1. Field of the Invention
The present invention relates to an organic light emitting diode display and a driving method thereof, and more particularly to an organic light emitting diode display that is adaptive for changing a data corresponding to an image property to prevent a reduction of a life span of an organic light emitting diode device and to improve a picture quality, and a driving method thereof.
2. Description of the Related Art
Recently, there have been many developments in flat panel display devices capable of reducing weight and bulk which are disadvantages of a cathode ray tube. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”), a field emission display (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”), and an electro-luminescence display device (hereinafter, referred to as “EL”), etc.
The PDP has an advantage of light weight and thin profile, and has been highlighted as a display device that is adaptive for making a large-dimension screen owing to its characteristics of a simple structure and a simple manufacturing process. However, the PDP has a disadvantage of a low luminous efficiency, a low brightness, and high power consumption. On the other hand, since an active matrix LCD to which a thin film transistor (hereinafter, referred to as “TFT”) as a switching terminal is applied, uses a semiconductor process, it is difficult to make a large-dimension screen. The active matrix LCD has a disadvantage in that power consumption is increased by a backlight unit.
On the other hand, the EL display device is largely classified into an inorganic light emitting diode display and an organic light emitting diode display depending upon a material of a light emitting layer and is a self-luminous device that is capable of light-emitting for itself. Furthermore, the EL display device has an advantage of a fast response speed, a high luminous efficiency, a high brightness, and a wide viewing angle. The inorganic EL display has high power consumption and cannot obtain a high brightness compared to the organic EL display device. Furthermore, the inorganic light emitting diode display cannot emit a variety of colors such as an R color, a G color, and a B color. On the other hand, the organic light emitting diode display is driven at a low DC voltage of dozens of volts, has a fast response speed, and can obtain a high brightness. As a result, the organic light emitting diode display can emit a variety of colors such as an R color, a G color, and a B color, and is adaptive for a post-generation flat panel display.
The organic light emitting diode display has an organic light emitting diode device as shown in FIG. 1. If a voltage is applied between an anode 100 of the organic light emitting diode device and a cathode 70 of the organic light emitting diode device, an electron generated from the cathode 70 moves toward an organic light emitting layer 78c via an electron injection layer 78a and an electron transport layer 78b. Further, a hole generated from the anode 100 moves forward the organic light emitting layer 78c via a hole injection layer 78e and a hole transport layer 78d. Thus, an electron and a hole are collided with each other to be re-combined to generate a light in the organic light emitting layer 78c. Herein, the electron and the hole are supplied from the electron transport layer 78b and the hole transport layer 78d, respectively. As a result, the light is emitted to the exterior via the anode 100 to display an image.
FIG. 2 is a block diagram schematically showing the organic light emitting diode display of the related art. Referring to FIG. 2, the organic light emitting diode display of the related art includes an OLED panel 20, a gate driving circuit 22, a data driving circuit 24, a gamma voltage generator 26, and a timing controller 27. Herein, the OLED panel 20 includes pixels 28 arranged at an area where is defined by a crossing of a gate line GL and a data line DL. The gate driving circuit 22 drives the gate lines GL of the OLED panel 20. The data driving circuit 24 drives the data lines DL of the OLED panel 20. The gamma voltage generator 26 supplies a plurality of gamma voltages to the data driving circuit 24. The timing controller 27 controls the data driving circuit 24 and the gate driving circuit 22.
The pixels 28 are arranged in a matrix type at the OLED panel 20. Further, a supply pad 10 and a ground pad 12 are disposed at the OLED panel 20. Herein, the supply pad 10 is supplied with a high-level potential voltage from a high-level potential voltage source VDD. The ground pad 12 is supplied with a ground voltage from a ground voltage source GND. In this case, the high-level potential voltage and the ground voltage are supplied to each pixel 28.
The gate driving circuit 22 supplies a gate signal to the gate lines GL to sequentially drive the gate lines GL.
The gamma voltage generator 26 supplies a plurality of analog gamma voltage to the data driving circuit 24. Herein, the gamma voltage generator 26 generates a positive polarity gamma voltage and a negative polarity gamma voltage which have a predetermined tilttilt corresponding to a characteristics of the OLED panel 20.
The timing controller 27 generates a data control signal which controls the data driving circuit 24 and a gate control signal which controls the gate driving circuit 22 using a plurality of synchronizing signals. A data control signal generated from the timing controller 27 is supplied to the data driving circuit 24 to control the data driving circuit 24. A gate control signal generated from the timing controller 27 is supplied to the gate driving circuit 22 to control the gate driving circuit 22. Furthermore, the timing controller 27 re-arranges a digital data which is supplied from a scaler in accordance with a resolution of the OLED panel 20 to supply it to the data driving circuit 24.
When a gate signal is supplied to the gate line GL, each pixel 28 is supplied with a data signal from the data line DL to generate a light corresponding to the data signal. To this end, each pixel 28 includes an organic light emitting diode device OLED and a cell driving circuit 30 as shown in FIG. 3. Herein, the organic light emitting diode device OLED has a cathode which is connected to the ground voltage source GND. The cell driving circuit 30 is connected to the gate line GL, the data line DL, and the high-level potential voltage source VDD and is connected to an anode of the organic light emitting diode device OLED to drive the organic light emitting diode device OLED. The cell driving circuit 30 includes a switching TFT T1, a driving TFT T2, and a capacitor C. When a gate signal is supplied to the gate line GL, the switching TFT T1 is turned-on to supply a data signal which is supplied to the data line DL to a node N. A data signal which is supplied to the node N is charged into the capacitor C and is supplied to a gate terminal of the driving TFT T2. The driving TFT T2 controls a current amount I which is supplied to the organic light emitting diode device OLED from the high-level potential voltage source VDD in response to a data signal with which the gate terminal is supplied to adjust a light emitting amount of the organic light emitting diode device OLED. Since a data signal is discharged from the capacitor C although the switching TFT T1 is turned-off, the driving TFT T2 supplies a current I from the high-level potential voltage source VDD to the organic light emitting diode device OLED to allow the organic light emitting diode device OLED to keep a light emitting until a data signal of the next frame is supplied. Herein, an actual cell driving circuit 30 may be set in a variety of structures other than the above-mentioned structure.
The data driving circuit 24 converts a data with which thereof is supplied into an analog gamma voltage (data signal) corresponding to a gray scale value in response to a data control signal from the timing controller 27, and supplies the data signal to the data lines DL. Herein, the data driving circuit 24 generates a data signal using any one analog gamma voltage corresponding to a data among a plurality of analog gamma voltages which are supplied from the gamma voltage generator 26. More specifically, the data driving circuit 24 selects any one voltage value among the analog gamma voltages which are supplied from the gamma voltage generator 26 corresponding to a gray scale of a data, and supplies the selected voltage signal to the data lines DL as a data signal. As a result, an image having brightness corresponding to a gray scale of a data is displayed at the OLED panel 20.
On the other hand, since a forward current, that is, a current which flows from an anode to a cathode is always applied to the organic light emitting diode device OLED, a degradation of the organic light emitting layer 78c is aggravated by a stress which is generated by an applying current as a driving time is increased. If the degradation of the organic light emitting layer 78c is aggravated, a life span of the organic light emitting diode device OLED is reduced. Specifically, since brightness of a display image is in proportion to an amount of a current which is applied to the organic light emitting diode device OLED, the above-mentioned problem is remarkable at a high brightness image having data of a high gray scale range.
On the other hand, in the organic light emitting diode display of the related art, the switching TFT T1 and the driving TFT T2 include a semiconductor layer having a poly-silicon p-Si for good electric field effect mobility. The p-Si thin film transistor is formed by a low temperature poly si LTPS through a laser annealing using an amorphous silicon a-Si. If the LTPS is used, the manufacturing cost is reduced. However, a tit stain is generated at a display image by a laser scan. Moreover, since such a stain is remarkably shown at a low brightness image having data of a low gray scale range, the organic light emitting diode display of the related art has an disadvantage in that an uniformity of an image is deteriorated in the low gray scale range.