Organic electroluminescent (EL) display device and method for driving the same

An organic EL display device for compensating for a reduction of the voltage between the gate and source of a driving transistor occurring due to a voltage drop of the source voltage caused by the resistance component of a power source line, and a method for driving the organic EL display device. The organic EL display device has a data driver for receiving digital image data and applying the digital image data and a data voltage corresponding to the position of a pixel circuit. The data driver outputs different data voltages depending on the position of the pixel circuit even when the same digital image data are received. When the driving transistor is a P-type transistor, the data driver applies a higher data voltage to a pixel circuit that is closer to an external voltage source than that applied to a farther one even when the same digital data are received. When the driving transistor is an N-type transistor, the data driver applies a lower data voltage to a pixel circuit that is closer to an external voltage source than that applied to a farther one even when the same digital data are received.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to application Ser. No. 2002-0019932, filed in the Korean Intellectual Property Office on Apr. 12, 2002, the disclosure of which is incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an organic electroluminescent (hereinafter, referred to as “EL”) display device, and a method for driving the organic EL display device. More specifically, the present invention relates to an organic EL display device capable of compensating for a reduction of the voltage between the gate and source of a driving transistor that occurs due to a voltage drop of the source voltage caused by the resistance component of a power source line, and a method for driving the organic EL display device.

(b) Description of the Related Art

In general, an organic EL display device is a display device that electrically excites a fluorescent organic compound to emit light, and drives N×M organic luminescent cells to display an image. Typically the techniques for driving the organic luminescent cells include, the passive matrix method and the active matrix method using thin film transistors (TFTs).

Compared with the passive matrix method that uses positive and negative electrodes lying at right angles to each other and selectively drives the electrode lines, the active matrix method connects TFTs and capacitors to the individual ITO (Indium Tin Oxide) pixel electrodes to maintain a voltage through capacitance.

FIG. 1is a circuit diagram of a conventional pixel circuit for driving an organic EL device using TFTs, in which one of N×M pixels is shown.

Referring toFIG. 1, a P-type driving transistor M1is connected to the organic EL device OELD to supply a current for emitting light. The current of the driving transistor M1is controlled by a data voltage applied through a P-type switching transistor M2. Between the source and gate of the transistor M1, a capacitor Cstis connected for maintaining the applied voltage for a predetermined period of time. The gate of the transistor M2is connected to the n-th scan line Scan[n], and the source of the transistor M2is connected to the m-th data line Data[m].

Now, the operation of the above-constructed pixel circuit will be described. With a scanning signal applied to the gate of the switching transistor M2to turn on the transistor M2, data voltage VDATAis applied to the gate (node A) of the driving transistor M1via the data lines. As the data voltage VDATAis applied to the gate, the current flows to the organic EL device OELD via the transistor M1to emit lights.

The current flowing to the organic EL device is given by the following equation:

IOELD=β2⁢(VGS-VTH)2=β2⁢(VDD-VDATA-VTH)2[Equation⁢⁢1]
In the above equation, IOELDis the current flowing to the organic EL device; VGSis the voltage between the source and gate of the transistor M1; VDDis the source voltage applied to the source of the transistor M1; VTHis the threshold voltage of the transistor M1; VDATAis the data voltage; and β is a constant value.

As can be seen from Equation 1, the current corresponding to the data voltage VDATAapplied to the pixel circuit shown inFIG. 1is sent to the organic EL device OELD, which then emits light. Here, the data voltage VDATAhas a multilevel value in a predetermined range, for representing gradation.

According to the conventional pixel circuit, virtually all the source voltage VDDis applied to the source of a driving transistor M1that is closely connected, via a power source line, to an external source outputting the source voltage VDD. But a voltage VDD′that is lower than the source voltage due to the resistance component of the power source line is applied to a source of the driving transistor M1that is connected far away from the external voltage source via the power source line.

This can be described as follows in further detail with reference toFIGS. 2 and 3.

In the pixel circuit ofFIG. 2, it is assumed that an external power source (not shown) is positioned adjacent to the first row of the pixel circuit.

InFIG. 2, the source voltage VDDis applied directly to the driving transistor M1of the pixel circuit in the first row, and, via a resistance Rp, to the driving transistor of the pixel circuit in the n-th row.

Assuming that data voltage V1is applied to the gate of the driving transistor of the pixel circuit in the first row and data voltage V2is applied to the gate of the driving transistor of the pixel circuit in the n-th row, the driving transistor M1is turned on as shown in the equivalent circuit diagram ofFIG. 3.

As shown inFIG. 3, the voltage VDDis applied to the source (denoted by ‘A’) of the driving transistor of the pixel circuit in the first row, but the voltage VDD′that is lower than VDDis applied to the source (denoted by ‘B’) of the driving transistor of the pixel circuit in the n-th row due to a voltage drop caused by the resistance Rp.

Accordingly, when the same data voltage is applied in order to represent the same gradation in the first and n-th rows, i.e., V1=V2, the voltage VDDapplied to the source of the driving transistor in the first row differs from the voltage VDD′applied to the source of the driving transistor in the n-th row. Hence a current of a different magnitude flows to the organic EL device as can be seen from Equation 1. Thus the conventional organic EL display device exhibits different gradations according to the position of the pixel even with the same data voltage, and therefore it has difficulty in accurately representing gradation.

Particularly, the difference of the source voltage caused by the resistance component of the power source line becomes greater with an increase in the distance from the external voltage source, and, for a high resolution (greater than SVGA) organic EL display device, a current of up to several amperes flows to the whole panel during a full white driving operation, resulting in a deterioration of the luminance by scores of grays.

SUMMARY OF THE INVENTION

An embodiment of the present invention may be used to solve the problems with the prior art and to provide an organic EL display device capable of compensating for a reduction of the voltage between the gate and source of a driving transistor occurring due to a voltage drop of the source voltage caused by the resistance component of a power source line, and a method for driving the organic EL display device.

In one embodiment of the present invention, there is provided an organic EL display device including: an organic EL panel comprising a plurality of data lines for transferring a data voltage representing a picture signal, a plurality of scan lines for transferring a scanning signal, and a pixel circuit formed by a plurality of pixels defined by the data and scan lines, the pixel circuit having an organic EL device and a driving transistor for driving the organic EL device; a scan driver for selectively applying the scanning signal to the scan lines; and a data driver for receiving digital image data and applying the digital image data and a data voltage corresponding to the position of the pixel circuit to the data lines.

The data driver outputs different data voltages depending on the position of the pixel circuit even when the same digital image data are received. More specifically, when the driving transistor is a P-type transistor, the data driver applies a higher data voltage to a pixel circuit that is closer to an external voltage source than that applied to a farther one even when the same digital data are received. Otherwise, when the driving transistor is an N-type transistor, the data driver applies a lower data voltage to a pixel circuit that is closer to an external voltage source than that applied to a farther one even when the same digital data are received.

In one embodiment of the present invention, there is provided an apparatus for driving an organic EL display device, which includes a plurality of data lines for transferring a data voltage representing a picture signal, a plurality of scan lines for transferring a scanning signal, and a pixel circuit formed by a plurality of pixels defined by the data and scan lines and having an organic EL device and a driving transistor for driving the organic EL device. The apparatus includes: a scan driver for selectively applying the scanning signal to the scan lines; a data driver for receiving RGB data as digital image data, and applying the digital image data and a data voltage corresponding to the position of the pixel circuit to the data lines; a graphic controller for generating the RGB data inherently or based on a picture signal that is externally applied; and a timing controller for generating horizontal and vertical sync signals from the RGB data, and sending the generated horizontal and vertical sync signals to the scan driver and sending the horizontal and vertical sync signals and the received RGB data to the data driver.

The data driver includes: a counter for detecting frame start information from the vertical sync signal and then counting the horizontal sync signal to output position data determining a scan line corresponding to a pixel circuit to which the RGB data are applied; a reference voltage adjuster for receiving the position data, and outputting a reference voltage corresponding to the position data; a voltage divider circuit comprising a plurality of resistances connected in series between a source voltage and the reference voltage; a switching section for selecting one of contact voltages each formed between the resistances of the voltage divider circuit; and a switch controller for receiving the horizontal and vertical sync signals and the RGB data, and controlling a switching operation of the switching section to select one contact voltage corresponding to the RGB data.

In one embodiment of the present invention, there is provided a method for driving an organic EL display device which includes a plurality of data lines for transferring a data voltage representing a picture signal, a plurality of scan lines for transferring a scanning signal, and a pixel circuit formed by a plurality of pixels defined by the data and scan lines and having an organic EL device and a driving transistor for driving the organic EL device. The method including: detecting the position of the pixel circuit from RGB data as digital image data; and (b) applying the RGB data and a data voltage corresponding to the position of the pixel circuit to the data lines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, as will be realized, the disclosed embodiment of the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.

FIG. 4is a diagram showing an organic EL display device in accordance with an embodiment of the present invention.

As shown inFIG. 4, the organic EL display device according to an embodiment of the present invention comprises an organic EL display panel10, a data driver20, a scan driver30, a timing controller40, and a graphic controller50.

The organic EL display panel10comprises a plurality of data lines D1, D2, D3, . . . and Dmfor transferring a data voltage representing picture signals, a plurality of scan lines S1, S2, S3, . . . and Snfor transferring scanning signals, and a pixel circuit11formed by a plurality of pixels each defined by the data and scan lines.

The pixel circuit11may comprise, as shown inFIG. 1, an organic EL device OELD, a P-type driving transistor M1, a switching transistor M2, and a capacitor Cst. Alternatively, the pixel circuit11may comprise, as shown inFIG. 5, an organic EL device OELD, an N-type driving transistor M3, a switching transistor M4, and a capacitor Cst.

The driving transistors M1and M3are connected to the organic EL device OELD to supply a current for emitting lights. The currents of the driving transistors M1and M3are controlled by the data voltage applied through the switching transistors M2and M4. The capacitor Cstfor maintaining the applied voltage for a predetermined period of time is connected between the source and gate of the transistors M1and M3.

The graphic controller50generates digital image data, i.e., RGB data, inherently or based on picture signals that are externally received.

The timing controller40generates horizontal sync signals Hsyncand vertical sync signals Vsyncfrom the RGB data to output the sync signals Vsyncand Hsyncto the scan driver30, or to output the sync signals Hsyncand Vsyncand the RGB data to the data driver20.

The method for generating horizontal sync signals Hsyncand vertical sync signals Vsyncfrom the RGB data is well known to those skilled in the art and will not be described herein.

The data driver20receives the sync signals Hsync and Vsync and the RGB data from the timing controller40generates a compensated data voltage with respect to scan lines in order to compensate for a reduction of the voltage between the gate and source of the driving transistors caused by a voltage drop of the power source line, and applies the compensated data voltage to the data lines. Here, the data driver20according to the embodiment of the present invention outputs different data voltages depending on the position of the pixel circuit, even when the same RGB data is received.

As will be described later, when with the same RGB data are received, the data driver20applies a higher data voltage to a pixel circuit that is closer to the external power source when using a P-type driving transistor, as shown inFIG. 1, or a lower data voltage to a pixel circuit that is closer to the external power source when using an N-type driving transistor, as shown inFIG. 5.

The scan driver30sequentially applies, to the plural scan lines, the scanning signals in synchronization with the sync signals received from the timing controller40.

FIG. 6is a detailed diagram of the data driver20in accordance with an embodiment of the present invention.

As shown inFIG. 6, the data driver20according to the embodiment of the present invention comprises a counter21, a reference voltage adjuster22, a voltage divider circuit24, a switching section25, a switch controller23, a shift register26, and a data buffer27.

The counter21receives the vertical sync signal Vsyncand the horizontal sync signal Hsyncand outputs information about the scan line corresponding to the pixel circuit to which the RGB data will be applied. Namely, the counter21detects frame start information from the vertical sync signal Vsyncand counts the horizontal sync signals Hsyncto output the position data that determines a scan line corresponding to the pixel circuit to which the RGB data will be applied.

The reference voltage adjuster22receives the position data from the counter21and outputs a reference voltage Vbcorresponding to the position data. The reference voltage is to compensate for a reduction of the voltage between the gate and source of the driving transistor caused by a voltage drop of the power source line. More specifically, the reference voltage adjuster22outputs a lower reference voltage to a pixel circuit that is farther from the external power source when using a P-type driving transistor, as shown inFIG. 1, or a higher data voltage to a pixel circuit that is farther from the external power source when using an N-type driving transistor, as shown inFIG. 5.

The voltage divider circuit24comprises i resistances R1, R2, . . . and Riconnected in series between a source voltage Vaand the reference voltage Vbof the reference voltage adjuster22. Contact voltages each formed between the resistances provide the respective gradation voltage levels.

The contact voltage Vxbetween the resistances is calculated by the following Equation 2:

As is apparent from Equation 2, the contact voltage Vxof the voltage divider circuit24becomes higher as the Vbincreases, i.e., the pixel circuit is nearer to the external power voltage source. The switching section25selects one of the contact voltages each formed between the resistances and sends the selected contact voltage to the shift register.

According to the voltage divider circuit shown inFIG. 6, the one voltage Vais constant (referred to as “source voltage” inFIG. 6) and the other voltage Vboutput from the reference voltage adjuster is variable depending on the position of the pixel circuit. Alternatively, the both voltages Vaand Vbcan be output from the reference voltage adjuster and controlled to be variable.

The switch controller23receives the horizontal sync signals Hsync, the vertical sync signals Vsync, and the RGB data, and controls the switching operation of the switching section25to select one contact voltage corresponding to the RGB data.

The shift register26sequentially shifts the selected contact voltage, and after shifting all the data voltages to be applied to the respective data lines, sends the voltages to the data buffer.

The data buffer27applies the data voltage, stored in synchronization with control signals (not shown), to the data lines.

According to one embodiment of the present invention, in order to compensate for a reduction of the voltage between the gate and source of the driving transistor due to a voltage drop of the power source line, a lower reference voltage is output to a pixel circuit that is farther from the external power voltage source than that applied to a closer one in the case of a P-type driving transistor. Thus even when RGB data of a same gradation level are output from the graphic controller, the embodiment of the present invention solves the problem regarding a reduction of the voltage difference between the gate and source of the driving transistor caused by a voltage drop of the power source line, since the data voltage applied to a pixel circuit far from the external power voltage source is lower than the data voltage applied to a pixel circuit that is adjacent to the external power voltage source.

For example, although the driving transistor of a pixel circuit has the same conductivity type as the switching transistor in the embodiment of the present invention, the transistors may differ from each other in the conductivity type.

As described above, according to one embodiment of the present invention, the reference voltage applied to the voltage divider circuit generating the data voltage is variable depending on the position of the pixel circuit, thereby compensating for a reduction of the voltage between the gate and source of the driving transistor occurring due to a drop of the source voltage caused by the resistance component of the power source line.