Display and driving method thereof

This application claims priority to and benefit of Korea Patent Application No. 2003-52603 filed on Jul. 30, 2003 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference.

(a) Field of the Invention

The present invention relates to a display and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display and a driving method thereof.

(b) Description of the Related Art

In general, an organic EL display is a display device for electrically exciting fluorescent and organic compounds and emitting light therefrom, and voltage or current drives (M×N) organic emission cells to represent images. An organic emission cell includes an anode (indium tin oxide: ITO), an organic thin film, and a metallic cathode layer. The organic thin film includes an emission layer (EML), an electron transport layer (ETL), a hole transport layer (HTL) for balancing electrons and holes to improve emission efficacy, and additionally includes an electron injection layer (EIL) and a hole injection layer (HIL).

Methods for driving the above-configured organic emission cells include the passive matrix method for allowing anodes and cathodes to cross each other according to an addressing method, selecting a line, and driving the line, and the active matrix method for connecting a thin film transistor (TFT) and a capacitor with each ITO pixel electrode so as to maintain a voltage by a capacitance. The active matrix method includes a voltage programming method and a current programming method according to signal types (a voltage or a current) used by a driving circuit.

An organic EL display comprises an organic EL display panel, a scan driver, and a data driver. The organic EL display panel includes a plurality of data lines for transmitting data signals that represent image signals, a plurality of scan lines for transmitting select signals, and pixel circuits each formed at a pixel area defined by two adjacent scan lines and two adjacent data lines. When the scan driver applies a select signal to a scan line, a transistor is turned on by the select signal, data signals for representing image signals are applied through the data lines to a gate of the transistor from the data driver, and a current flows to an organic EL element through the transistor to emit light.

In order to reduce the number of data driver ICs, demultiplexers or shift registers are used, and a current sample/hold circuit is used when the panel is driven according to the current programming method. It is usual for the current programming method that requires much time of programming data to pixels to use the current sample/hold circuit so as to realize high resolution. However, since the conventional sample/hold circuit uses TFTs as active devices for sampling and holding the current, the available current range is limited because of the TFT characteristics. Also, the sampled current and the held current are different because of the kickback phenomenon that is caused when switching the TFTs, and the output current of the driver IC and the current for driving the data lines become different.

Control the output current of the driver IC is then so as to eliminate the current variation, and hence, a gamma correction process is needed. A deviation of the hold current caused by the deviation of the TFT characteristics is generated during the gamma correction process, and the images may not be uniform.

In accordance with the present invention an EL display is provided using a demultiplexer based analog switch, and a driving method thereof.

In one aspect of the present invention, a display apparatus includes a display area having first and second data lines for transmitting data signals that represent video signals, a first scan line for transmitting a select signal, a first pixel circuit coupled to the first data line and the first scan line, and a second pixel circuit coupled to the second data line and the first scan line; a data driver for outputting data signals corresponding to the first and second data lines through signal lines; a scan driver for outputting the select signal; and a demultiplexer for transmitting the data signals from the signal lines to the respective first and second data lines. A single frame includes first and second fields. The data signal from the first data line is programmed to the first pixel circuit during a first period of the first field, and the data signal from the second data line is programmed to the second pixel circuit during a second period of the second field.

In another aspect of the present invention, a display apparatus includes a display area having first and second data lines formed in one direction, first and second adjacent scan lines formed to cross the first and second data lines, a first pixel circuit coupled to the first data line and the first scan line, a second pixel circuit coupled to the second data line and the first scan line, a third pixel circuit coupled to the first data line and the second scan line, and a fourth pixel circuit coupled to the second data line and the second scan line; a data driver for outputting data signals corresponding to the first and second data lines through signal lines; and a demultiplexer for transmitting the data signals from the signal lines to the first and second data lines. A single frame includes first and second fields. The first pixel circuit displays images during the first field. The second pixel circuit displays images during the second field. The third pixel circuit displays images during a third field generated by moving the second field for a predetermined time. The fourth pixel circuit displays images during a fourth field generated by moving the first field for a predetermined time.

In still another aspect of the present invention, a display apparatus includes a display area having first and second data lines formed in one direction, first and second adjacent scan lines formed to cross the first and second data lines, a first pixel circuit coupled to the first data line and the first scan line, a second pixel circuit coupled to the second data line and the first scan line, a third pixel circuit coupled to the first data line and the second scan line, and a fourth pixel circuit coupled to the second data line and the second scan line; a data driver for outputting data signals corresponding to the first and second data lines through signal lines; and a demultiplexer for transmitting the data signals from the signal lines to the first and second data lines. A single frame includes first and second fields. The first pixel circuit displays images during the first field. The second pixel circuit displays images during the second field. The third pixel circuit displays images during a third field generated by moving the first field for a predetermined time. The fourth pixel circuit displays images during a fourth field generated by moving the second field for a predetermined time.

In still yet another aspect of the present invention, in a driving method of a display apparatus including a plurality of first data lines, second data lines formed between the adjacent first data lines, first scan lines, second scan lines formed between the adjacent first scan lines, and pixels formed by one of the first and second data lines and one of the first and second scan lines and each having an emission element, the driving method divides the data signals applied through signal lines to the first and second data lines, and applies the divided data signals. A frame is divided into a plurality of fields, and the fields are driven. Data signals are applied to a pixel through the first data line during a first field among the fields, the pixel being formed along the first data line and formed on an area defined by the first scan line and the first data line. Data signals are applied to a pixel through the second data line during a second field among the fields, the pixel being formed along the second data line and formed on an area defined by the first scan line and the second data line.

In a still further aspect of the present invention, a display apparatus includes a plurality of groups formed of a plurality of data lines and scan lines, and pixels coupled to respective data lines and scan lines. The display apparatus includes a data driver for outputting data signals corresponding to the respective data lines of each group through a signal line; and a demultiplexer for transmitting the data signals from the signal line to the data lines. A single frame includes first and second fields. The pixel circuit coupled to a first data line among the data lines and a first scan line among the scan lines in the group displays images during the first field. The pixel circuit coupled to a second data line and the first scan line in the group displays images during the second field. The pixel circuit coupled to a third data line among the data lines and a second scan line in the group displays images during a third field generated by moving the first field for a predetermined time. The pixel circuit coupled to a fourth data line among the data lines except the third data line and the second scan line displays images during a fourth field generated by moving the second field for a predetermined time.

In one embodiment according to the present invention, the third data line is the second data line. In another embodiment, the fourth data line is a data line other than the first and second data lines.

FIG. 1 shows a brief diagram of an organic EL display according to an exemplary embodiment of the present.

FIG. 2 shows a demultiplexer coupled to a data driver of the organic EL display according to an exemplary embodiment of the present.

FIG. 3 shows two pixel circuits coupled to the demultiplexer of FIG. 2 .

FIGS. 4A , 4 B, 5 A, and 5 B show timing diagrams of signals that follow the organic EL display driving method, and pixels that are turned on according to a first exemplary embodiment of the present invention.

FIGS. 6A , 6 B, 7 A, and 7 B show timing diagrams of signals that follow the organic EL display driving method, and pixels that are turned on according to a first exemplary embodiment of the present invention.

FIG. 1 shows a brief diagram of an organic EL display according to an exemplary embodiment of the present invention.

As shown, the organic EL display comprises organic EL display panel 100 , scan drivers 200 and 300 , data driver 400 , and demultiplexer 500 .

Organic EL display panel 100 comprises a plurality of data lines Data[1] to Data[n] for transmitting data signals that represent video signals, a plurality of scan lines select1[1] to select1[m] and select2[1] to select2 [m], and a plurality of pixel circuits 110 . Data lines Data[1] to Data[n] transmit data signals that represent video signals to pixel circuits 110 , scan lines select1[1] to select1[m] transmit select signals for selecting pixel circuits 110 to pixel circuits 110 , and scan lines select2[1] to select2 [m] transmit emit signals for emitting light to pixel circuits 110 . Pixel circuits 110 are respectively formed at a plurality of pixels surrounded by data lines Data[1] to Data[n] and scan lines select1[1] to select1[m]. Organic EL display panel 100 forms a display area, and scan drivers 200 and 300 , data driver 400 , and/or demultiplexer 500 can be formed on organic EL display panel 100 .

Scan driver 200 sequentially applies select signals to scan lines select1[1] to select1[m], and scan driver 300 sequentially applies emit signals to scan lines select2[1] to select2 [m]. Data driver 400 applies data signals for representing video signals to demultiplexer 500 , and demultiplexer 500 has a 1:2 format, and divides the data signals sequentially applied through a single line from data driver 400 into two data lines.

FIG. 2 shows a representative portion of demultiplexer 500 coupled to the data driver of the organic EL display according to an exemplary embodiment of the present invention.

Demultiplexer 500 is coupled to data driver 400 through signal lines SP 1 to SPn′, and transmits the data signals sequentially applied through a single signal line SP 1 to SPn′ to two data lines Data[2i−1] and Data[2i]. One signal line SP 1 to SPn′ is coupled to two switches, and the two switches are respectively coupled to a single data line. For example, two switches S 1 and S 2 are coupled to signal line SPi, and switches S 1 and S 2 are respectively coupled to data lines Data[2i−1] and Data[2i]. The switches are alternately turned on to transmit the data signal from signal line SPi to respective data lines Data[2i−1] and Data[2i]. Switches S 1 and S 2 can be formed with NMOS transistors, PMOS transistors, or other similar types of transistors.

Referring to FIG. 3 , a method for driving the organic EL display using demultiplexer 500 according to an exemplary embodiment of the present invention will be described. For ease of description, two pixel circuits 110 a and 110 b coupled to (2i−1) th and 2i th data lines Data[2i−1] and Data[2i] and a j th scan line select1[j] will be depicted.

Data lines Data[2i−1] and Data[2i] of two pixel circuits 110 a and 110 b are coupled to signal line SPi from data driver 400 through switches S 1 and S 2 . Pixel circuit 110 a comprises four transistors M 1 to M 4 , capacitor Cst, and organic EL element OLED. Pixel circuit 110 b comprises four transistors M 1 ′ to M 4 ′, capacitor Cst′, and an organic EL element OLED′.

When switch S 1 is turned on while transistors M 1 , M 2 , M 1 ′, and M 2 ′ are turned on by the select signal from scan line select1[j], a data signal from signal line SPi is applied to pixel circuit 110 a through data line Data[2i−1]. Since transistor M 3 is diode-connected by turned-on transistors M 1 and M 2 , a voltage corresponding to the data signal from data line Data[2i−1] is programmed to capacitor Cst.

When switch S 2 is turned on, the data signal from signal line SPi is applied to pixel circuit 110 b through data line Data[2i]. The voltage corresponding to the data signal from data line Data[2i] is programmed to capacitor Cst′ since transistor M 3 ′ is diode-connected by turned-on transistors M 1 ′ and M 2 ′. In this instance, since switch S 1 is turned off and the current of 0 A is transmitted through data line Data[2i−1], the voltage (a blank signal) corresponding to 0 A is programmed to capacitor Cst.

Therefore, when transistors M 4 and M 4 ′ are turned on by an emit signal from scan line select2[j] to allow pixel circuits 110 a and 110 b to emit light, current of 0 A from pixel circuit 110 a flows to organic EL element OLED. That is, pixel circuit 110 a fails to display the original gray and enters into a blank state.

To solve this problem, an additional scan line can be used for pixel circuits 110 a and 110 b , but the additional usage of a scan line increases wiring, reduces an aperture ratio, and further needs a scan driver for controlling the added scan line, thereby raising the cost.

To remedy this problem, the method for driving the organic EL display divides a single frame into first and second fields, and drives switches S 1 and S 2 coupled to signal line SPi so that two adjacent pixels coupled to two data lines Data[2i−1] and Data[2i] to which the data signal output from single signal line SPi is applied may be respectively turned on during the first and second fields. A driving method for the organic EL display according to the exemplary embodiment of the present invention will be described.

FIGS. 4A , 4 B, 5 A, and 5 B show timing diagrams of signals that follow the organic EL display driving method, and pixels that are turned on according to a first exemplary embodiment of the present invention.

Referring to FIGS. 4A and 4B , switches S 1 and S 2 are alternately turned on and off during the first field so that the data signal may be alternately applied to two adjacent data lines Data[2i−1] and Data[2i] while the select signal output through scan driver 200 is sequentially applied to respective scan lines select1[1] to select1[m].

That is, when the select signal is applied to scan line select1[1], switch S 1 is turned on, and switch S 2 is turned off. As a result, the data signal is applied to data line Data[2i−1], and supply of the data signal to data line Data[2i] is intercepted. Therefore, when an emit signal is applied to scan line select2[1], the pixel circuit coupled to scan line select1[1] and data line Data[2i−1] emits light, and the pixel circuit coupled to scan line select1[1] and data line Data[2i] becomes blank and emits no light. It is desirable to apply the emit signal to scan line select2[1] after an enable period of the select signal applied to scan line select1[1] is finished. In another way, it is possible to allow the pixel circuit to emit light when the enable period of the select signal is finished, by eliminating scan lines select2[1] to select2[m] for emit signal transmission, changing transistors M 4 and M 4 ′ in the pixel circuit of FIG. 3 with NMOS transistors, and driving the NMOS transistors through scan lines select1[1] to select1[m].

Next, when the select signal is applied to scan line select1[2], switch S 2 is turned on, and switch S 1 is turned off. As a result, the data signal is applied to data line Data[2i], and supply of the data signal to data line Data[2i−1] is intercepted. Therefore, when an emit signal is applied to scan line select2[2], the pixel circuit coupled to scan line select1[2] and data line Data[2i] emits light, and the pixel circuit coupled to scan line select1[2] and data line Data[2i−1] becomes blank and emits no light.

Switches S 1 and S 2 are alternately turned on/off to sequentially apply a data signal to data lines Data[2i−1] and Data[2i] while a select signal is applied to scan lines select[3] to select[m]. Accordingly as shown in FIG. 4B , the data signal is programmed to the pixel circuit coupled to odd scan line select1[2j−1] and odd data line Data[2i−1] and the pixel circuit coupled to even scan line select1[2j] and even data line Data[2i] in the first field. The pixel circuits to which the data signal is programmed emit light until they become blank by a second field, that is, during a half of the single frame period. Further, the emission period of the pixel circuits can be reduced by controlling the timing of the emit signal.

Next, referring to FIGS. 5A and 5B , in the second field, switches S 1 and S 2 are alternately turned on/off in the opposite manner of the first field case as shown in FIG. 5A so that the data signal may be applied to two adjacent data lines Data[2i−1] and Data[2i] while the select signal output through scan driver 200 is sequentially applied to respective scan lines select1[1] to select1[m].

As shown in FIG. 5A , when a select signal is applied to scan line select1[1], switch S 2 is turned on, and switch S 1 is turned off. As a result, the data signal is applied to data line Data[2i], and supply of the data signal to data line Data[2i−1] is intercepted. Therefore, when an emit signal is applied to scan line select2[1], the pixel circuit coupled to scan line select1[1] and data line Data[2i] emits light, and the pixel circuit coupled to scan line select1[1] and data line Data[2i−1] becomes blank and emits no light. As described in the operation of the first field, it is desirable to apply the emit signal to scan line select2[1] after an enable period of the select signal applied to scan line select1[1] is finished. In another way, it is possible to allow the pixel circuit to emit light when the enable period of the select signal is finished, by eliminating scan lines select2[1] to select2[m] for emit signal transmission, changing transistors M 4 and M 4 ′ in the pixel circuit of FIG. 3 with NMOS transistors, and driving the NMOS transistors through scan lines select1[1] to select1[m].

Next, when the select signal is applied to scan line select1[2], switch S 1 is turned on, and switch S 2 is turned off. As a result, the data signal is applied to data line Data[2i−1], and supply of the data signal to data line Data[2i] is intercepted. Therefore, when an emit signal is applied to scan line select2[2], the pixel circuit coupled to scan line select1[2] and data line Data[2i−1] emits light, and the pixel circuit coupled to scan line select1[2] and data line Data[2i] becomes blank and emits no light.

Switches S 1 and S 2 are alternately turned on/off to sequentially apply a data signal to data lines Data[2i−1] and Data[2i] while a select signal is applied to scan lines select[3] to select[m]. Accordingly as shown in FIG. 5B , the data signal is programmed to the pixel circuit coupled to odd scan line select1[2j−1] and even data line Data[2i] and the pixel circuit coupled to even scan line select1[2j] and odd data line Data[2i−1] in the second field. The pixel circuits to which the data signal is programmed emit light after they become blank by the first field, that is, during a half the single frame period. Further, the emission period of the pixel circuits can be reduced by controlling the timing of the emit signal.

As described, since the driving method according to the first exemplary embodiment uses the duty driving method for emitting light during the half period of a single frame, the data signal (the current) is doubled compared to the conventional driving methods, thereby solving the problem of the reduced available programming time. Also, the duty driving method according to the first exemplary embodiment improves the flicker phenomenon generated from the conventional duty driving methods since it sequentially emits the odd pixels and the even pixels.

Also, the organic EL display driving method according to the first exemplary embodiment alternately turns on/off switches S 1 and S 2 so that the data signal may be alternately applied to the two adjacent data lines Data[2i−1] and Data[2i] while the select signal output through scan driver 200 is sequentially applied to respective scan lines select1[1] to select1[m], and in addition, the method can turn on/off switches S 1 and S 2 so that the data signal may be programmed to the pixel circuit coupled to odd scan line select1[2j−1] and even data line select[2i] and the pixel circuit coupled to even scan line select [2j] and odd data line Data[2i−1] during the first field, and the data signal may be programmed to the pixel circuit coupled to odd scan line select1[2j−1] and odd data line Data[2i−1] and the pixel circuit coupled to even scan line select[2j] and even data line Data[2i] during the second field.

The organic EL display driving method according to the second exemplary embodiment alternately allows the pixel circuit coupled to the odd data line and the pixel circuit coupled to the even data line to emit light respectively during the first and second fields.

FIGS. 6A , 6 B, 7 A, and 7 B show timing diagrams of signals that follow the organic EL display driving method, and pixels that are turned on according to a second exemplary embodiment of the present invention.

Referring to FIGS. 6A and 6B , switch S 1 is turned on and switch S 2 is turned off during the first field while the select signal output through scan driver 200 is sequentially applied to respective scan lines select1[1] to select1[m]. As a result, the data signal is applied to data line Data[2i−1], and supply of a data signal to data line Data[2i] is intercepted. Therefore, when the emit signal is sequentially applied to scan lines select2[1] to select2[m], the pixel circuits coupled to scan lines select1[1] to select1[m] and odd data line Data[2i−1] sequentially emit light, and the pixel circuits coupled to scan lines select1[1] to select1[m] and even data line Data[2i] become blank and emit no light as shown in FIG. 6B .

It is desirable to apply the emit signal to scan lines select2[1] to select2[m] when the enable period of the select signal applied to scan lines select1[1] to select1[m] is finished. Further, it is possible to eliminate scan lines select2[1] to select2[m] for transmitting the emit signal, change the transistors M 4 and M 4 ′ of FIG. 3 with NMOS transistors, drive the NMOS transistors through scan lines select1[1] to select1[m], and thereby allow the pixel circuit to emit light when the enable period of the select signal is finished.

Also, the pixel circuits to which the data signal is programmed emit light until they become blank by the second field described later, that is, during about a half the single frame. Further, the period during which the pixel circuits emit light can be reduced by controlling the timing of the emit signal.

Referring to FIGS. 7A and 7B , switch S 1 is turned off and switch S 2 is turned on in the second field opposite to the case of the first field while the select signal output through scan driver 200 is sequentially applied to respective scan lines select1[1] to select1[m]. Therefore, when the emit signal is sequentially applied to scan lines select2[1] to select2[m], the pixel circuits coupled to scan lines select1[1] to select1[m] and even data line Data[2i] sequentially emit light, and the pixel circuits coupled to scan lines select1[1] to select1[m] and odd data line Data[2i−1] become blank and emit no light.

It is desirable to apply the emit signal to scan lines select2[1] to select2[m] when the enable period of the select signal applied to select1[1] to select1[m] is finished. Further, it is possible to allow the pixel circuit to emit light when the enable period of the select signal is finished, by eliminating scan lines select2[1] to select2[m] for emit signal transmission, changing the transistors M 4 and M 4 ′ in the pixel circuit of FIG. 3 with NMOS transistors, and driving the NMOS transistors through scan lines select1[1] to select1[m].

Also, the pixel circuits to which the data signal is programmed emit light after they become blank by the first field, that is, during about a half the single frame period. Further, the emission period of the pixel circuits can be reduced by controlling the timing of the emit signal.

Since the data signal is applied to even data line Data[2i] after it is applied to odd data line Data[2i−1], and the duty driving method is used in the second exemplary embodiment, the time for programming the data signal can be shortened by doubling the data current.

Also, by turning on switch S 2 and turning off switch S 1 while the select signal output through scan driver 200 is sequentially applied to respective scan lines select1[1] to select1[m] in the first field, and turning on switch S 1 and turning off switch S 2 in the second field, the organic EL display driving method according to the second exemplary embodiment allows the pixel circuits coupled to scan lines select1[1] to select1[m] and even data line Data[2i] to emit light during the first field, and allows the pixel circuits coupled to scan lines select1[1] to select1[m] and odd data line Data[2i−1] to emit light during the second field.

While this invention has been described in connection with what is presently considered to be 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.

For example, a 1:2 demultiplexer is shown in the exemplary embodiments, but the present invention can be applied to 1:N demultiplexers. In the case of using a 1:3 demultiplexer, a single frame would be divided into first to third fields, and three switches (not illustrated) coupled to a signal line SPi would be driven so that three adjacent pixels coupled to three data lines to which the data signal output from single signal line SPi emit light respectively during the first to third fields.

Further, by the driving method according to the first exemplary embodiment, the data signal would be programmed to the pixel circuit coupled to scan line select1[3j−2] and data line Data[3i−2], the pixel circuit coupled to scan line select1[3j−1] and data line Data[3i−1], and the pixel circuit coupled to scan line select1[3j] and data line Data[3i] during the first field, the data signal would be programmed to the pixel circuit coupled to scan line select1[3j−2] and data line Data[3i−1], the pixel circuit coupled to scan line select1[3j−1] and data line Data[3i], and the pixel circuit coupled to scan line select1[3j] and data line Data[3i−2] during the second field, and the data signal would be programmed to the pixel circuit coupled to scan line select1[3j−2] and data line Data[3i], the pixel circuit coupled to scan line select1[3j−1] and data line Data[3i−2], and the pixel circuit coupled to scan line select1[3j] and data line Data[3i−1] during the third field.

Also, by the driving method according to the second exemplary embodiment, the pixel circuits coupled to scan lines select1[1] to select1[m] and data line Data[3i−2] sequentially emit light during the first field, the pixel circuits coupled to scan lines select1[1] to select1[m] and data line Data[3i−1] sequentially emit light during the second field, and the pixel circuits coupled to scan lines select1[1] to select1[m] and data line Data[3i] sequentially emit light during the third field.

As described above, since the current supply is doubled by driving the organic EL display apparatus according to the present invention, the problem of reduced programming time is overcome. In addition, the flicker phenomenon caused by the duty driving is improved by dividing a single frame into a plurality of fields, and the time for programming the data is shortened.