Abstract:
An organic light emitting display and a driving method thereof capable of reducing power consumption. A driving transistor controls a current through an organic light emitting diode of the display. A voltage controller supplies a first voltage to the anode of the OLED of at least one specific pixel and controls the cathode voltage of the OLED in correspondence to a second current through the OLED, such that the cathode voltage corresponds to the first voltage supplied to the OLED. Thus, the driving transistor can be driven in saturation mode with consistent current in spite of process variations, with a reduced power consumption.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2007-0123375, filed on Nov. 30, 2007, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to an organic light emitting display and a driving method thereof. 
         [0004]    2. Description of Related Art 
         [0005]    Recently, various flat panel display devices having reduced weight and volume in comparison to a cathode ray tube (CRT) have been developed. Examples of flat panel display devices include liquid crystal displays, field emission displays, plasma display panels, organic light emitting displays, etc. 
         [0006]    Among these examples, the organic light emitting display displays an image utilizing organic light emitting diodes (OLEDs) that generate light by the recombination of electrons and holes. An organic light emitting display generally has a rapid response speed and a low power consumption. 
         [0007]      FIG. 1  is a circuit diagram illustrating a pixel of a conventional organic light emitting display. 
         [0008]    Referring to  FIG. 1 , a pixel  4  of a conventional organic light emitting display includes an organic light emitting diode OLED and a pixel circuit  2  that is coupled to a data line Dm and a scan line Sn to control the organic light emitting diode OLED. 
         [0009]    An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit  2 , and a cathode electrode thereof is coupled to a second power ELVSS. The organic light emitting diode OLED generates light having a brightness (which may be predetermined) corresponding to a current supplied from the pixel circuit  2 . 
         [0010]    The pixel circuit  2  controls an amount of current supplied to the organic light emitting diode OLED in accordance with a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. To this end, the pixel circuit  2  includes a second transistor M 2  coupled between a first power ELVDD and the organic light emitting diode OLED, and a first transistor M 1  coupled to the second transistor M 2 , the data line Dm and the scan line Sn, and a storage capacitor Cst coupled between a gate electrode and a first electrode of the second transistor M 2 . 
         [0011]    A gate electrode of the first transistor M 1  is coupled to the scan line Sn, and a first electrode thereof is coupled to the data line Dm. And, a second electrode of the first transistor M 1  is coupled to one terminal of the storage capacitor Cst. Herein, the first electrode is set as one of a source electrode or a drain electrode, and the second electrode is set as an electrode different from the first electrode. For example, if the first electrode is a source electrode, the second electrode is a drain electrode, and vice versa. The first transistor M 1  coupled to the scan line Sn and the data line Dm supplies a data signal on the data line Dm to the storage capacitor Cst by being turned on when the scan signal is supplied from the scan line Sn. At this time, the storage capacitor Cst is charged with a voltage corresponding to the data signal. 
         [0012]    A gate electrode of the second transistor M 2  is coupled to one terminal of the storage capacitor Cst, and a first electrode thereof is coupled to the other terminal of the storage capacitor Cst and the first power ELVDD. And, a second electrode of the second transistor M 2  is coupled to an anode electrode of the organic light emitting diode OLED. The second transistor M 2  controls an amount of current flowing from the first power ELVDD, through the organic light emitting diode OLED, to the second power ELVSS in accordance with a voltage stored in the storage capacitor Cst. At this time, the organic light emitting diode OLED generates light corresponding to the amount of current supplied by the second transistor M 2 . 
         [0013]    In the conventional pixel  4 , the second transistor M 2  is driven as a substantially constant current source supplying a current (e.g., a predetermined current) to the organic light emitting diode OLED in accordance with the voltage stored in the storage capacitor Cst. Herein, the transistor M 2  should be driven in its saturation region in order that the second transistor M 2  drives a substantially constant current. Therefore, the voltage of the first power ELVDD and the second power ELVSS are set so that the second transistor M 2  is driven in the saturation region. 
         [0014]    In more detail, the voltage between the first power ELVDD and the second power ELVSS can be expressed as shown in the following Equation 1: 
         [0000]        ELVDD−ELVSS&gt;Vds   —   sat+Voled+Vmt+Vmo    Equation 1 
         [0015]    In Equation 1, Vds_sat represents a minimum voltage between the first electrode and the second electrode (e.g., the source and the drain) of the second transistor M 2  for driving the second transistor M 2  in the saturation region when a maximum current (i.e., the saturation current of the second transistor M 2  when the data value representing the highest gray level is supplied on the data line Dm and stored in the storage capacitor Cst) flows from the pixel circuit  2  to the organic light emitting diode OLED. Voled represents a voltage applied to the organic light emitting diode OLED when the maximum current is supplied. 
         [0016]    Vmt represents voltage margin due to a process deviation of the second transistor M 2 , and Vmo represents a voltage margin corresponding to the process deviation and the temperature characteristics of the organic light emitting diode OLED. 
         [0017]    Actually, in the organic light emitting diode OLED, the voltage margin Vmo corresponding to the temperature changes even in the case where the same current is supplied. Therefore, Vmo is set such that the pixel  4  can be stably driven in consideration of the temperature characteristics of the organic light emitting diode OLED. 
         [0018]    Meanwhile, when the voltages of the first power ELVDD and the second power ELVSS are set as shown in Equation 1, power consumption may be undesirably high. In particular, the voltage margin Vmo that is added in consideration of the temperature characteristics may result in 20% to 30% of the power consumption. Therefore, a method capable of reducing power consumption by lowering the margin voltage of Vmo is desired. 
       SUMMARY 
       [0019]    To address these and other issues, an organic light emitting display and a driving method thereof having features of an exemplary embodiment of the present invention is capable of reducing power consumption by lowering the voltage margin corresponding to process deviations and the temperature characteristics of the organic light emitting diode OLED. 
         [0020]    According to an exemplary embodiment of the present invention, an organic light emitting display includes first and second power generators for generating first and second powers, respectively. A plurality of pixels within the display each include an organic light emitting diode (OLED). At least some pixels among the plurality of pixels are in a display region of the organic light emitting display, and the at least some pixels each include a driving transistor for controlling a first current through the OLED. A voltage controller supplies a second current to the OLED of at least one specific pixel of the plurality of pixels, and controls a voltage of the second power supply in correspondence to a first voltage of the OLED provided when the second current is supplied to the OLED. 
         [0021]    In a further exemplary embodiment, the voltage controller includes a controller for controlling a turn-on and a turn-off of the first transistor, the controller comprising a memory for storing first data representing a saturation voltage for driving the driving transistor in a saturation region, and a margin voltage corresponding to a range of process deviation of the driving transistor when the driving transistor supplies a maximum current; and a register for generating second data, wherein the register is configured to adjust a value of the second data in accordance with a comparator output; a first digital-analog converter for converting the first data into a second voltage; a current source for supplying the second current to the OLED when the first transistor is turned on; an adder for adding the first voltage and the second voltage to generate a third voltage; a comparator for comparing the third voltage with a voltage of the first power, and for supplying the comparator output; and a second digital-analog converter for converting the second data to an analog voltage. 
         [0022]    According to another exemplary embodiment of the present invention, a method is provided for driving an organic light emitting display including a first power, a second power, an organic light emitting diode, and a pixel circuit comprising a driving transistor for controlling a current through the organic light emitting diode. In this embodiment, the method includes storing first data representing a saturation voltage for driving the driving transistor in a saturation region, and a margin voltage corresponding to a range of process deviation of the driving transistor when the driving transistor supplies a current corresponding to a highest gray level; supplying a first current to an OLED of at least one specific pixel; comparing a third voltage with a voltage of the first power, the third voltage comprising a sum of a first voltage extracted from the OLED while supplying the first current and a second voltage generated by converting the first data to an analog signal; and controlling a voltage of the second power in accordance with a result of comparing the third voltage with the voltage of the first power. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The accompanying drawings, together with the specification illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
           [0024]      FIG. 1  is a circuit diagram illustrating a conventional pixel in the related art; 
           [0025]      FIG. 2  is a block diagram illustrating an organic light emitting display according to a first exemplary embodiment of the present invention; 
           [0026]      FIG. 3  is a block diagram illustrating an organic light emitting display according to a second exemplary embodiment of the present invention; and 
           [0027]      FIG. 4  is simplified schematic diagram illustrating the voltage controller and the pixel of  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    Hereinafter, certain exemplary embodiments of the present invention will be described with reference to the accompanying drawings. Herein, when a first element is described as being coupled to a second element, the first element may be directed coupled to the second element, or it may be indirectly coupled to the second element via a third element. Further, some of the elements that may not be essential for a complete understanding of the invention have been omitted for clarity. Like reference numerals refer to like elements throughout. 
         [0029]    Hereinafter, certain exemplary embodiments of the present invention, which can be easily carried out by those skilled in the art, will be described with reference to the accompanying  FIGS. 2 through 4 . 
         [0030]      FIG. 2  is a block diagram illustrating an organic light emitting display according to an exemplary embodiment of the present invention. 
         [0031]    Referring to  FIG. 2 , the organic light emitting display according to the exemplary embodiment of the present invention displays images during a plurality of frames, and includes a display region (or display unit)  30  that includes a plurality of pixels  40  and  42  coupled to scan lines S 1 -Sn and data lines D 1 -Dm, a scan driver  10  that drives the scan lines S 1 -Sn, a data driver  20  that drives the data lines D 1 -Dm, and a timing controller  50  that controls the scan driver  10  and the data driver  20 . 
         [0032]    The organic light emitting display according to the exemplary embodiment of the present invention further includes a first power generator  60  that generates first power ELVDD, a voltage controller  80  that controls a second power generator  70  corresponding to a voltage extracted from a pixel (e.g., a specific pixel)  42 , and a second power generator  70  that generates a second power ELVSS under the control of the voltage controller  80 . 
         [0033]    In the display region  30 , the first power ELVDD from the first power generator  60  and the second power ELVSS from the second power generator  70  are coupled to the pixels  40  and  42 . When the scan driver  10  supplies the scan signal, the respective pixels  40  and  42  coupled to the first power ELVDD and the second power ELVSS are selected, and emit light at a brightness corresponding to the data signal supplied by the data driver  20 . 
         [0034]    To this end, the respective pixels  40  and  42  include an organic light emitting diode (not illustrated in  FIG. 2 ) and a pixel circuit (not illustrated in  FIG. 2 ) that supplies current to the organic light emitting diode. The pixel circuit, which typically includes at least one transistor and capacitor, controls an amount of current supplied from the first power ELVDD to the second power ELVSS via the organic light emitting diode, in accordance with the data signal. The organic light emitting diode emits red, green, or blue light in accordance with the amount of current supplied from the pixel circuit. 
         [0035]    The scan driver  10  sequentially supplies the scan signals to the scan lines S 1 -Sn. If the scan signals are sequentially supplied to the scan lines S 1 -Sn, rows of pixels  40  and  42  are sequentially selected. 
         [0036]    The data driver  20  generates data signals using data supplied from the timing controller  50 , and supplies the generated data signals to the data lines D 1 -Dm whenever the scan signals are supplied. Then, the data signals are supplied to the pixels  40  and  42  selected by the scan signals. 
         [0037]    The timing controller  50  generates a data driving control signal DCS and a scan driving control signal SCS that correspond to synchronization signals supplied from the outside. The data driving control signal DCS generated from the timing controller  50  is supplied to the data driver  20 , and the scan driving control signal SCS generated therefrom is supplied to the scan driver  10 . And, the timing controller  50  rearranges data supplied from the outside to supply it to the data driver  20 . 
         [0038]    The voltage controller  80  is coupled to at least one specific pixel  42  included in the display region  30 . The voltage controller  80  extracts a voltage applied to the organic light emitting diode of the specific pixel  42 , while supplying a reference current (e.g., a predetermined current) to the specific pixel  42 . At this time, the voltage extracted from the organic light emitting diode includes voltage information applied to the organic light emitting diode corresponding to the temperature currently driven (i.e., Vmo+Voled). The voltage controller  80  extracting a voltage of the pixel  42  controls the second power generator  70  to minimize or reduce power consumption. 
         [0039]    The second power generator  70  generates second power ELVSS corresponding to the signal from the voltage controller  80  (described below) and supplies the generated second power ELVSS to the pixels  40  and  42 . 
         [0040]    The first power generator  60  generates first power ELVDD and supplies the generated first power ELVDD to the pixels  40  and  42 . 
         [0041]    In  FIG. 2 , the voltage controller  80  is illustrated as being coupled to the specific pixel  42  included in the display region  30 , but the present invention is not limited thereto. In practice, as shown in  FIG. 3 , the voltage controller  80  may be coupled to at least one dummy pixel  44  positioned in a region (i.e., a non-display region) other than the display region  30 . 
         [0042]      FIG. 4  is simplified schematic diagram illustrating the voltage controller  80  and the pixel  42 ,  44  of  FIGS. 2 and 3 . 
         [0043]    Referring to  FIG. 4 , the pixel  42 ,  44  includes a pixel circuit  48  that supplies current to an organic light emitting diode OLED, whereby the organic light emitting diode OLED emits light corresponding to the current supplied from the pixel circuit  48 , and a first transistor M 3  coupled between an anode electrode of the organic light emitting diode OLED and a voltage controller  80 . 
         [0044]    Herein, when the pixel as shown in  FIG. 4  is the dummy pixel  44 , the first transistor M 3  is turned on every i th  (i is a natural number) frame period. When the first transistor M 3  is turned on, the voltage controller  80  supplies a current, e.g., a maximum current corresponding to the brightest gray level, to the organic light emitting diode OLED. At this time, the pixel circuit  48  blocks an electrical coupling between a first power ELVDD and the organic light emitting diode OLED. Actually, when a pixel as shown in  FIG. 4  is the dummy pixel  44 , the pixel circuit  48  and the first power ELVDD may be omitted. 
         [0045]    Whenever the first transistor M 3  is turned on, the voltage controller  80  controls a voltage of a second power ELVSS in accordance with a voltage applied to the organic light emitting diode OLED. Herein, if a period in between times that the first transistor M 3  is turned on is a short period (for example, i=2), the voltage of the second power ELVSS is frequently changed, resulting in frequent changes in the brightness of a panel, which may negatively affect a user&#39;s viewing experience. Therefore, i is experimentally determined in consideration of the size and resolution of the panel such that the changes in the brightness of the panel are not necessarily observed by a viewer. 
         [0046]    Meanwhile, when the pixel as shown in  FIG. 4  is the specific pixel  42  within the display region  30 , the first transistor M 3  is turned on when the specific pixel does not perform a display operation. For example, the first transistor M 3  included in the specific pixel  42  is turned on during a period when the specific pixel displays black. In this case, the voltage controller  80  is supplied with data from a timing controller  50  to the specific pixel  42 , and turns on the first transistor M 3  when the data displays black (e.g., in the case of having “00000000” bits). As described above, the first transistor M 3  is turned on during the period that the specific pixel  42  displays black, thereby not causing a collision between the current (e.g., the predetermined current) supplied from the voltage controller  80  and the current supplied form the pixel circuit  48 . 
         [0047]    Meanwhile, in the exemplary embodiment described, the voltage controller  80  does not unconditionally turn on the first transistor M 3  when the specific pixel  42  displays black. In other words, the voltage controller  80  controls a point of time when the first transistor M 3  is turned on such that the change in voltage of the second power ELVSS is not observed by a viewer. 
         [0048]    The voltage controller  80  includes a current source  81 , an adder  82 , a comparator  83 , a first digital-analog converter  84  (hereinafter, referred to as “first DAC”), a second DAC  85 , and a controller  86 . 
         [0049]    The current source  81  supplies a current (e.g., a predetermined current) to the organic light emitting diode (OLED) corresponding to a current when the pixels  40  emit light at the highest brightness. 
         [0050]    The adder  82  adds a first voltage Vsamp applied to the organic light emitting diode OLED with a second voltage Vtft supplied from the first DAC  84  when the current source  81  supplies the current to the organic light emitting diode OLED, and supplies the sum as a third voltage to the comparator  83 . 
         [0051]    The comparator  83  compares the third voltage with the voltage of the first power ELVDD, and provides the comparative result to the controller  86 . 
         [0052]    The controller  86  controls turn-on and turn-off of the first transistor M 3 . The controller  86  includes a memory  87  and a register  88 . 
         [0053]    A first data corresponding to a total voltage of VDS_sat and Vmt is stored in the memory. In this exemplary embodiment, VDS_sat and Vmt are set as fixed values in every panel so that they can be previously stored in the memory  87 . 
         [0054]    The first DAC  84  converts the first data supplied from the memory  87  to the second voltage (Vtft=VDS_sat+Vmt) to supply it to the adder  82 . 
         [0055]    The register  88  supplies a second data of j (j is a natural number) bits, the value of which increases or decreases in accordance with the comparative result of the comparator  83 , to the second DAC  85 . 
         [0056]    The second DAC  85  converts the second data supplied from the register  88  to analog voltage FBV to supply it to a second power generator  70 . 
         [0057]    The second power generator  70  generates the second power ELVSS using the analog voltage FBV supplied from the second DAC  85 . Herein, the second power ELVSS is generated as shown in the following equation 2: 
         [0000]        ELVSS=α×FBV+ΔV    Equation 2 
         [0058]    In Equation 2, α represents a real number larger than 0 and ΔV represents a voltage, and is also a real number. In Equation 2, α and ΔV are previously and experimentally determined in order that the second power ELVSS can be stably generated from the analog voltage FBV. Herein, α and ΔV are set as fixed values so that the voltage of the second power ELVSS is determined by the analog voltage FBV. 
         [0059]    Explaining an exemplary operation process in detail, first the first data stored in the memory  87  is supplied to the first DAC  84 . The first DAC  84  converts the first data supplied form the memory  87  to the second voltage Vtft to supply it to the adder  82 . 
         [0060]    The first transistor M 3  is turned on by controlling the controller  86 . At this time, current is not supplied from the pixel circuit  48  to the organic light emitting diode OLED. If the first transistor M 3  is turned on, a current (e.g., a predetermined current) from the current source  81  is supplied to the organic light emitting diode OLED. At this time, the first voltage Vsamp is applied to the organic light emitting diode OLED. Herein, the value of the first voltage Vsamp varies depending on the temperature currently experienced. For example, the first voltage Vsamp may be about 4V at a high temperature (e.g., 80° C.) and may be about 8V at a low temperature (e.g., −30° C.). 
         [0061]    The first voltage Vsamp applied to the organic light emitting diode OLED is supplied to the adder  82 . At this time, the adder  82  generates the third voltage by adding the first voltage Vsamp and the second voltage Vtft, and supplies the generated third voltage to the comparator  83 . 
         [0062]    The comparator  83  supplied with the third voltage compares the third voltage with the voltage value of the first power ELVDD and supplies the comparative result to the register  88 . For example, when the first power ELVDD has a high voltage, the comparator  83  supplies a first control signal to the register  88 , and when the third voltage has a high voltage, the comparator  83  supplies a second control signal to the register  88 . 
         [0063]    The register  88  increases or decreases the value of the second data in accordance with the control signal supplied from the comparator  83 . For example, when the first control signal is input, the comparator  83  increases the value of the second data, and when the second control signal is input, the comparator  83  decreases the value of the second data. In other words, the comparator  83  increases or decreases the value of the second data in order that the third voltage output from the adder  82  has a similar value with the first power ELVDD. 
         [0064]    The second DAC  85  converts the second data into the analog voltage FBV to supply it to the second power generator  70 . 
         [0065]    The second power generator  70  generates the second power ELVSS by using the analog voltage FBV supplied from the second DAC  85 . Thereafter, the voltage controller  80  generates an optimal voltage of the second power ELVSS corresponding to the temperature currently driven, repeating the processes as described above. 
         [0066]    In summary, the organic light emitting display according to the exemplary embodiment of the present invention extracts the voltage applied to the organic light emitting diode OLED corresponding to the temperature, and controls the voltage of the second power ELVSS corresponding to the extracted voltage. As described above, the voltage of the second power ELVSS is controlled using the voltage extracted from the organic light emitting diode OLED, making it possible to reduce or minimize power consumption. In other words, the voltage of Vmo as shown in Equation 1 is controlled to correspond to the temperature currently driven so that there is no need for an unnecessarily wide margin. 
         [0067]    Meanwhile, in another exemplary embodiment of the present invention the voltage controller  80  can be coupled to at least two specific pixels  42  or dummy pixels  44 . In this case, the voltage controller  80  repeats the processes as described above in the specific pixel  42  or the dummy pixel  44 . And, the register  88  controls the voltage of the second power generator  70  only when the same result is obtained in both or all the specific pixels  42  or the dummy pixels  44 , that is, only when the same control signal (the first control signal or the second control signal) is generated in all the specific pixels  42  or the dummy pixels  44 . 
         [0068]    The organic light emitting display and the driving method thereof according to various exemplary embodiments of the present invention sets the voltage value of the second power ELVSS to correspond to the temperature currently driven, making it possible to reduce power consumption. 
         [0069]    While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.