to the data line DLj, charges corresponding to Vref−PV 1 _ h +|Vth| are stored in both electrodes of the second capacitor Cpr, and charges corresponding to the threshold voltage (|Vth|) are stored in both electrodes of the first capacitor Cst. In this state, when the data voltage Vdata is applied to the data line DLj, the voltage of the first node N 1  may vary by a value proportional to the difference between the data voltage Vdata and the reference voltage Vref. For example, the voltage of the first node N 1  may vary by Cst/(Cst+Cpr)*(Vdata−Vref). Since the voltage of the first node N 1  is PV 1 _ h −|Vth| in the fifth period T 5 , when the pixel Pxij receives the data voltage Vdata, the voltage of the first node N 1  may be PV 1 _ h −|Vth|+Cst/(Cst+Cpr)*(Vdata−Vref). 
     Each data voltage Vdata may be written into the first node N 1  of the plurality of pixels PX connected to the data line DLj in this manner. When the sixth period T 6  is finished, the second transistor M 2  of all the pixels PX is turned off. 
     When the seventh period T 7 , i.e., the light emission period, starts, the fourth level voltage PV 2 _ l  is applied to the second power supply line PL 2 . The fourth level voltage PV 2 _ l  may be applied to the second power supply line PL 2  continuously from the start of the seventh period T 7  until the start of the first period T 1  of the next frame. During the seventh period T 7 , the first level voltage PV 1 _ h  is applied to the first power supply line PL 1 , and the second and third transistors M 2  and M 3  are maintained in the turned-off state. 
     The first transistor M 1  outputs a driving current according to the gate voltage, that is, the voltage of the first node N 1 . The first transistor M 1  may output a driving current proportional to square of the value obtained by subtracting the threshold voltage (|Vth|) from the source-gate voltage of the first transistor M 1 . Since the source electrode of the first transistor M 1  is connected to the first power supply line PL 1 , the source voltage of the first transistor M 1  is equal to the first level voltage PV 1 _ h . Accordingly, the first transistor M 1  may output a driving current proportional to the square of Cst/(Cst+Cpr)*(Vdata−Vref). Since the driving current is determined regardless of the level of the first level voltage PV 1 _ h  and the level of the threshold voltage (|Vth|), the pixels PX of the display unit  110  may emit light of uniform luminance. 
     For example, in each of the pixels PX, the threshold voltages (|Vth|) of the first transistor M 1  may be different from each other due to a process error or the like. However, according to the present embodiment, the deviation of the threshold voltage (|Vth|) is not reflected in the intensity of the driving current, so that the deviation of the threshold voltage (|Vth|) may be compensated. Further, when the pixels PX connected to the first power supply line PL 1  consume a large amount of current, a voltage lower than the target level of the first level voltage PV 1 _ h  may be transmitted to the pixels PX connected to the end of the first power supply line PL 1 . However, according to the present embodiment, the level of the voltage transmitted through the first power supply line PL 1  is not reflected in the intensity of the driving current, so that the organic light emitting display device  100  according to the present embodiment may have a uniform display quality. 
     According to a comparative example, a pixel may be connected to an initialization voltage line to which an initialization voltage Vinit having a plurality of levels is transmitted, and a first capacitor may be connected between the initialization voltage line and the gate electrode of the driving transistor. The turn-on and turn-off operations of the driving transistor may be more freely controlled by adjusting the level of the initialization voltage Vinit. However, in order to drive the pixel, the initialization voltage line is required to be in the display unit and another driving circuit for driving the initialization voltage line is further required. According to the present embodiment, since the initialization voltage line is not included, the size of the pixel PX, i.e., the area of the pixel PX can be further reduced, so that a larger number of pixels PX can be arranged in the same space. In addition, according to the present embodiment, another driving circuit for driving the initializing voltage line is not required, so that the manufacturing cost and the maintenance cost can be reduced. 
     The pixel PX according to an embodiment, although it includes only three transistors, can initialize the first transistor M 1  to remove hysteresis characteristic, can compensate the threshold voltage Vth of the first transistor M 1 , and can insure the organic light emitting diode (OLED) fully emits light. Therefore, the organic light emitting display device  100  including the plurality of pixels PX may be manufactured to have an ultra-high resolution of 1200 ppi or higher, for example, approximately 1600 ppi, so that a video image with a clearer image quality can be displayed. In particular, the organic light emitting display device  100  may be useful when the viewer&#39;s eyes and a screen are very close to each other, e.g., a head-mounted display. The organic light emitting display device  100  may be implemented as a head-mounted display. 
       FIG. 4  is a timing chart for driving the pixel of  FIG. 2  according to another embodiment. Referring to  FIG. 4 , first and second power supply voltages PV 1  and PV 2 , a control signal GC, first to nth scan signals S 1  to Sn, and a data signal Dj are shown. 
     Referring to the timing chart shown in  FIG. 4 , the third period T 3  and the fourth period T 4  shown in  FIG. 3  may be repeated a plurality of times. That is, after the second period T 2 , the third period T 3   a  and the fourth period T 4   a  proceed, and the third period T 3   b  and the fourth period T 4   b  may proceed again. When the fourth period T 4   b  is finished, the fifth to seventh periods T 5  to T 7  may be sequential as in the timing chart shown in  FIG. 3 . Thus, the voltage level of the first node N 1  can be more reliably lowered than the second level voltage PV 1 _ l.    
       FIG. 5  is a perspective view of a head-mounted display, which is an example of a display device according to an embodiment.  FIG. 6  is a use state view of the head-mounted display of  FIG. 5 .  FIG. 7  is a partial exploded perspective view of the head-mounted display of  FIG. 5 . 
     Referring to  FIGS. 5 and 6 , a head-mounted display  200  is a device worn on the head of a user. The head-mounted display  200  may include a case  210 , a strap unit  220 , and a cushion unit  230 . The head-mounted display  200  may include or be coupled with a display panel according to various embodiments of the present disclosure. 
     The case  210  may be worn on the head of a user USER. A display panel according to an embodiment and an acceleration sensor may be accommodated inside the case  210 . The acceleration sensor may sense the motion of the user USER and may transmit a predetermined signal to the display panel. Accordingly, the display panel may provide an image corresponding to a change in the line of sight of the user USER. Therefore, the user USER can experience a virtual reality like an actual reality. 
     In addition to the display panel and the acceleration sensor, components having various functions may be accommodated in the case  210 . For example, a proximity sensor for determining whether a user USER wears the case  210  may be accommodated in the case  210 . Further, an operation unit (not shown) for adjusting a volume, screen brightness, or the like may be additionally disposed outside the case  210 . The operation unit may be provided as a physical button, or may be provided in the form of a touch sensor or the like. 
     The strap unit  220  may be coupled with the case  210  to allow the user USER to easily wear the head-mounted display  200 . The strap unit  220  may include a main strap  221  and an upper strap  222 . 
     The main strap  221  may be worn along the periphery of the head of the user USER. The main strap  221  may fix the case  210  to the user such that the case  210  is be brought into close contact with the head of the user USER. The upper strap  222  may connect the case  210  and the main strap  221  along the upper portion of the head of the user USER. The upper strap  222  can prevent the case  210  from sliding down. The upper strap  222  can further improve the fitness of the user USER by dispersing the load of the case  210 . 
     Although it is shown in  FIG. 5  that each of the main strap  221  and the upper strap  222  has a length adjustable portion, the present disclosure is not limited thereto. For example, according to another embodiment, each of the main strap  221  and the upper strap  222  has elasticity, and thus the length adjustable portion may be omitted. 
     If the case  210  may be fixed to the user USER, the strap unit  220  may be modified into various forms in addition to those shown in  FIGS. 5 and 6 . For example, according to another embodiment, the upper strap  222  may be omitted. According to still another embodiment, the strap unit  220  may be modified into various shapes such as a helmet coupled with the case  210  and a pair of glass legs coupled with the case  210 . 
     The cushion unit  230  may be between the case  210  and the head of the user USER. The cushion unit  230  may be made of a material that is freely deformable in shape. For example, the cushion unit  230  may include a polymer resin (e.g., polyurethane, polycarbonate, polypropylene, or polyethylene) or may be formed of a sponge obtained by foam-molding a rubber liquid, a urethane-based material or an acrylic-based material. However, the present disclosure is not limited thereto. 
     The cushion unit  230  allows the case  210  to be brought into close contact with the user, thereby improving the fitness of the user USER. The cushion unit  230  may be detached from the case  210 . According to another embodiment, the cushion unit  230  may be omitted. 
     Referring to  FIG. 7 , the case  210  may be separated into a body  211  and a lid  212 . a mounting space DPS for mounting a display panel DP is provided between the body  211  and the lid  212 , and the lid may cover the mounting space DPS. Although it is illustratively shown in  FIG. 7  that the body  211  and the lid  212  are separated from each other, alternatively, the body  211  and the lid  212  may be provided integrally. 
     The display panel DP may be in the mounting space DPS between the body  211  and the lid  212 . The display panel DP may include pixels PX each having the pixel circuit shown in  FIG. 2 , and the pixels PX may be controlled or driven in accordance with the timing chart shown in  FIG. 3  or  FIG. 4 . The display panel DP is integrally mounted in the head-mounted display  200  to provide an image. 
     According to another embodiment, a display device (e.g., a portable terminal) may be coupled with the head-mounted display  200  to provide an image. The display device may include pixels PX each having the pixel circuit shown in  FIG. 2 , and the pixels PX may be controlled or driven in accordance with the timing chart shown in  FIG. 3  or  FIG. 4 . 
     In  FIG. 7 , a case where a left-eye image and a right-eye image are displayed through one display panel DP will be described as an example. The display panel DP may be divided into a left-eye image display area L_DA for displaying a left-eye image and a right-eye image display area R_DA for displaying a right-eye image. The left-eye image display area L_DA and the right-eye image display area R_DA may be driven by separate drivers. According to another embodiment, both the left-eye image display area L_DA and the right-eye image display area R_DA may be driven by one driver. According to still another embodiment, the display panel DP may include a left-eye display panel and a right-eye display panel that are separate from each other. 
     The display panel DP generates an image corresponding to the input image data. The display panel DP may include pixels PX each having the pixel circuit shown in  FIG. 2 . Each of the pixels PX may include three transistors and two capacitors, and may be connected to the first and second power supply lines PL 1  and PL 2 , the data line DL, the scan line SL, and the control line CL. The pixels PX may be controlled or driven in accordance with the timing chart shown in  FIG. 3  or  FIG. 4 . 
     An optical system OL may be inside the body  211  of the case  210 . The optical system OL can enlarge an image provided from the display panel DP. Since the image displayed on the display panel DP is enlarged by the optical system OL and recognized by the user USER, a high-quality image can be provided to the user USER only when the resolution of the display panel DP is very high. The display panel DP according to an embodiment includes pixels PX each having a pixel circuit including three transistors, two capacitors, and a light emitting element. Therefore, the organic light emitting display device  100  including the pixels PX can be manufactured with an ultra-high resolution of 1200 ppi or higher, for example, about 1600 ppi, and thus an image having a clearer image quality can be displayed. 
     The optical system OL may be spaced apart from the display panel DP in the first direction DR 1 . The optical system OL may be between the display panel DP and the eye of the user USER. The distance between the optical system OL and the display panel DP may be adjusted depending on the visual acuity of the user USER. 
     The optical system OL may include a right eye optical system OL_R and a left eye optical system OL_L. The left eye optical system OL_L may enlarge an image and provide the enlarge image to the left pupil of the user USER, and the right eye optical system OL_R may enlarge an image and provide the enlarged image to the right pupil of the user USER. The left eye optical system OL_L and the right eye optical system OL_R may be spaced apart from each other in the second direction DR 2  intersecting the first direction DR 1 . The distance between the right eye optical system OL_R and the left eye optical system OL_L may be adjusted corresponding to the distance between the two eyes of the user USER. 
     The optical system OL may be a convex aspherical lens. Each of the left eye optical system OL_L and the right eye optical system OL_R may be formed of only one lens. Alternatively, each of the left eye optical system OL_L and the right eye optical system OL_R may include a plurality of lenses. 
     According to various embodiments of the present disclosure, a pixel circuit includes only two switching transistors in addition to the driving transistor, and is connected to only one control line in addition to the scanning line and the data line. Therefore, the area of a pixel can be reduced, and the resolution of the display device including such a pixel can be increased. Further, the pixel circuit according to the various embodiments of the present disclosure can simultaneously solve a problem of non-uniformity of the threshold voltage of the driving transistor, a problem that the driving transistor has hysteresis characteristic, and a problem that the organic light emitting diode slightly emits light. Accordingly, the display device according to various embodiments of the present disclosure can display an image of ultra-high resolution. 
     Example 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 and scope of the present invention as set forth in the following claims.