1. Field of the Invention
The present invention relates to an electro luminescence display device, and more particularly to an organic electro luminescence display device using a pre-charge, and a driving method thereof.
2. Description of the Related Art
Recently, there have been developed various flat panel displays that can reduce their weight and size which are the disadvantages of the cathode ray tube CRT. The flat panel display includes the liquid crystal display LCD, the field emission display FED, the plasma display panel PDP, the electro luminescence EL display device and etc.
The PDP is relatively simple in its structure and fabricating process, thus it is most advantageous in being made into large screen, but there is a big disadvantage in that its luminous efficiency and brightness is low and its power consumption is high.
The LCD is mainly used as a display device of a notebook computer and its demand is increasing. However, the LCD is fabricated by a semiconductor process, thus there is difficulty in being made into large screen. And, the LCD is not a self luminous device, and as such requires a separate light source, thus there is a disadvantage in that its power consumption is high due to the light source. Further, the LCD has a disadvantage in that its viewing angle is narrow and light loss is great due to optical devices such as a polarizing sheet, a prism sheet, a diffusion plate and etc.
The EL display device is generally classified into an inorganic EL display device and an organic EL display device, and it has an advantage in that its response speed is fast and its luminous efficiency, brightness and viewing angle is big. The EL display device can display a picture at a high brightness of tens of thousands of [cd/m3] by a voltage of about 10[V], and it has been applied to most of the EL display devices which are commonly used.
A unit element of the organic EL display device, as shown in FIG. 1, includes an anode 2 formed of a transparent conductive material on a glass substrate 1; and a hole injection layer 3, a light-emitting layer 4 formed of an organic material and a cathode 5 formed of a metal having a low work function are deposited thereon. If an electric field is applied between the anode 2 and the cathode 5, holes within the hole injection layer 3 and electrons within the metal are progressed to and combined together in the light-emitting layer 4. Then, a phosphorous material within the light-emitting layer 4 is excited and transited to generate a visible light. At this moment, the brightness is in proportion to a current between the anode 2 and the cathode 5.
The organic EL display device is divided into passive type and active types.
FIG. 2 is a circuit diagram equivalently representing part of a passive type organic EL display device, and FIG. 3 is a waveform representing scan signal and data signal waveforms of the passive type organic EL display device.
Referring to FIGS. 2 and 3, the passive type EL display device includes a plurality of data lines D1 to D3 and a plurality of scan lines S1 to S3 which cross each other; and organic EL elements OLED formed at intersections between the data lines DL1 to D3 and the scan lines S1 to S3.
The data lines D1 to D3 are connected to the anode of the organic EL element OLED to supply data currents Id to the anode of the organic EL element OLED.
The scan lines S1 to S3 are connected to the cathode of the organic EL element OLED to supply scan pulses SP1 to SP3 synchronized with the data currents Id to the cathode of the organic EL element OLED.
The organic EL element OLED emits light in proportion to the current flowing between the anode and the cathode for a display period DT when the scan pulses SP1 to SP3 are applied.
The organic EL elements OLED of the organic EL display device are charged with the current for a response time RT which is delayed by resistance components of the data lines D1 to D3 and a capacitance which is in the organic EL elements OLED, thus there is a problem in that the response speed and brightness are low. In order to compensate the low response speed of the organic EL elements OLED, there has recently been a trend that a pre-charge period PCHA is provided as a non-display period between the display periods DT and the organic EL elements OLED are pre-charged during the pre-charge period PCHA. However, even though the organic EL display device is pre-charged, there is a problem in that the response time RT of the organic EL elements OLED is lengthened in a low gray level as in FIG. 4.
Further, in a driving method of pre-charging the organic EL display device, there is a problem in that the response speed is fast but the organic EL elements OLED are over-charged by an overshoot in a high gray level. This is because the pre-charged current Ipre is fixed to be the value of gray level of data x the pre-charged constant “10” regardless of the gray level of data as in FIG. 5. The current Ioled of the organic EL elements OLED which is charged with the fixed pre-charged current Ipre increases exponentially as the gray level increases as in FIG. 6. As a result, if the organic EL elements OLED are driven by the pre-charging method, its brightness is not changed linearly but increases exponentially as the gray level increases, thus there is a problem in that the gray level expression ability becomes low.
Besides, most of the pre-charging method used currently has a problem in that power consumption is high and the current is not sufficiently pre-charged to the organic EL element OLED in the low gray level range such that the gray level expression ability is low in the low gray level.