Electroluminescent display and method of driving same

An electroluminescent (EL) display device has pixels spatially arranged in a matrix, each pixel having first, second, third and fourth EL elements, gate lines each coupled to a respective row of pixels, first data lines each coupled to the first EL elements of a respective column of pixels, second data lines each coupled to the second EL elements of a respective column of pixels, first switches each coupled to a respective third EL element, second switches each coupled to a respective fourth EL element, a switch control line coupled to the first and second switches for providing a switching control signal to set the first and second switches to be in an ON state or an OFF state, and third data lines each coupled to the first and second switches corresponding to a respective column of pixels for transmitting a data signal.

FIELD OF THE INVENTION

The present invention relates generally to an electroluminescent (EL) display, and more particularly to an EL display device with each pixel having four light emitting elements and methods of driving same.

BACKGROUND OF THE INVENTION

Display devices employing electroluminescent display elements, such as organic light emitting diodes (OLEDs), have become a popular choice among flat panel displays. EL displays are used as television screens, computer monitors, portable electronic systems such as mobile phones and personal digital assistants (PDAs). An OLED is a light emitting diode (LED) in which the emissive electroluminescent (EL) layer is a film of an organic material which emit light in response to an electric current. This layer of organic material is situated between two electrodes. Generally, at least one of these electrodes is transparent. An EL display functions without a backlight. Thus, it can display deep black levels and can also be thinner and lighter than other flat panel displays such liquid crystal displays (LCDs).

An EL display panel may comprise a plurality of pixels spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns in a display area. For a color EL display panel, each pixel may comprise several OLEDs each emitting light of a different color in the visible spectrum from which various colors may be composed.FIG. 8(a) shows schematically a typical pixel810for a color EL display panel that has three light emitting elements812, each emitting red (R), green (G), and blue (B) light, respectively. The traditional organic material used for blue OLEDs has a relatively low luminous efficiency. Consequently, a relatively large electrical current is required in order for a blue OLED to emit light with a certain intensity. Operating an OLED at the large electrical currents would have the adverse effect of shortening the lifetime of the OLED.

Recently, a new material of emitting yellow-green light with a higher luminous efficiency has been reported. Table 1 summarizes the luminous efficiencies for four EL materials used in color EL displays. In this table, R stands for an EL material for red OLEDs, G stands for an EL material for green OLEDs, B2stands for a traditional EL material for blue OLEDs, and B1stands for a new EL material for yellow-green OLEDs. The 1931 CIE color space (x, y) parameters associated with the light emitted by these materials are also listed. As can be seen in Table 1, the new EL material B1for yellow-green OLEDs has a luminous efficiency that is more than four times that of the traditional EL material B2for blue OLEDs. The new material B1may be used in addition to the traditional material B2for producing blue light in a color EL display panel.

FIG. 8(b) shows a pixel820for a color EL display panel that includes four light emitting elements822, one emitting red light (R), one emitting green light (G), one emitting yellow-green light (B1), and one emitting blue light (B2). The four light emitting elements822are spatially arranged as a 2×2 sub-array.

FIG. 9shows schematically an EL display panel900that has such a pixel arrangement asFIG. 8(b). The EL display panel900includes a plurality of pixels910spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns. Each pixel910includes four light emitting elements912labeled as R, G, B1, and B2, respectively, according to the color of the light emitted by the elements. The EL display panel900includes a plurality of gate lines GAn and GBn (n=1,2, . . . ) and a plurality of data lines SAn and SBn (n=1,2, . . . ). Each gate line GAn is coupled to the R and B1elements of a respective row of pixels, and each gate line GBn is coupled to the G and B2elements of a respective row of pixels. A gate driver outputs scanning signals to the plurality of gate lines GAn and GBn sequentially so that the elements in the pixels are sequentially driven in a row-by-row fashion. Each data line SAn is coupled to the R and G elements of a respective column of pixels, and each data line SBn is coupled to the B1and B2elements of a respective column of pixels. Therefore, in this configuration, two gate lines GAn and GBn are required for each row of pixels and two data lines SAn and SBn are required for each column of pixels. For example, for a display panel with a 240×320 resolution, there would be 480 total data lines and 640 total gate lines. In comparison, for a display with the same resolution but comprising RGB pixels as shown inFIG. 8(a), the required data lines and gate lines would be 720 and 320, respectively. Therefore, the display panel900requires twice the number of gate lines as that required in a display panel with the same resolution comprising RGB pixels. Consequently, the scan driver is more complicated which in turn results in higher cost.

SUMMARY OF THE INVENTION

The present invention, in one aspect, relates to an electroluminescent (EL) display device. In one embodiment, the EL display device includes a plurality of pixels spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns, each pixel has a first EL element operably emitting light of a first color, a second EL element operably emitting light of a second color, a third EL element operably emitting light of a third color, and a fourth EL element operably emitting light of a fourth color. In one embodiment, the first color is a red color, the second color is a green color, the third color is a blue color and the fourth color is a yellow-green color. In one embodiment, the first EL element, the second EL element, the third EL element and the fourth EL element of each pixel are spatially arranged as a 2×2 sub-array.

The EL display device also includes a plurality of gate lines. Each gate line is coupled to a respective row of pixels.

The EL display device further includes a plurality of first data lines, a plurality of second data lines, and a plurality of third data lines. Each first data line is coupled to the first EL elements of a respective column of pixels. Each second data line is coupled to the second EL elements of a respective column of pixels.

Additionally, the EL display device includes a plurality of first switches, a plurality of second switches, and a plurality of switch control lines. Each first switch coupled to a respective third EL element. Each second switch coupled to a respective fourth EL element. The first switches coupled to the third EL elements of a respective column of pixels are configured to be cooperatively in an ON state or an OFF state. The second switches coupled to the fourth EL elements of a respective column of pixels are configured to be cooperatively in an ON state or an OFF state. Each switch control line is coupled to the first switches and the second switches corresponding to a respective column of pixels for transmitting a switching control signal to set the first switches and the second switches to be in the ON state or the OFF state, such that when the first switches are cooperatively in the ON state, the second switches are cooperatively in the OFF state, and vice versus.

Further, each third data line coupled to the first switches and the second switches corresponding to a respective column of pixels for transmitting a data signal, such that when the first switches are cooperatively in the ON state, the data signal is provided to the third EL elements of the respective column of pixels, and when the second switches are cooperatively in the ON state, the data signal is provided to the fourth EL elements of the respective column of pixels.

In one embodiment, the EL display device may also have an image processor for processing an image to be displayed according to the pixel matrix of the EL display device such that the processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element, the second EL element, the third EL element and the fourth EL element, respectively, of the respective pixel, and a selector configured to compare the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel, to assign a logic value of 1 to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal, and to count the number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of each column of pixels.

In one embodiment, the switching control signal for a respective column of pixels is configured to set the first switches and the second switches corresponding to the respective column of pixels in the ON state and the OFF state, respectively, if NB1≧NB2 for the respective column of pixels, and the first switches and the second switches in the OFF state and the ON state, respectively, if NB1<NB2 for the respective column of pixels.

In one embodiment, the switching control signal for a respective column of pixels is characterized with a waveform having a low voltage and a high voltage. When the switching control signal for the respective column of pixels has one of the low voltage and the high voltage, the first switches and the second switches corresponding to the respective column of pixels are set in the ON state and the OFF state, respectively. When the switching control signal for the respective column of pixels has the other of the low voltage and the high voltage, the first switches and the second switches corresponding to the respective column of pixels are set in the OFF state and the ON state, respectively. In one embodiment, each of the plurality of first switches comprises one of an N-type thin film transistor (TFT) and a P-type TFT, and each of the plurality of second switches comprises the other of the N-type TFT and the P-type TFT, wherein each of the N-type TFT and the P-type TFT has a gate, a source and a drain. The gate, source and drain of each N-type TFT are respectively coupled to a corresponding switching control line, a corresponding third data line and one of the third EL element and the fourth EL element of a corresponding pixel. The gate, source and drain of each P-type TFT are respectively coupled to the corresponding switching control line, the corresponding third data line and the other of the third EL element and the fourth EL element of the corresponding pixel.

In another aspect, the present invention relates to a method of driving the EL display device disclosed above. In one embodiment, the method includes the steps of processing an image to be displayed according to the pixel matrix of the EL display device such that the processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element, the second EL element, the third EL element and the fourth EL element, respectively, of the respective pixel, and comparing the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel, assigning a logic value of 1 to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal, counting the number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of each column of pixels, and configuring the switching control signal for a respective column of pixels so as to set the first switches and the second switches corresponding to the respective column of pixels in the ON state and the OFF state, respectively, if NB1≧NB2 for the respective column of pixels, and the first switches and the second switches in the OFF state and the ON state, respectively, if NB1<NB2 for the respective column of pixels.

In yet another aspect, the present invention relates to an EL display device. In one embodiment, the EL display device has a plurality of pixels spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns, each pixel comprising a first EL element operably emitting light of a first color, a second EL element operably emitting light of a second color, a third EL element operably emitting light of a third color, and a fourth EL element operably emitting light of a fourth color, a plurality of gate lines, each gate line coupled to a respective row of pixels, a plurality of first data lines, each first data line coupled to the first EL elements of a respective column of pixels, a plurality of second data lines, each second data line coupled to the second EL elements of a respective column of pixels, a plurality of first switches, each first switch coupled to a respective third EL element, wherein the plurality of first switches are configured to be cooperatively in an ON state or an OFF state, a plurality of second switches, each second switch coupled to a respective fourth EL element, wherein the plurality of second switches are configured to be cooperatively in an ON state or an OFF state, a switch control line coupled to the plurality of first switches and the plurality of second switches for providing a switching control signal to set the first switches and the second switches to be in the ON state or the OFF state, such that when the plurality of first switches are cooperatively in the ON state, the plurality of second switches are cooperatively in the OFF state, and vice versus, and a plurality of third data lines, each third data line coupled to the first switches and the second switches corresponding to a respective column of pixels for transmitting a data signal, such that when the first switches are cooperatively in the ON state, the data signal is provided to the third EL elements of the respective column of pixels, and when the second switches are cooperatively in the ON state, the data signal is provided to the fourth EL elements of the respective column of pixels.

In one embodiment, the EL display device further has an image processor for processing an image to be displayed according to the pixel matrix of the EL display device such that the processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element, the second EL element, the third EL element and the fourth EL element, respectively, of the respective pixel, and a selector configured to compare the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel, to assign a logic value of 1 to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal, and to count the number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of the plurality of pixels.

In one embodiment, the switching control signal is configured to set the plurality of first switches in the ON state and the plurality of second switches in the OFF state, respectively, if NB1≧NB2, and the plurality of first switches in the OFF state and the plurality of second switches in the ON state, respectively, if NB1<NB2.

In one embodiment, the switching control signal is characterized with a waveform having a low voltage and a high voltage. When the switching control signal has one of the low voltage and the high voltage, the plurality of first switches and the plurality of second switches are set in the ON state and the OFF state, respectively. When the switching control signal has the other of the low voltage and the high voltage, the plurality of first switches and the plurality of second switches are set in the OFF state and the ON state, respectively.

In one embodiment, each of the plurality of first switches comprises one of an N-type TFT and a P-type TFT, and each of the plurality of second switches comprises the other of the N-type TFT and the P-type TFT, wherein each of the N-type TFT and the P-type TFT has a gate, a source and a drain. The gate, source and drain of each N-type TFT are respectively coupled to the switching control line, a corresponding third data line and one of the third EL element and the fourth EL element of a corresponding pixel. The gate, source and drain of each P-type TFT are respectively coupled to the switching control line, the corresponding third data line and the other of the third EL element and the fourth EL element of the corresponding pixel.

In one embodiment, the first color is a red color, the second color is a green color, the third color is a blue color and the fourth color is a yellow-green color. In one embodiment, the first EL element, the second EL element, the third EL element and the fourth EL element of each pixel are spatially arranged as a 2×2 sub-array.

In a further aspect, the present invention relates to a method of driving the EL display device. In one embodiment, the method has the steps of processing an image to be displayed according to the pixel matrix of the EL display device such that the processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element, the second EL element, the third EL element and the fourth EL element, respectively, of the respective pixel, comparing the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel, assigning a logic value of 1 to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal, counting the number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of the plurality of pixels, and configuring the switching control signal so as to set the plurality of first switches in the ON state and the plurality of second switches in the OFF state, respectively, if NB1≧NB2, and the plurality of first switches in the OFF state and the plurality of second switches in the ON state, respectively, if NB1<NB2.

DETAILED DESCRIPTION OF THE INVENTION

The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms that are used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the invention. The use of examples anywhere in this specification, including examples of any terms discussed herein, is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.

The description will be made as to the embodiments of the present invention in conjunction with the accompanying drawings inFIGS. 1-7. In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to an EL display device with each pixel having four light emitting elements and a method of driving the same.

Referring toFIG. 1, an EL display device100is schematically shown according to one embodiment of the present invention. The EL display device100includes a plurality of pixels110spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns. Each pixel110has four light emitting elements111-114spatially arranged as a 2×2 sub-pixel array. The four light emitting elements111-114in one embodiment correspond to a first EL element111operably emitting light of a first color, a second EL element112operably emitting light of a second color, a third EL element113operably emitting light of a third color, and a fourth EL element114operably emitting light of a fourth color. The first, second and third colors are corresponding to red (R), green (G) and yellow-green (B1) colors, respectively, while the fourth color in one embodiment is corresponding to a blue (B2) color. Each of four light emitting elements111-114is formed of a different EL material. In one embodiment, the EL material forming the light emitting element (B1)113has a higher luminous efficiency than that of the EL material in the light emitting element (B2)114.

The EL display device100also includes a plurality of gate lines, {Gj}, j=1, 2, 3, . . . , M, M being a positive integer and the row number of the pixel matrix. Each gate line Gj is coupled to the four light emitting elements111-114of a respective row of pixels110. In this arrangement, only one gate line is required for each row of pixels, which substantially reduces the number of the gate lines required for each row of pixels in a conventional EL display device900shown inFIG. 9. In operation, a plurality of scanning signals generated from a gate driver (not shown) is respectively applied the plurality of gate lines {Gj } for sequentially driving the pixels in a row-by-row fashion.

The EL display device100further includes a plurality of first data lines {SRk}, k=1, 2, 3, . . . , N, N being a positive integer and the column number of the pixel matrix, a plurality of second data lines {SGk}, and a plurality of third data lines {SBk}. Each first data line SRk is coupled to the first EL elements (R)111of a respective column of pixels. Each second data line SGk is coupled to the second EL elements112of a respective column of pixels. Each third data line SBk is coupled to the third EL elements (B1)113and the fourth EL elements (B2)114of a respective column of pixels.

The EL display device100also includes a plurality of first switches121, a plurality of second switches122, and a plurality of switch control lines {Qk}.

As shown inFIG. 1, each first switch121is coupled to a respective third EL element (B1)113, while each second switch122is coupled to a respective fourth EL element (B2)114. In this exemplary embodiment, the first switches121coupled to the third EL elements (B1)113of a respective column of pixels are configured to be cooperatively in an ON state or an OFF state. The second switches122coupled to the fourth EL elements (B2)114of a respective column of pixels are configured to be cooperatively in an ON state or an OFF state. Each switch control line Qk is coupled to the first switches121and the second switches122corresponding to a respective column of pixels for transmitting a switching control signal to set the first switches121and the second switches122to be in the ON state or the OFF state, such that when the first switches121are cooperatively in the ON state, the second switches122are cooperatively in the OFF state, and vice versus.

Further, each third data line SBk is coupled to the first switches121and the second switches122corresponding to a respective column of pixels for transmitting a data signal, such that when the first switches121are cooperatively in the ON state, the data signal is provided to the third EL elements113of the respective column of pixels, and when the second switches122are cooperatively in the ON state, the data signal is provided to the fourth EL elements114of the respective column of pixels.

According to this embodiment shown inFIG. 1, only three data lines SRk, SGk and SBk are required for each column of pixels, same as in a conventional color EL display device with each pixel having three light emitting elements. Only one of the two light emitting elements B1and B2will emit light at a given time.

The plurality of first data lines {SRk}, the plurality of second data lines {SGk}, and the plurality of third data liens {SBk} are coupled to a source driver (not shown) that outputs pixel data signals of an image to be displayed on the display panel100.

FIG. 2schematically shows a block diagram of a system200of for driving an EL display device according to one embodiment of the present invention. The system200includes an image process (not shown) for processing an image to be displayed on the EL display panel100according to the pixel matrix of the EL display device100, which is input to an input line buffer210. The processed image has a plurality of pixel data signals. Each pixel data signal is associated with a respective pixel in the EL display panel100and includes a first sub-pixel data signal indicating an intensity level for the red light emitting element R of the respective pixel, a second sub-pixel data signal indicating an intensity level for the green light emitting element G of the respective pixel, a third sub-pixel data signal indicating an intensity level for the yellow-green light emitting element B1of the respective pixel, and a fourth sub-pixel data signal indicating an intensity level for the blue light emitting element B2of the respective pixel.

The processed image is then input to a data processor230. The data processor230includes a selector232and a driver integrated circuit (IC)234. The selector232then compares the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel. If the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, the selector232assigns a logic value of 1 to the corresponding pixel. Otherwise, the selector232assigns a logic value of 0 to the corresponding pixel. Then, the selector232counts the number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of each column of pixels.

If NB1≧NB2 for a respective column of pixels, the switching control signal for the respective column of pixels is configured to set the first switches121and the second switches122corresponding to the respective column of pixels in the ON state and the OFF state, respectively. If NB1<NB2 for the respective column of pixels, the first switches121and the second switches122corresponding to the respective column of pixels in the OFF state and the ON state, respectively.

The driver IC234converts, for each pixel, the first sub-pixel data signal to a first voltage to be applied to a corresponding first data line SRk, the second sub-pixel data signal to a second voltage to be applied to a corresponding second data line SGk, the third sub-pixel data signal if the corresponding pixel has a logic value of 1 or the fourth sub-pixel data signal if the corresponding pixel has a logic value of 0 to a third data voltage to be applied to a corresponding third data line SBk. In addition, the system200includes a driving system220that outputs a clock signal CLK, a vertical synchronizing signal VS, a horizontal synchronizing signal HS to the data processor230, so that the plurality of pixel data signals of the image is driven to the EL display panel100sequentially in a row-by-row fashion controlled by the plurality of scanning signals.

FIG. 3shows schematically a time chart for a scanning signal G and a plurality of switching control signals, {Qk} (k=1, 2, 3, . . . , N), for driving an EL display device according to one embodiment of the present invention. Each switching control signal Qk for a respective column of pixels is characterized with a waveform having a low voltage, L, and a high voltage, H, for controlling the first switches121and the second switches122corresponding to the respective column of pixels. The scanning signal G (a high voltage pulse) activates a corresponding row of pixels in the EL display panel100so that data signals are applied to the plurality of pixels in the corresponding rows.

Each switching control signal Qk determines whether the yellow-green light emitting elements B1or the blue light emitting elements B2of a corresponding pixel in the corresponding row of pixel receives a third sub-pixel data voltage. For example, in one embodiment, the first switches121and second switches122are configured such that when the switching control signal Qk for the respective column of pixels has the high voltage H, the first switches121and the second switches122corresponding to the respective column of pixels are set in the ON state and the OFF state, respectively. Therefore, the third sub-pixel data voltage is applied to the yellow-green light emitting elements B1. When the switching control signal for the respective column of pixels has the low voltage L, the first switches121and the second switches122corresponding to the respective column of pixels are set in the OFF state and the ON state, respectively. Accordingly, the third sub-pixel data voltage is applied to the blue light emitting elements B2.

Other configuration of the first switches121and the second switches122can also be utilized to practice the invention. For example, when the switching control signal Qk for the respective column of pixels has the low voltage L, the first switches121and the second switches122corresponding to the respective column of pixels are set in the ON state and the OFF state, respectively, whereby the third sub-pixel data voltage is applied to the yellow-green light emitting elements B1. When the switching control signal for the respective column of pixels has the high voltage H, the first switches121and the second switches122corresponding to the respective column of pixels are set in the OFF state and the ON state, respectively, whereby the third sub-pixel data voltage is applied to the blue light emitting elements B2.

The first switches121and the second switches122can be any types of two state switches, such as thin film transistors (TFTs), as shown inFIG. 4.

FIG. 4shows schematically an EL display device400according to one embodiment of the present invention, where each of the plurality of first switches421is an N-type TFT, while each of the plurality of second switches422is a P-type TFT. The gate, source and drain of each N-type TFT421are respectively coupled to a corresponding switching control line Qk, a corresponding third data line SBk and the third EL element (B1)413of a corresponding pixel. The gate, source and drain of each P-type TFT422are respectively coupled to the corresponding switching control line Qk, the corresponding third data line SBk and the fourth EL element (B2)414of the corresponding pixel. For such an arrangement, when a switching control signal Qk for a respective column of pixels has the low voltage L, the N-type TFTs421of the respective column of pixels are turned on (in the ON state), while the P-type TFTs422of the respective column of pixels are turned off (in the OFF state). Accordingly, the third sub-pixel data signal provided from the respective third data line SBk is applied to the third EL elements (B1)413of the respective column of pixels. Otherwise, when the switching control signal Qk for the respective column of pixels has the high voltage H, the N-type TFTs421of the respective column of pixels are turned off (in the OFF state), while the P-type TFTs422of the respective column of pixels are turned on (in the ON state). Accordingly, the third sub-pixel data signal provided from the respective third data line SBk is applied to the fourth EL elements (B2)414of the respective column of pixels.

In an alternative embodiment, each of the plurality of first switches421is a P-type TFT, while each of the plurality of second switches422is an N-type TFT. In this example, when a switching control signal Qk for a respective column of pixels has the low voltage L, the N-type TFTs of the respective column of pixels are turned off (in the OFF state), while the P-type TFTs of the respective column of pixels are turned on (in the ON state). Accordingly, the third sub-pixel data signal provided from the respective third data line SBk is applied to the fourth EL elements (B2) of the respective column of pixels. When the switching control signal Qk for the respective column of pixels has the high voltage H, the N-type TFTs of the respective column of pixels are turned on (in the ON state), while the P-type TFTs of the respective column of pixels are turned off (in the OFF state). Accordingly, the third sub-pixel data signal provided from the respective third data line SBk is applied to the third EL elements (B1) of the respective column of pixels.

Accordingly, one aspect of the present invention provides a method of driving such an EL display device. The method includes the following steps. At first, an image to be displayed is processed according to the pixel matrix of the EL display device such that the processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element, the second EL element, the third EL element and the fourth EL element, respectively, of the respective pixel. Then, the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel are compared. A logic value of 1 is assigned to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 assigned to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal. The number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of each column of pixels are counted subsequently. Next, the switching control signal for a respective column of pixels is configured to set the first switches and the second switches corresponding to the respective column of pixels in the ON state and the OFF state, respectively, if NB1≧NB2 for the respective column of pixels, and the first switches and the second switches in the OFF state and the ON state, respectively, if NB1<NB2 for the respective column of pixels.

While each of the EL display devices100and400shown inFIGS. 1 and 4, respectively, has fewer gate lines and data lines than a conventional EL display device, it still has 2×M×N switches that require N switching control signals/lines Qk (k=1, 2, 3, . . . N), where M and N are equal to the numbers of row and columns, respectively, of pixels. Therefore the electrical layout of the display devices is still relatively complicated and consequently results in higher cost.

FIG. 5schematically shows an EL display device500according to another embodiment of the present invention in which the number of switching control signals/lines is reduced to one. Similar to the EL display device100shown inFIG. 1, the EL display device500has a plurality of pixels510spatially arranged in a form of a matrix with a plurality of rows and a plurality of columns. Each pixel510has a first EL element (R)511operably emitting light of a red color, a second EL element (G)512operably emitting light of a green color, a third EL element (B1)513operably emitting light of a yellow-green color, and a fourth EL element (B2)514operably emitting light of a blue color. The EL display device500is adapted for displaying an image. Operably, the image is processed by an image processor, according to the pixel matrix of the EL display device500such that the processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element (R)511, the second EL element (G)511, the third EL element (B1)513and the fourth EL element (B2)514, respectively, of the respective pixel.

The EL display device500also has a plurality of gate lines {Gj}, j=1, 2, 3, . . . M, M being the row number of the pixel matrix, a plurality of first data lines, {SRk}, k=1, 2, 3, . . . N, N being the column number of the pixel matrix, a plurality of second data lines, {SGk}, a plurality of third data lines, {SBk}, a plurality of first switches521, a plurality of second switches522, and a signal switch control line, Q.

Each gate line Gj is coupled to a respective row of pixels. Each first data line SRk is coupled to the first EL elements (R)511of a respective column of pixels. Each second data line SGk is coupled to the second EL elements (G)512of a respective column of pixels. Each third data line SBk is coupled, via the first switches521and the second switches522, to the third EL elements (B1)513and the fourth EL elements (B2)514of a respective column of pixels.

Each first switch521is coupled to a respective third EL element (B1)511, while each second switch522is coupled to a respective fourth EL element (B2)512. The single switch control line Q is coupled to all of the first switches521and the second switches522for providing a switching control signal to set the first switches521and the second switches522to be in the ON state or the OFF state. In this exemplary embodiment, all of the first switches511are configured to be cooperatively in an ON state or an OFF state, while all of the second switches are configured to be cooperatively in the OFF state or the ON state.

Accordingly, when the first switches521are cooperatively in the ON state, the third sub-pixel data signal from a respective third data line SBk is provided to the third EL elements (B1)513of the respective column of pixels, and when the second switches522are cooperatively in the ON state, the third sub-pixel data signal from the respective third data line SBk is provided to the fourth EL elements (B2)514of the respective column of pixels.

To determine which switches are turned on or off, a selector is adapted and configured to compare the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel, to assign a logic value of 1 to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal, and to count the number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of the plurality of pixels. If NB1≧NB2, the switching control signal Q is configured to set the first switches521in the ON state and the second switches522in the OFF state, respectively. Otherwise, if NB1<NB2, the switching control signal Q is configured to the first switches521in the OFF state and the second switches522in the ON state, respectively, if NB1<NB2.

In one embodiment, the switching control signal Q is characterized with a waveform having a low voltage, L, and a high voltage, H, for controlling the first switches521and the second switches522, as shown inFIG. 6. For example, when the switching control signal Q has the low voltage L, the first switches521and the second switches522are set in the ON state and the OFF state, respectively, and when the switching control signal Q has the high voltage H, the first switches521and the second switches522are set in the OFF state and the ON state, respectively. Now, instead of N switching control signals, the single switching control signal Q controls whether each third data line SBk is coupled to the third light emitting element (B1)513or to the fourth light emitting element (B2)514of a respective pixel.

FIG. 7shows schematically an EL display panel700according to one embodiment of the present invention. The display panel700has a similar layout as that of the display panel500. However, each first switch721includes an N-type TFT, while each second switch722includes a P-type TFT. The gate, source and drain of each N-type TFT are respectively coupled to the switching control line Q, a corresponding third data line SBk and the third EL element (B1)713of a corresponding pixel. The gate, source and drain of each P-type TFT are respectively coupled to the switching control line Q, the corresponding third data line SBk and the fourth EL element (B2)714of the corresponding pixel. For such an arrangement, when the switching control signal Q has the high voltage H, all of the first switches721are turned off and all of the second switches722are turned on. Accordingly, a third sub-pixel data signal is input to the fourth light emitting element (B2)714of a respective column of pixels. When the switching control signal Q has the low voltage L, all of the first switches721are turned on and all of the second switches722are turned off. Accordingly, a third sub-pixel data signal is input to the third light emitting element (B1)713of the respective column of pixels.

In an alternative embodiment, each first switch721is a P-type TFT and each second switch722is an N-type TFT.

Furthermore, another aspect of the present invention provides a method of driving such an EL display device500(or700). The method includes the following steps. At first, an image to be displayed is processed according to the pixel matrix of the EL display device. The processed image has a plurality of pixel data signals, each pixel data signal being associated with a respective pixel and having a first sub-pixel data signal, a second sub-pixel data signal, a third sub-pixel data signal and a fourth sub-pixel data signal indicating intensities for the first EL element, the second EL element, the third EL element and the fourth EL element, respectively, of the respective pixel.

Next, the third sub-pixel data signal and the fourth sub-pixel data signal of each pixel are compared. A logic value of 1 is assigned to the corresponding pixel if the third sub-pixel data signal is greater than or equal to the fourth sub-pixel data signal, or a logic value of 0 is assigned to the corresponding pixel if the third sub-pixel data signal is less than the fourth sub-pixel data signal. The number of the logic value of 1, NB1, and the number of the logic value of 0, NB2, of the plurality of pixels are counted.

Then, the switching control signal is configured to set the plurality of first switches721in the ON state and the plurality of second switches722in the OFF state, respectively, if NB1≧NB2, and the plurality of first switches721in the OFF state and the plurality of second switches722in the ON state, respectively, if NB1<NB2.