EP1845508B1

EP1845508B1 - System and method of providing driving voltages to an RGBW display panel

Info

Publication number: EP1845508B1
Application number: EP20060112633
Authority: EP
Inventors: Ching-Wei Lin
Original assignee: Chimei Innolux Corp
Current assignee: Innolux Corp
Priority date: 2006-04-13
Filing date: 2006-04-13
Publication date: 2012-04-11
Anticipated expiration: 2026-04-13
Also published as: EP1845508A1

Description

BACKGROUND

The invention relates to panel displays, and more particularly, to systems and methods for providing driving voltages to RGBW display panels.
Color image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal modulators and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). In a common OLED color image display device, a pixel includes red, green and blue colored subpixels. These light emitting colored subpixels define a color gamut, and by additively combining the illumination from each of these three subpixels, i.e. with the integrative capabilities of the human visual system, a wide variety of colors can be achieved. OLEDs may be used to generate color directly using organic materials to emit energy in desired portions of the electromagnetic spectrum, or alternatively, broadband emitting (apparently white) OLEDs may be attenuated with color filters to achieve red, green and blue output.
Images and data displayed on a color display device are typically stored and/or transmitted in three channels, that is, having these signals corresponding to a standard (e.g. RGB). It is also important to recognize that data typically is sampled to assume a particular spatial arrangement of light emitting elements. In an OLED display device, these light emitting elements are typically arranged side by side on a plane. Therefore, if incoming data is sampled for display on a color display device, the data will also be resampled for display on an OLED display having four subpixels per pixel rather than the three subpixels used in a three channel display device.
In this regard, Fig. 1A shows a conventional OLED subpixel driving circuit structure, and Fig. 1B shows RGBW subpixel arrangements of a conventional display panel. As shown in Fig. 1A, the subpixel is driven by the current 11 through the driving transistor T1. The driving transistor T1 outputs the current 11 according to the voltage V1.
Fig. 1C shows a conventional digital signal processing (DSP) structure for driving RGBW subpixels. As shown in Fig. 1C, RGB digital signals are sampled and held and output to a Gamma linear control unit. The Gamma linear control unit adjusts RGB digital signals for Gamma linearity and outputs to the conversion unit. The conversion unit converts the adjusted RGB digital signals to RGBW digital signals and outputs to a Gamma compensation unit. The Gamma compensation unit executes a Gamma compensation of the RGBW digital signals from the conversion unit for Gamma correction and outputs to a RGBW driver. The RGBW driver converts the RGBW digital signals to RGBW analog signals to drive corresponding RGBW subpixels.
Fig. 2A shows the relationship between the luminance of the OLED subpixel and the current I1. As shown, there is a linear relationship between the luminance of the OLED subpixel and the current I1. Fig. 2B shows the relationship between the current I1 of the driving transistor T1 and the voltage V1 to be non-linear. Fig. 2C shows the relationship between luminance of the OLED subpixel and observable brightness (gamma). Fig. 2D shows the relationship between observable brightness and voltage V1 applied to the driving transistor T1.
Thus, a gamma correction is required to compensate the non-linear relationship.
Conventionally, RGB data is converted to RGBW data through digital data processing (DSP). However, due to different optical characteristics (gamma correction) for each RGBW color, DSP typically requires a complicated algorithm to execute such conversion. Further, it may be difficult to obtain a precise analog output corresponding to the gamma correction for each color after using the complicated conversion algorithm.
For example, Fig. 3 shows a conventional method for converting RGB data to RGBW data. As shown in Fig. 3, the Min(R,G,B) is assumed to be W data, and R'G'B' data (driving the display device) can be obtained by removing the W component from the R,G,B components respectively. Fig. 4 shows another conventional method for converting RGB data to RGBW data. As shown in Fig. 4, the Min(R,G,B) is assumed to be W data, and the W component is converted to W' data in accordance with a characteristic of α*W, where α <1. The R'G'B' data are obtained by removing the W' component from the RGB components respectively. However, these two simple methods typically cannot precisely provide gamma correction for each color because of the non-linear relationship between driving voltage and observable brightness.
A method and system according to the preamble of the independent claims is described in the article "6-bit AMOLED with RGB Adjustable Gamma Compensation LTPS TFT Circuit" written by Y. Matsueda et al. and published in SID 05 Digest on pages 1352 - 1355. For displaying image the system includes a data driver comprising an average brightness extraction unit. The system further includes a reference voltage generation circuit adapted to provide first to third sets of reference voltages suitable for the red, green and blue sub-pixels, wherein the reference voltage generation circuit at least comprises first, second and third voltage generators. The system also comprises a digital-to-analog (D/A) conversion unit adapted to generate driving voltages to drive the red, green and blue sub-pixels and a display panel comprising the red, green and blue sub-pixels adapted to generate color images according to the driving voltages.
From EP 1 298 637 A2 a similar system is known, namely a "liquid crystal display" including a reference voltage generator changing the level of a first predetermined voltage based on a first signal to generate a reference voltage. The first signal varies depending on one of the brightness of the surroundings of the liquid crystal display, brightness of the on-screen images of the liquid crystal display and a user's manipulation (see par. [0006]).
Further methods for providing driving voltages of a system for displaying images are described in EP 0 547 603 A2 ; US 2004 / 0 113 875 A1 or US 6 593 934 B1 .
From the above mentioned documents RGB based display systems and methods are known which are adapted to select gamma characteristics based on the average image brightness which is extracted from the three color input signals (R. G, B). Thus a conversion of RGB data into RGBW has to be performed, if these known methods shall be applied to RGBW panels. However, in conventional systems the conversion requires DSP processing of a complicated algorithm due to gamma correction. Thus high efforts are needed to achieve an accurately controlled gamma correction for RGBW brightness.
It is therefore object of the present invention to provide a system and a method which can more easily provide an accurately controlled gamma correction for RGBW brightness.
The object is solved by a system having the features of claim 1 and by a method having the features of independent claim 10.
Accordingly the invention proposes to execute an AND logic operation to red, green and blue input signals controlling brightness of the red, green and blue sub-pixels respectively to extract the white component signal; to generate first to fourth sets of reference voltages suitable for the red, green, blue and white sub-pixels, wherein the first to third sets of reference voltages suitable for the red, green and blue sub-pixels are generated according to the white component signal; and to generate the driving voltages to drive the red, green, blue and white sub-pixels according to the first to fourth sets of reference voltages, the red, green and blue input signals and the white component signal. The system of the invention shall comprise a white component extraction unit which is adapted to execute an AND logic operation to the red, green and blue input signals controlling brightness of the red, green and blue sub-pixels respectively, to extract the white component signal. The reference voltage generation circuit shall be adapted to provide first to fourth sets of reference voltages suitable for the red, green, blue and white sub-pixels, wherein the first to third sets of reference voltages suitable for the red, green and blue sub-pixels are generated according to the white component signal, wherein the first, second and third voltage generators each comprise: first and second resistor strings connected to each other in series, each comprising a plurality of resistors and nodes. The first, second and third voltage generators shall further comprise a first de-multiplexer adapted to selectively make connections between a first power voltage and one of the nodes of the first resistor string according to the white component signal; and a second de-multiplexer adapter to selectively make connections between a second power voltage and one of the nodes of the second resistor string according to the white component signal. Finally the digital-to-analog (D/A) conversion unit shall be adapted to generate the driving voltages to drive the red, green, blue and white sub-pixels according to the first to fourth sets of reference voltages, the red, green and blue input signals and the white component signal.
In summary the invention discloses a system and a method for providing driving voltages of RGBW display panels.
An exemplary embodiment of such a system comprises a data driver with a reference voltage generation circuit providing reference voltages according to a white component signal (W) extracted from three color input signals (R,G,B), and a digital-to-analog (D/A) conversion unit to generate driving voltages according to the reference voltages, the three color input signals and the white component signal.
An exemplary embodiment of a method for providing driving voltages of a RGBW display panel, comprises generating reference voltages according to a white component signal (W) extracted from three color input signals (R,G,B); and generating driving voltages according to the reference voltages, the three color input signals and the white component signal.

DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:

Fig. 1A shows a conventional OLED subpixel driving circuit structure;
Fig. 1B shows RGBW pixel arrangements of conventional display panel;
Fig. 1C shows a conventional digital signal processing (DSP) structure for driving RGBW pixels;
Fig. 2A shows the relationship between the luminance of OLED and current;
Fig. 2B shows the relationship between current through the control transistor and driving voltage thereof;
Fig. 2C shows the relationship between luminance of the OLED and observable brightness;
Fig. 2D shows the relationship between observable brightness and driving voltage of driving transistor;
Fig. 3 shows a conventional method for converting RGB data to RGBW data;
Fig. 4 shows another conventional method for converting RGB data to RGBW data;
Fig. 5 shows an embodiment of a data driver;
Figs. 6A-6D show embodiments of a voltage generator;
Fig. 7 shows another embodiment of a data driver;
Figs. 8-1 and 8-2 show another embodiment of a data driver;
Fig. 9 is a schematic diagram of an embodiment of a display; and
Fig. 10 is a schematic diagram of an embodiment of an electronic device employing the display panel shown in Fig. 9.

DETAILED DESCRIPTION

Systems for providing driving voltages to display panels will now be described with reference to several exemplary embodiments. In this regard, an embodiment of a system providing driving voltages to an RGBW display panel is depicted in Fig. 5. As shown in Fig. 5, data driver 100A comprises a white component extraction unit 10, an analog reference voltage generation circuit 20 and N digital-to-analog (D/A) conversion units 30_1A∼30_NA.
The white component extraction unit 10 extracts a white component signal Wi from three color input signals Ri, Gi and Bi. For example, three color input signals Ri, Gi and Bi can be 6 bit digital data. If color input signals R1, G1 and B1 are 110111, 010111 and 000111 respectively, the white component signal W1 can be 000111. Alternately, white component extraction unit 10 can output a suppressed white component signal W1 of 000011 according to the color input signal R1, G1 and B1.
The white component signal Wi can be obtained by executing an AND logic operation to the three color input signals Ri, Gi and Bi. For example, when the color input signals R1, G1 and B1 are 110111, 010111 and 000111 respectively, the white component signal W1 can be 000111.
Conversely, the white component signal Wi can be obtained by executing an AND logic operation to M bits of the three color input signals Ri, Gi, Bi, and 0 < M < 6. For example, when M=2, a suppressed white component signal W1 of 000011 can be obtained according to the color input signal R1, G1 and B1.
The analog reference voltage generation circuit 20 generates four sets of reference voltages V0_R∼V63_R, V0_G∼V63_G, V0_B∼V63_B and V0_W∼V63_W for color input signal Ri, Gi and Bi and the white component signal Wi respectively, the reference voltages V0_R∼V63_R, V0_G∼V63_G and V0_B∼V63_B are generated according to the white component signal Wi.
The D/A conversion units 30_1A∼30_NA receive the reference voltages VO_R∼V63_R, V0_G∼V63_G, V0_B∼V63_B and V0_W∼V63_W from the analog reference voltage generation circuit 20 to generate corresponding driving voltages VA1_R∼VAN_R, VA1_G∼VAN_G, VA1_B∼VAN_B and VA1_W∼VAN_W according to the three color input signals Ri, Gi and Bi and the white component signal Wi. For example, the D/A conversion unit 30_1A receives the reference voltages V0_R∼V63_R, V0_G∼V63_G, V0_B∼V63_B and V0_W∼V63_W and generates corresponding driving voltages VA1_R, VA1_G, VA1_B and VA1_W according to the three color input signals R1, G1 and B1 and the white component signal W1 during a first period. The D/A conversion unit 30_2A receives the reference voltages V0_R∼V63_R, V0_G∼V63_G, V0_B∼V63_B and V0_W∼V63_W and generates corresponding driving voltages, VA2_R, VA2_G, VA2_B and VA2_W according to the three color input signals R2, G2 and B2 and the white component signal W2 during a second period, and so on. Namely, all D/A conversion units 30_1A∼30_NA employ the same type of analog reference voltage circuit which can generate different reference voltages V0_R∼V63_R, V0_G∼V63_G, V0_B∼V63_B and V0_W∼V63_W according to different white component signals Wi during different periods.
The D/A conversion units 30_1A~30_NA each comprise four sampling latches S1_R~S1_W, four holding latches H1_R~H1_W, four D/A converters DAC_R~DAC_W and four analog buffers AB_R~AB_W. The sampling latches S1_R~S1_W sample the color input signals Ri, Gi and Bi and the white component signal Wi at one time. The holding latches H1_R~H1_W hold the color input signals Ri, Gi and Bi and the white component signal Wi sampled by the sampling latches S1_R~S1_W. The D/A converters DAC_R~DAC_W convert the held color input signals Ri, Gi and Bi and the held white component signal Wi to corresponding analog voltages VA1_R~VA1_W according to the reference voltages V0_R~V63_R, V0_G~V63_G, V0_B~V63_B and V0_W~V63_W, and output the corresponding driving voltages VA1_R~VA1_W through the analog buffers AB_R~AB_W. Operation and structure of the D/A conversion units 30_2A~30_NA are similar to those of the D/A conversion unit 30_1A. In this embodiment, the data diver 100A can output four corresponding voltages to drive four data lines at one time.
The analog reference voltage generation circuit 20 comprises four voltage generators 22R, 22G, 22B and 22W shown in Figs. 6A~6D to generate reference voltages V0_R~V63_R, V0_G~V63_G, V0_B~V63_B and V0_W~V63_W. As shown in Fig. 6A, the voltage generator 22R generates the reference voltages V0_R~V63_R to D/A converters DAC_R of the D/A conversion units 30_1A~30_NA according to the white component signal Wi. The voltage generator 22R comprises two de-multiplexers 211 and 212 and two series-connected resistor strings 231 and 232. The resistor string 231 comprises resistors R0_R"~R62_R" connected in series, and the resistor string 232 comprises resistors R0_R~R64_R for red color grey level gamma correction. The de-multiplexer 211 selectively outputs a first power voltage VerfH to one node of the resistor string 231 according to the white component signals Wi, and the de-multiplexer 212 selectively outputs a second power voltage VrefL to one node of the resistor string 232 according to the white component signals Wi. The first power voltage VrefH exceeds the second power voltage VrefL, the resistors R0_R" and R0_R are the same, the resistors R1_R" and R1_R are the same, the resistors R2_R" and R2_R are the same, and so on.
For example, if the white component signal Wi extracted from the three color input signals Ri, Gi and Bi is 000000, the power voltage VrefL is forced to the node N0 of the resistor string 232, and the power voltage VrefH is forced to the node N3 of the resistor string 231. Alternately, if the white component signal Wi extracted from the three color input signals Ri, Gi and Bi is 000001, the power voltage VrefL is forced to the node N1 of the resistor string 232, and the power voltage VrefH is forced to the node N4 of the resistor string 231. Accordingly, the voltage level of the reference voltage V0_R~V63_R for the red input signal Ri can be lowered by a first voltage drop.
Alternately, if the white component signal Wi extracted from the three color input signals Ri, Gi and Bi is 000010, the power voltage VrefL is forced to the node N2 of the resistor string 232, and the power voltage VrefH is forced to the node N5 of the resistor string 231. Accordingly, the voltage level of the reference voltage V0_R~V63_R for the red input signal Ri can be lowered by a second voltage drop exceeding the first voltage drop. Thus, the voltage level of the reference voltage V0_R~V63_R for the red input signal Ri can be adjusted based on the white component signal Wi.
As shown in Fig. 6B, the voltage generator 22G generates the reference voltages V0_G~V63_G to D/A converters DAC_G of the D/A conversion units 30_1A~30_NA according to the white component signal Wi. The voltage generator 22R comprises two de-multiplexers 213 and 214 and two series-connected resistor strings 233 and 234. The resistor string 233 comprises resistors R0_G"~R62_G" connected in series, and the resistor string 234 comprises resistors R0_G~R64_G for green color grey level gamma correction. The de-multiplexer 213 selectively outputs the first power voltage VrefH to one node of the resistor string 233, and the de-multiplexer 214 selectively outputs the second power voltage VrefL to one node of the resistor string 234. The resistors R0_G" and R0_G are the same, the resistors R1_G" and R1_G are the same, the resistors R2_G" and R2_G are the same, and so on.
As shown in Fig. 6C, the voltage generator 22B generates the reference voltages V0_B~V63_B to D/A converters DAC_B of the D/A conversion units 30_1A~30_NA according to the white component signal Wi. The voltage generator 22B comprises two de-multiplexers 215 and 216 and two series-connected resistor strings 235 and 236. The resistor string 235 comprises resistors R0_B"~R62_B" connected in series, and the resistor string 236 comprises resistors R0_B~R64_B for blue color grey level gamma correction. The de-multiplexer 215 selectively outputs the first power voltage VrefH to one node of the resistor string 235, and the de-multiplexer 216 selectively outputs the second power voltage VrefL to one node of the resistor string 236. The resistors R0_B" and R0_B are the same, the resistors R1_B" and R1_B are the same, the resistors R2_B" and R2_B are the same, and so on. Operation of the voltage generator 22G and 22B is similar to that of the voltage generator 22R., . The resistors R0_R~R64_R, R0_G~R64_G and R0_B~R62_B can be different from others, depending on design.
As shown in Fig. 6D, the voltage generator 22W comprises a resistor string 237 comprising a plurality of resistors R0_W~R63_W connected in series for white color grey level gamma correction. The power voltages VrefH and VrefL are forced to two ends of the resistor string 237, such that the reference voltages V0_W~V63_W are generated according to difference resistances of the resistors R0_W~R63_W.
In this embodiment , the voltage level of the reference voltages V0R~V63R, V0G~V63G and V0B~V63B for three color input signals Ri, Gi and Bi can be adjusted based on the white component signal Wi. The lower voltage level of the reference voltages V0_R~V63_R, V0_G~V63_G and V0_B~V63_B, the lower driving voltage VA1_R~VAN_R, VA1_G~VAN_G and VA1_B~VAN_B generated by D/A conversion units 30_1A~30_NA. Namely, the voltage level of the driving voltages VA1_R~VAN_R, VA1_G~VAN_G and VA1_B~VAN_B generated by D/A conversion units 30_1A~30_NA can be adjusted according to the extracted white component signal Wi. When N-type transistors are used as driving devices of pixels, the RGB brightness of the subpixels on a display device is lowered as the driving voltage decreases based on the white component signal Wi. In some embodiments, when P-type transistors are used as driving devices of pixels, the RGB brightness of the pixels on a display device is lowered as the driving voltage increases based on the white component signal Wi. Thus, gamma correction for RGBW brightness can be accurately controlled.
Alternately, in some embodiments, the de-multiplexers 211, 213 and 215 selectively output the second power voltage VrefL to one node of the resistor string 231, 233 and 235, and the de-multiplexer 212, 214 and 216 selectively output the first power voltage VrefH to one node of the resistor string 232, 234 and 236.
Fig. 7 shows another embodiment of a data driver. As shown, the data driver 100B is similar to the data driver 100A shown in Fig. 5, with the exception of analog sampling and holding latches ASH_R~ASH_W coupled between the analog buffers AB_R~AB_W and the D/A converters DAC_R~DAC_W in each D/A conversion unit 30_1B~30_NB. Description of the same structure shown in Fig. 5 is omitted for simplification. In the data driver 100B, the driving voltages VA1_R~VAN_R, VA1_G~VAN_G, VA1_B~VAN_B and VA1_W~VAN_W generated by the D/A conversion units 30_1B~30_NB during different periods can be sampled and held by the analog sampling and holding latches ASH_R~ASH_W. Thus, the data driver 100B can output the corresponding voltages to drive one row of data lines in one time.
Figs. 8-1 and 8-2 show another embodiment of a data driver. As shown, the data driver 100C is similar to the data driver 100A shown in Fig. 5, with the exception of N analog reference voltage generation circuits 20_1~20_N coupled to the D/A conversion units 30_1C~30_NC. Description of the same structure shown in Fig. 7 is omitted for simplification. In the data driver 100C, the N analog reference voltage generation circuits 20_1~20_N each correspond to one of the D/A conversion units 30_1C~30_NC. For example, the analog reference voltage generation circuit 20_1 corresponds to the D/A conversion unit 30_1C, the analog reference voltage generation circuit 20_2 corresponds to the D/A conversion unit 30_2C, and so on. The color input signals Ri, Gi, Bi and the extracted white component signal Wi are sampled by the sampling latches S1_R~S1_W and held by the holding latches H1_R~H1_W in the D/A conversion units 30_1C~30_NC during each period. For example, the color input signals R1, G1, B1 and the extracted white component signal W1 are sampled and held in the D/A conversion units 30_1C during a first period, the color input signals R2, G2, B2 and the extracted white component signal W2 are sampled and held in the D/A conversion units 30_2C during a second period, and so on.
All held color input signals Ri, Gi, Bi and the white component signal Wi can be output to the corresponding D/A converters DAC_R~DAC_W and the corresponding analog reference voltage circuit at one time. For example, the white component signal W1 is output to analog reference voltage generation circuit 20_1, such that the reference voltages V0_R~V63_R, V0_G~V63_G, V0_B~V63_B and V0_W~V63_W are output to the D/A converters DAC_R~DAC_W. Accordingly, the D/A converters DAC_R~DAC_W receive the reference voltages V0_R~V63_R, V0_G~V63_G, V0_B~V63_B and V0_W~V63_W and generate the driving voltage VA1_R~VA1_W according to the three color input signals R1, G1, B1 and W1. Similarly, the D/A conversion units 30_2C~30_NC generate the driving voltages VA2_R~VAN_R, VA2_G~VAN_G and VA2_B~VAN_B at the same time. Namely, the data driver 100C can output the corresponding voltages to drive one row of data lines in one time.
Fig. 9 is a schematic diagram of another embodiment of a system, in this case a display panel, for providing driving voltages. As shown in Fig. 9, the display device 300 comprises a data driver such as data drvier100A/100B/100C, a pixel array 200 and a gate driver 210. The pixel array 200 comprises RGBW color pixels arranged in matrix, a plurality of data lines and a plurality of scan lines. The data driver generates analog driving voltages to the pixel array 200, and the gate driver 210 provides scan signals to the pixel array 200 such that the scan lines are asserted or de-asserted. The pixel array 200 generates color images according to the analog driving voltages from the data driver. While the display panel can be an organic light emitting panel, an electroluminescent panel or a liquid crystal display panel for example, various other technologies can be used in other embodiments.
Fig. 10 schematically shows an embodiment of yet another system, in this case an electronic device for providing driving voltages. In particular, electronic device 600 employs a display panel such as display panel 600 shown in Fig. 9. The electronic device 600 may be a device such as a PDA, notebook computer, digital camera, tablet computer, cellular phone or a display monitor device, for example.
Generally, the electronic device 600 comprises a housing 500, a display panel 300 and a DC/DC converter 400, although it is to be understood that various other components can be included, such components not shown or described here for ease of illustration and description. In operation, the DC/DC converter 400 powers the display panel 300 so that the display panel 300 can display color images.

Claims

A system for displaying images according to a red (Ri), green (Gi) and blue (Bi) input signal, the system comprising:
a display panel (300) comprising red, green and blue sub-pixels adapted to generate color images according to driving voltages;

the display panel further comprising a data driver (100) comprising:
a reference voltage generation circuit (20) adapted to provide sets of reference voltages suitable for the sub-pixels, wherein the reference voltage generation circuit (20) at least comprises first, second and third voltage generators (22R, 22G, 22B); and

a digital-to-analog (D/A) conversion unit (30_1A) adapted to generate the driving voltages to drive the sub-pixels according to the input signals (Ri, Gi, Bi) and the sets of reference voltages;

characterized in that

the display panel further comprises white sub-pixels;

the data driver further comprises a white component extraction unit (10) adapted to execute an AND logic operation to the red, green and blue input signals (Ri, Gi, Bi) controlling brightness of the red, green and blue sub-pixels respectively, to extract a white component signal (Wi) controlling brightness of a white sub-pixel;

the reference voltage generation circuit (20) is adapted to provide first to third sets of reference voltages (V0_R - V63_R , V0_G - V63_G, V0_B - V63_B) respectively suitable for the red, green blue and sub-pixels according to the white component signal (Wi), wherein the first, second and third voltage generators (22R, 22G, 22B), respectively providing the first to third sets of reference voltages, each comprise:

first and second resistor strings (231, 232; 233, 234; 235, 236) connected to each other in series, each comprising a plurality of resistors (R0R - R62R; R0G - R62G; ROB - R62B) and nodes (N4 - N5; N0 - N3);

a first de-multiplexer (211; 213; 215) adapted to selectively make connections between a first power voltage (V_refH) and one of the nodes of the first resistor string (231; 233; 235) according to the white component signal; and

a second de-multiplexer (212; 214; 216) adapted to selectively make connections between a second power voltage (V_refL) and one of the nodes of the second resistor string (232; 234; 236) according to the white component signal;

wherein the digital-to-analog (D/A) conversion unit (30) is adapted to generate the driving voltages (VAN_R, VAN_G, VAN_B, VAN_W) to drive the red, green, blue and white sub-pixels according to the first to fourth sets of reference voltages, the red, green and blue input signals and the white component signal; and
wherein the reference voltage generation circuit (20) further comprises a fourth voltage generator (22W) that is adapted to generate the fourth set of reference voltages suitable for the white sub-pixels.
The system as claimed in claim 1, wherein the driving voltages at least comprise a red driving voltage (VAN_R), a green driving voltage (VAN_G), a blue driving voltage (VAN_B) and a white driving voltage (VAN_W), and the digital-to-analog conversion unit (30_1) comprises:
a first digital-to-analog converter (DAC_R) adapted to generate the red driving voltage according to the first set of reference voltages and the red input signal;

a second digital-to-analog converter (DAC_G) adapted to generate the green driving voltage according to the second set of reference voltages and the green input signal;

a third digital-to-analog converter (DAC_B) adapted to generate the blue driving voltage according to the third set of reference voltages and the blue input signal, and

a fourth digital-to-analog converter (DAC_W) adapted to generate the white driving voltage according to the fourth set of reference voltages and the white component signal.
The system as claimed in claim 2, wherein the digital-to-analog conversion unit (30_1) further comprises a plurality of digital holding units (H/L) connected to the inputs of the digital-to-analog converters (DAC_R - DAC_W) and adapted to hold the red, green and blue input signals and the white component signal.
The system as claimed in claim 2, wherein the digital-to-analog conversion unit (30_1) further comprises a plurality of analog holding units (S/H) adapted to hold the red, green, blue and white driving voltages output by the digital-to-analog converters (DAC_R - DAC_W).
The system as claimed in claim 1, wherein the fourth voltage generator (22W) comprises a third resistor string (237) connected between the first power voltage and the second power voltage.
The system as claimed in claim 1, wherein the display panel (300) is a liquid crystal display panel.
The system as claimed in claim 1, wherein the display panel (300) is an electroluminescent panel.
The system as claimed in claim 1, wherein the display panel (300) is an organic light emitting panel.
The system as claimed in claim 1, wherein the system is implemented as a PDA, a display monitor, a digital camera, a notebook computer, a tablet computer or a cellular phone.
A method of displaying images according to a red (Ri), green (Gi) and blue (Bi) input signal, comprising:
generating first to third sets of reference voltages suitable for red, green and blue sub-pixels, respectively;

generating driving voltages to drive the red, green, and blue sub-pixel according to the input signals (Ri, Gi, Bi) and the sets of reference voltages; and

displaying color images by driving the sub-pixels according to the driving voltages;

characterized by the steps of:
extracting a white component (Wi) controlling brightness of a white sub-pixel by executing an AND logic operation to the red, green and blue input signals (Ri, Gi, Bi) controlling brightness of the red, green and blue sub-pixels respectively ;

generating the first to third sets of reference voltages (V0_R - V63_R, V0_G - V63_G , V0_B- V63_B) respectively suitable for the red, green and blue sub-pixels according to the white component signal (Wi);

generating a fourth set of reference voltages (V0_W - V63_W) suitable for the white sub-pixels; and

generating the driving voltages (VAN_R. VAN_G , VAN_B , VAN_W) to drive the red, green, blue and white sub-pixels according to the first to fourth sets of reference voltages, the red, green and blue input signals and the white component signal.
The method as claimed in claim 10, wherein the driving voltages at least comprise a red driving voltage generated according to the first set of reference
voltages and the red input signal, a green driving voltage generated according to the second set of reference voltages and the green input signal, a blue driving voltage generated according to the third set of reference voltages and the blue input signal and a white driving voltage generated according to the fourth set of reference voltages and the white component signal.
The method as claimed in claim 10, further comprising holding the white component signal (Wi) and the red, green and blue input signals (Ri, Gi, Bi) before generating the driving voltages.
The method as claimed in claim 10, wherein the white component signal (Wi) and the red, green and blue input signals (Ri, Gi, Bi) each is a digital data comprising N bits.
The method as claimed in claim 10, wherein the white component signal (Wi) and the red, green and blue input signals (Ri, Gi, Bi) each is a digital data comprising N bits, and the white component signal (Wi) is obtained by executing the AND logic operation to M bits of the red, green and blue input signals (Ri, Gi, Bi), and 0 < M < N.
The method as claimed in claim 10, further comprising holding the generated driving voltages.
The method as claimed in claim 10, wherein the system comprises a display device and the display device is an organic light emitting device, a liquid crystal display device or an electroluminescent device.