Abstract:
This invention relates to a method for driving an electro-luminescence display device. The method includes the steps of selecting a scan line by applying a scan signal to any one of a plurality of scan lines; and switching between a constant voltage and a constant current to apply data to a plurality of data lines crossing the scan lines. The method switches between the constant voltage source and the constant current source to drive the data lines. As a result, it increases the brightness uniformity and brightness. Therefore, the picture quality can be sustained at a high level.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electro-luminescence display device, and more particularly to a method and apparatus for driving an electro-luminescence display device that is adaptive for increasing picture quality. 
   2. Description of the Related Art 
   Recently, there has been developed various flat display devices, which can be reduced in weight and bulk where a cathode ray tube CRT has a disadvantage. Such flat display panel includes a liquid crystal display, a field emission display, a plasma display panel, and electro-luminescence (hereinafter, EL) display device. 
   The structure and fabricating process of the PDP is relatively simple, thus the PDP is most advantageous to be made large-sized, but the light emission efficiency and brightness thereof is low and its power dissipation is high. It is difficult to make the LCD large-sizes because of using a semiconductor process, but since it is mainly used as a display device of a notebook computer, the demand for it increases, however there is a disadvantage that the LCD can hardly be made into a large-sized one and that power dissipation is high due to a backlight unit. Further, light loss by optical devices such as a polarizing filter, a prism sheet and diffusion plate is high and a viewing angle is narrow in the LCD. As compared with this, the EL display device is generally classified into an inorganic EL and an organic EL, and there is an advantage that its response speed is fast, its light-emission efficiency and brightness are high, and it has wide viewing angle. The organic EL display device can display a picture in a high brightness of several ten thousands [cd/m 2 ] with a voltage of about 10[V]. 
   In the organic EL display device, as shown in  FIG. 1 , there is formed an anode  2  of transparent conductive material on a glass substrate  1 , and there are deposited a hole injection layer  3 , a light-emission layer  4  of organic material, an electron injection layer  5  and a cathode  6  of metal on top of it. If an electric field is applied between the anode  2  and the cathode  6 , holes in the hole injection layer  3  and electrons in the electron injection layer  5  respectively progress toward the light-emission layer  4  to be combined in the light-emission layer. Then, a fluorescent material in the light-emission layer  4  gets excited and transferred to generate a visible light. At this moment, the brightness is not proportional to a voltage between the anode  2  and the cathode  6  but is proportional to a current. Accordingly, an apparatus for driving the organic EL display device generally drives the organic EL display device by a constant current source. 
   Referring to  FIG. 2 , the apparatus for driving an organic display device of the related art includes a constant current source  21  applying current to data lines DL 1  to DLm, and switching devices  22  and  23  applying a scan high voltage Vhigh and a ground voltage GND to each of scan lines SL 1  to SLn. 
   The data lines DL 1  to DLm act as the cathodes in  FIG. 1 , and the scan lines SL 1  to SLn act as the anodes in FIG.  1 . There are formed (m×n) number of pixel cells  20  at intersections of m number of data lines DL 1  to DLm and n number of scan lines SL 1  to SLn. The constant current source  21  is realized as two or more switching devices and a current mirror including the current source. The constant current source  21  synchronized with scan pulses applied to the scan lines SL 1  to SLn in accordance with input data applies the constant current to the data lines DL 1  to DLm. The switching devices  22  and  23  are realized as transistor devices such as MOS-FET. The switching devices  22  and  23  connected to the scan lines SL 1  to SLn sequentially apply negative scan voltages to the scan lines SL 1  to SLn to select the scan line where data are displayed. To this end, the switching devices  22  connected to the ground voltage source GND are turned on in response to a control signal T 1  to apply the ground voltage GND to the selected scan line, and the switching devices  23  connected to the scan high voltage source Vhigh is turned on in response to a control signal T 2  to apply the scan high voltage Vhigh to an unselected scan line. 
     FIG. 3  represents scan pulses applied to the scan lines SL 1  to SLn, and data pulses applied to the data lines applied to the data lines DL 1  to DLm. 
   Referring to  FIG. 3 , scan pulses SCAN are sequentially applied as negative voltages, i.e., forward voltage, to the scan lines SL 1  to SLn, and data pulses DATA synchronized with the scan pluses SCAN are applied as positive current to the data lines DL 1  to DLm. At this moment, light is emitted only at the pixel cells DATA to which the positive current is applied in accordance with the data among the pixel cells DATA connected to the scan lines SL 1  to SLn to which the negative voltage is applied. 
   On the other hand, charges of reverse direction are charged in both ends of the pixel cell  20  connected to the unselected scan line. In such a state, if the scan line is selected when the negative voltage is applied to the unselected scan line, the pixel cells  20  charged with the reverse charges takes a considerable delay time Δt for being charged to a desired positive data current level as in a data RDATA applied to an actual EL panel of FIG.  4 . This is because the input current applied to the pixel cells  20  charged with the reverse charges is wasted by the reverse charge. 
   The data delay of the organic EL display device can be explained more particularly through Formula 1. When the equivalent capacitance of the pixel cell  20  is C, the voltage charged in the pixel cell  20  is V, the amount of charges charged in the pixel cell  20  is Q, and the current inputted to the pixel cell  20  is I, the charge amount charged in the pixel  20  is determined as in the following Formula 1.
 
 Q=C×V=I×t   [FORUMULA 1]
 
   If the current is uniform in accordance with time, the time t taken to charge the pixel cell  20  to a desired voltage is (C×V)/I. For example, if C is 2.4 [nF] and I is 200 [ ], the time taken to charge the pixel cell  20  to 10[V] is (2.4 [nF]×10[V])/200 [μA]=120 [μs]. Such a charging time is a considerably long time as compared with the light-emission time of a scan line in the organic EL display device. 
   Such a delay time deteriorates the effective response speed and brightness of the pixel cells  20 . In order to compensate the deterioration of the response speed, the current should be increased, but it causes another problem of increasing power dissipation to occur because the driving voltage of each pixel  20  should be increased. 
   Further, in the driving apparatus of the EL display device of the relate art, the brightness between the data lines DL 1  to DLm is difficult to make uniform because the data lines DL 1  to DLm is driven by the constant current source  21 . In order to make the brightness between the data lines DL 1  to DLm uniform, the current applied to each data line DL 1  to DLm must be the same. To this end, it is required to minimize the current deviation scope of a plurality of data driving integrated circuits IC each including the constant current source  21 . For example, the current deviation scope of each data driving IC must be limited to within 50±0.5 [μA] for making the brightness of each data lines DL 1  to DLm uniform to be about 20 [nit]. In realizing an actual circuit, designing and fabricating the data driving IC with the current deviation of within 1% not only increases the IC unit price, but also it is difficult to drive each data driving IC in within the desired current deviation even in case that the driving IC&#39;s are applied to the actual EL panel. 
   As a result, the related art EL display device drives the data lines DL 1  to DLn by the constant current source  21  to cause the brightness and the brightness uniformity to be decreased, thereby decreasing picture quality. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a method and apparatus for driving an electro-luminescence display device that is adaptive for increasing picture quality. 
   In order to achieve these and other objects of the invention, a method for driving an electro-luminescence display device according to an aspect of the present invention includes selecting a scan line by applying a scan signal to any one of a plurality of scan lines, wherein the scan signal falls down to a voltage higher than a ground voltage; and switching between a constant voltage and a constant current to apply data to a plurality of data lines crossing the scan lines. 
   In the method, a switching is carried out between the constant voltage and the constant current to drive the data lines in accordance with a brightness of a display device that can be controlled by a user. 
   In the method, the data lines are driven by the constant voltage in a low brightness of the display device, and the data lines are driven by the constant current in a high brightness of the display device. 
   In the method, the data lines are charged with the constant current in a charging time of the data, and the data lines are driven by the constant voltage when a pixel cell emits light after completion of charging the data. 
   In the method, the electro-luminescence display device is a passive matrix type. 
   A driving apparatus for an electro-luminescence display device according to another aspect of the present invention includes a scan driver selecting a scan line by applying a scan signal to any one of a plurality of scan lines, wherein the scan signal falls down to a voltage higher than a ground voltage; and a data driver switches between a constant voltage and a constant current to apply data to a plurality of data lines crossing the scan lines. 
   The data driver includes a constant voltage source generating the constant voltage; a constant current source generating the constant current; and a switching device connecting any one of the constant voltage source and the constant current source to the data line. 
   Herein, the data driver switches between the constant voltage and the constant current to drive the data lines in accordance with a brightness of a display device that can be controlled by a user. 
   Herein, the data driver drives the data lines by the constant voltage in a lower brightness of the display device, and drives the data lines by the constant current in a high brightness of the display device. 
   Herein, the data driver charges the data lines with the constant current in a charging time of the data, and drives the data lines by the constant voltage when a pixel cell emits light after completion of charging the data. 
   Herein, the data driver varies a supply time of a voltage and a current applied to the data lines in accordance with a gray level value of an input data. 
   Herein, the scan driver includes a first switching device for switching a current path between the scan lines and a ground voltage source that generates the ground voltage; a second switching device for switching a current path between the scan lines and a voltage source that generates a specific scan high voltage; and a third switching device for switching a current path between the scan lines and the first switching device. 
   The scan driver further includes a comparator comparing a voltage in the scan line with a specific reference voltage; and a switching device controlling the voltage in the scan line by control of the comparator. 
   Herein, the reference voltage is set to be higher than the ground voltage. 
   Herein, the electro-luminescence display device is a passive matrix type. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects of the invention will be apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings, in which: 
       FIG. 1  is a sectional view briefly representing an organic electro-luminescence display device of the related art; 
       FIG. 2  is a plan view representing a driving apparatus and an electrode arrangement of an organic electro luminescence display device of the related art; 
       FIG. 3  is a waveform representing driver signals outputted from the driving apparatus shown in  FIG. 2 ; 
       FIG. 4  is a waveform representing the delay of data shown in  FIG. 3 ; 
       FIG. 5  is a plan view representing a driving apparatus and an electrode arrangement of an organic electro luminescence display device according to the first embodiment of the present invention; 
       FIG. 6  is a waveform diagram representing a scan pulse and a data pulse outputted from the driving apparatus shown in  FIG. 5 ; 
       FIG. 7  is a plan view representing a driving apparatus and an electrode arrangement of an organic electro luminescence display device according to the second embodiment of the present invention; 
       FIG. 8  is a plan view representing a driving apparatus and an electrode arrangement of an organic electro luminescence display device according to the third embodiment of the present invention; and 
       FIG. 9  is a waveform diagram representing a scan voltage controlled by a comparator and a third switching device shown in FIGS.  7  and  8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIGS. 5  to  9 , embodiments of the present invention will be explained as follows. 
   Referring to  FIG. 5 , a driving apparatus of an EL panel according to the first embodiment of the present invention includes a passive matrix type EL panel, a constant current source  54  for applying currents to data lines DL 1  to DLm, a constant voltage source  51  for applying voltages to data lines DL 1  to DLm, and a switching device  52  connecting any one of the constant voltage source  51  and the constant current source  54  to the data line DL 1  to DLm, switching devices  53  and  55  for applying a scan high voltage Vhigh and a ground voltage GND to each scan line SL 1  to SLn, and a timing controller  56  for controlling each of the switching devices  52 ,  53  and  55 . 
   The EL panel is formed in a passive matrix type. There are formed (m×n) number of pixel cells  50  at intersections of m number of data lines DL 1  to DLm and n number of scan lines SL 1  to SLn in the EL panel. 
   The constant current source  54  applies constant currents to the data lines DL 1  to DLm while the data lines DL 1  to DLm are charged. Further, the constant current source  54  applies the constant current to the data lines DL 1  to DLm when displaying a gray level data with big current consumption, e.g., a data, the gray level of which is in an upper half of the whole gray level range. Further, the constant current source  54  applies the current to the data lines DL 1  to DLm in the event of a brightness mode with big current consumption, e.g., in the event that brightness mode is adjusted to be high by a user to make the average brightness of a picture adjusted to be several hundreds [cd/m 2 ] or more. 
   The constant voltage source  51  applies constant voltages to the data lines DL 1  to DLm after completion of charging the current. Further, the constant voltage source  51  applies the constant voltage to the data lines DL 1  to DLm in a picture with low brightness uniformity, e.g., in a gray level scope that is the lower half of the whole expressible gray levels. And the constant voltage source  51  applies the voltage to the data lines DL 1  to DLm in the event that the brightness mode is adjusted to be low by the user to make the average brightness of the picture is adjusted to be low. 
   The first switching device  52  connects any one of the constant voltage source  51  and the constant current source  54  to the data line DL 1  to DLm in response to a control signal Φ 1  from the timing controller  56 . 
   The first switch  52  and the constant current source  54  are integrated in a data driving IC. The data driving IC further includes only the first switching device  52  in addition to the circuit configuration of the data driving IC of constant current driving scheme applied to an EL panel driving circuit of the related art, thus it is easy to design and fabricate this data driving IC. The error range for the voltage deviation of such a data driving IC can be easily controlled in 0.1[V] or less. 
   The switching devices  53  and  55  connected to the scan lines SL 1  to SLn sequentially apply negative scan voltages to the scan lines SL 1  to SLn to select the scan line where data are displayed. To this end, second switching devices  53  connected to the ground voltage source GND are turned on in response to a control signal Φ 2  to apply a ground voltage GND to the selected scan lines, and third switching devices  55  connected to a scan high voltage source Vhigh are turned on in response to a control signal Φ 3  to apply a scan high voltage Vhigh to the unselected scan lines. Each of the second and third switching devices  53  and  55  is integrated in a scan driving IC. 
   The timing controller  56  receives a video data and a vertical/horizontal synchronization signal H and V, generates control signals Φ 1 , Φ 2 , Φ 3  necessary for the first to third switching devices  52 ,  53  and  55 , and applies the generated control signals Φ 1 , Φ 2 , Φ 3  to the control terminals of the switching devices. 
   The method and apparatus for driving the EL according to the present invention has the data lines DL 1  to DLm charged with a current determined in accordance with a constant voltage level from the constant voltage source  51  when displaying a data in a gray level range where brightness uniformity decreases easily or a data of low brightness mode, thus the brightness uniformity can be sustained at a high level. Further, the method and apparatus for driving the EL according to the present invention has the data lines DL 1  to DLm charged with a current from the constant current source  54  when displaying a data in a gray level range where a sufficient current is required or a data of high brightness mode, thus the brightness of a picture can be increased. 
     FIG. 6  represents a scan pulse applied to scan lines SL 1  to SLn and a data pulse applied to data lines DL 1  to DLm shown in FIG.  5 . 
   Referring to  FIG. 6 , scan pulses SCAN are sequentially applied as negative voltages, i.e., forward voltages, to the scan lines SL 1  to SLn, and data pulses DATA synchronized with the scan pluses SCAN are applied as positive voltages to the data lines DL 1  to DLm. The width W of the data pulse DATA increases and decreases in accordance with the gray level value of an input data. In other words, the method and apparatus for driving the EL according to the present invention controls the light-emission time of the pixel cell  50  by a pulse width modulation method PWM to express the gray level. 
     FIG. 7  represents a driving apparatus of an EL panel according to the second embodiment of the present invention. 
   Referring to  FIG. 7 , a driving apparatus of an EL panel according to the second embodiment of the present invention includes a passive matrix type EL panel, a constant current source  54  for applying currents to data lines DL 1  to DLm, a constant voltage source  51  for applying voltages to data lines DL 1  to DLm, and a first switching device  52  connecting any one of the constant voltage source  51  and the constant current source  54  to the data line DL 1  to DLm, a second and a third switching device  53  and  55  for applying a scan high voltage Vhigh and a ground voltage GND to each scan line SL 1  to SLn, a comparator  70  comparing a specific reference voltage Vref with a voltage in the scan line SL 1  to SLn, a fourth switching device  57  for switching a current path between the scan line SL 1  to SLn and the ground voltage source GND, and a timing controller  56  for controlling the first to third switching devices  52 ,  53  and  55 . 
   The constant current source  54  applies constant currents to the data lines DL 1  to DLm while the data lines DL 1  to DLm are charged. Further, the constant current source  54  applies the current to the data lines DL 1  to DLm in data of a gray level range with big current consumption and in a high brightness mode with big current consumption. 
   The constant voltage source  51  applies constant voltages to the data lines DL 1  to DLm after completion of charging the current. Further, the constant voltage source  51  applies the voltage to the data lines DL 1  to DLm in data of a gray level range with low brightness uniformity and in a brightness mode with low brightness uniformity. 
   The first switching device  52  connects any one of the constant voltage source  51  and the constant current source  54  to the data line DL 1  to DLm in response to a control signal Φ 1  from the timing controller  56 . 
   The first and second switching devices  53  and  55  sequentially apply negative scan voltages to the scan lines SL 1  to SLn to select the scan line where data are displayed. To this end, the second switching devices  53  connected to the ground voltage source GND are turned on in response to a control signal Φ 2  to discharge the selected scan line to a ground potential GND, and the third switching devices  55  connected to a scan high voltage source Vhigh are turned on in response to a control signal Φ 3  to apply a scan high voltage Vhigh to the unselected scan lines. 
   The timing controller  56  receives a video data and a vertical/horizontal synchronization signal H and V, generates control signals Φ 1 , Φ 2 , Φ 3  necessary for the first to third switching devices  52 ,  53  and  55 , and applies the generated control signals Φ 1 , Φ 2 , Φ 3  to the control terminals of the switching devices. 
   The non-inversion input terminals of the comparators  70  are connected to the scan lines SL 1  to SLn, and the inversion input terminals of the comparators  70  are connected to a reference voltage source Vref. The output terminals of the comparators  70  are connected to the control terminals, i.e., the gate terminals, of the fourth switching devices  57 . Each comparator  70  compares the reference voltage Vref with a voltage in the scan line SL 1  to SLn and generates an output signal of low logic when the voltage in the scan line SL 1  to SLn is lower than the reference voltage Vref. And then, the generated output signal is applied to the control terminal of the fourth switching device  57 . If the voltage in the scan line SL 1  to SLn is equal to or higher than the reference voltage Vref, each comparator  70  generates an output signal of high logic to apply the generated output signal to the control terminal of the fourth switching device  57 . The fourth switching devices  57  cut off a current path between the drain terminal and the source terminal when the voltage in the scan line SL 1  to SLn is lower than the reference voltage Vref in response to the output signal of low logic of the comparator. If the voltage in the scan line SL 1  to SLn is equal to or higher than the reference voltage Vref, the fourth switching devices  57  allows the current path to conduct between the drain terminal and the source terminal in response to the output signal of high logic of the comparator. 
   As a result, the comparators  70  and the fourth switching devices  57 , as in  FIG. 9 , drop the voltage in the scan lines SL 1  to SLn not to the ground voltage GND but to the reference voltage Vref in the same manner. In other words, the comparators  70  and the fourth switches  57  act to make the voltage in the scan lines SL 1  to SLn drop not to the ground voltage but to a designated reference voltage Vref when scan pulses SCAN are applied to the scan lines SL 1  to SLn. This is because the voltage in the scan lines SL 1  to SLn rises higher than the ground voltage GND and the deviation of the rising voltage can be different in each scan line SL 1  to SLn by causes such as the current deviation of each scan driving IC and the deviation of the current applied to the scan driving IC through the data line DL 1  to DLm and the pixel cell  50  when the voltage in the scan line SL 1  to SLn drops. To this end, the reference voltage Vref is set to be the maximum voltage rising value of the scan line SL 1  to SLn when the scan pulse is applied in consideration of the allowable current of the scan driving IC. The reference voltage Vref is set to be 0.5[V] or more, preferably about 2[V], assuming that ground voltage GND is 0[V]. 
   The comparators  70  can be replaced with a common comparator  80  as shown in FIG.  8 . The common comparator  80  substantially has the same function as the comparators  70  shown in FIG.  7 . 
   As described above, the method and apparatus of the EL of the present invention drives the data lines DL 1  to DLm using the constant voltage source  51  and the constant current source  54  at the same time. As a result, the method and apparatus of the EL of the present invention increases the brightness uniformity and brightness, thus the picture quality can be sustained at a high level. 
   Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.