Patent Grant
8922597
The present application claims priority from Japanese application JP 2011-052649 filed on Mar. 10, 2011, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular, a data-line driving circuit included in a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device uses a potential difference between a pixel electrode and a common electrode that are included in each of the pixel circuits to control a transmittance of liquid crystal. In the case where a time average of a potential applied to a pixel electrode of any one of the pixel circuits deviates from a common potential applied to the common electrode (this case is referred to as the application of a DC component to the liquid crystal), the relation between the transmittance of the liquid crystal and the potential difference is not maintained any more to result in the generation of a ghost image. In the liquid crystal display device, polarity of the potential applied to the pixel electrode changes for each frame to prevent the generation of the ghost image. The polarity indicates that the potential applied to the pixel electrode or a data line is higher or lower than the common potential. A positive-polarity potential indicates that the potential is higher than the common potential, whereas a negative-polarity potential indicates that the potential is lower than the common potential.
Even when an average of the positive-polarity potential and the negative-polarity potential applied to the data line for a certain level of grayscale is equal to the common potential, the ghost image is disadvantageously generated in some cases. This is because the average of the positive-polarity potential and the negative-polarity potential applied to the pixel electrode differs from the common potential in this case. Therefore, conventionally, control for shifting the average of the positive-polarity potential and the negative-polarity potential applied to the data line from the common potential by a predetermined value is performed.
Japanese Patent No. 3704716 discloses a liquid crystal display device for shifting positive-polarity and negative-polarity precharge potentials applied to a data line from a central potential of a data voltage amplitude by a predetermined value. Japanese Patent Application Laid-open No. 2004-219824 discloses a liquid crystal display device for controlling whether or not to perform precharging for a pixel circuit in accordance with a temperature.
The inventors of the present invention observed the ghost image more carefully than conventionally done. Then, it was found that the ghost image was sometimes generated even when the average of the potential of a positive-polarity signal and the potential of a negative-polarity signal applied to the data line was shifted from the common potential by a predetermined value (fixed optimal value).
The present invention has been made to solve the problem described above, and has an object to provide a liquid crystal display device which is capable of suppressing the generation of a ghost image as compared with the case where an average of a potential of a positive-polarity signal and a potential of a negative-polarity signal, which are applied to a data line, is shifted from a common potential by a predetermined value.
Representative aspects of the present invention disclosed in this application are briefly described as follows.
(1) A liquid crystal display device, including: a plurality of pixel circuits arranged in matrix; a plurality of data lines provided so as to correspond to rows of the plurality of pixel circuits; a plurality of scanning lines provided so as to correspond to columns of the plurality of pixel circuits; a data-line driving circuit for providing a signal to the plurality of data lines; and a scanning-line driving circuit for providing a scanning signal to the plurality of scanning lines, in which: each of the plurality of pixel circuits includes: a pixel capacitance having one end provided with a common potential; and a pixel transistor having a gate electrode provided with the scanning signal from one of the plurality of scanning lines, corresponding to the pixel circuit, and a source electrode and a drain electrode, one of the source electrode and the drain electrode being connected to another end of the pixel capacitance and another of the source electrode and the drain electrode being connected to one of the plurality of data lines, corresponding to the pixel circuit; the data-line driving circuit selectively outputs a positive-polarity signal and a negative-polarity signal to corresponding one of the plurality of data lines in accordance with a grayscale value for one of the plurality of pixel circuits; and the data-line driving circuit outputs the positive-polarity signal and the negative-polarity signal so that an average of a potential of the positive-polarity signal and a potential of the negative-polarity signal corresponding to the grayscale value changes in accordance with any one of the grayscale value, a temperature, a distance to the corresponding one of the plurality of pixel circuits from the scanning-line driving circuit, and a distance to the corresponding one of the plurality of pixel circuits from the data-line driving circuit.
(2) The liquid crystal display device according to the above-mentioned item (1), in which: the data-line driving circuit selectively outputs anyone of a combination of a positive-polarity precharge signal and an image signal subsequent to the positive-polarity precharge signal and a combination of a negative-polarity precharge signal and an image signal subsequent to the negative-polarity precharge signal to the corresponding one of the plurality of data lines in accordance with the grayscale value for the corresponding one of the plurality of pixel circuits; and the data-line driving circuit outputs the positive-polarity precharge signal and the negative-polarity precharge signal so that an average of a potential of the positive-polarity precharge signal and a potential of the negative-polarity precharge signal corresponding to the grayscale value changes in accordance with any one of the grayscale value, the temperature, the distance to the corresponding one of the plurality of pixel circuits from the scanning-line driving circuit, and the distance to the corresponding one of the plurality of pixel circuits from the data-line driving circuit.
(3) The liquid crystal display device according to the above-mentioned item (2), in which the data-line driving circuit outputs the positive-polarity precharge signal and the negative-polarity precharge signal so that the average of the potential of the positive-polarity precharge signal and the potential of the negative-polarity precharge signal corresponding to the grayscale value increases or decreases monotonously when the grayscale value increases from the smallest value to any one value within a range of the grayscale value or as the temperature decreases.
(4) The liquid crystal display device according to the above-mentioned item (2) or (3), in which: a potential of the image signal is determined in accordance with the grayscale value; and the data-line driving circuit outputs the precharge signals so that a potential difference between the potential of the precharge signal and the potential of the image signal, each signal having any one of the positive polarity and the negative polarity, corresponding to the grayscale value changes as the grayscale value increases until the grayscale value becomes equal to a change limit grayscale value corresponding to a grayscale value at which the potential difference becomes equal to a predetermined value and a change amount of the potential difference after the grayscale value exceeds the change limit grayscale value is smaller than before the grayscale value exceeds the change limit grayscale value.
(5) The liquid crystal display device according to the above-mentioned item (2) or (3), in which: a potential of the image signal is determined in accordance with the grayscale value; and the data-line driving circuit outputs the precharge signals so that a potential difference between the potential of the precharge signal and the potential of the image signal, each signal having any one of the positive polarity and the negative polarity, corresponding to the grayscale value changes as the distance to the corresponding one of the plurality of pixel circuits from the data-line driving circuit increases until the distance becomes equal to a border distance at which the potential difference becomes equal to a predetermined value and a change amount of the potential difference after the distance exceeds the border distance is smaller than before the distance exceeds the border distance.
(6) The liquid crystal display device according to the above-mentioned item (2) or (3), in which: a potential of the image signal is determined in accordance with the grayscale value; and the data-line driving circuit outputs the precharge signals so that a potential difference between the potential of the precharge signal and the potential of the image signal, each signal having any one of the positive polarity and the negative polarity, corresponding to the grayscale value changes as the distance to the corresponding one of the plurality of pixel circuits from the scanning-line driving circuit decreases until the distance becomes equal to a border distance at which the potential difference becomes equal to a predetermined value and a change amount of the potential difference after the distance becomes smaller than the border distance is smaller than before the distance becomes smaller than the border distance.
(7) The liquid crystal display device according to the above-mentioned item (2) or (3), in which: a potential of the image signal is determined in accordance with the grayscale value; and the data-line driving circuit outputs the precharge signals so that a potential difference between the potential of the precharge signal and the potential of the image signal, each signal having any one of the positive polarity and the negative polarity, corresponding to the grayscale value changes as the temperature decreases until the temperature becomes equal to a border temperature at which the potential difference becomes equal to a predetermined value and a change amount of the potential difference after the temperature becomes lower than the border temperature is smaller than before the temperature becomes lower than the border temperature.
(8) The liquid crystal display device according to any one of the above-mentioned items (2) to (7), in which the data-line driving circuit outputs the positive-polarity precharge signal and the negative-polarity precharge signal so that the average of the potential of the positive-polarity precharge signal and the potential of the negative-polarity precharge signal corresponding at least to the smallest grayscale value becomes equal to the common potential.
(9) The liquid crystal display device according to the above-mentioned item (8), in which: the data-line driving circuit selectively outputs the positive-polarity precharge signal and the negative-polarity precharge signal corresponding to the grayscale value and a previous grayscale value which is a grayscale value in a previous frame; and the data-line driving circuit outputs the positive-polarity precharge signal and the negative-polarity precharge signal so that the average of the potential of the positive-polarity precharge signal and the potential of the negative-polarity precharge signal becomes equal to the common potential at least when the previous grayscale value is smaller than the grayscale value.
According to the present invention, it is possible to suppress the generation of the ghost image as compared with the case where the average of the potential of the positive-polarity signal and the potential of the negative-polarity signal, which are applied to the data line, is shifted from the common potential by the predetermined value.
In the accompanying drawings:
Hereinafter, an embodiment of the present invention is described based on the accompanying drawings. The components having the same functions, which are described and illustrated in this specification, are denoted by the same reference character, and the description thereof is herein omitted.
A liquid crystal display device according to the embodiment of the present invention includes a liquid crystal display panel, a backlight unit for supplying light transmitting through the liquid crystal display panel, and a control board. In terms of a structure, the liquid crystal display panel includes an array substrate, a counter substrate, liquid crystal, and an integrated-circuit package. On the array substrate, pixel circuits PC are formed. The counter substrate is provided so as to be opposed to the array substrate. The liquid crystal is sealed between the array substrate and the counter substrate. The integrated-circuit package is provided on the array substrate. Polarizer plates are bonded onto the outer side of the array substrate and the outer side of the counter substrate. The liquid crystal display device according to this embodiment performs color display.
The scanning lines GL are aligned in the display region DA so as to extend in a horizontal direction in
Each of the pixel circuits PC includes a pixel transistor TR, a liquid-crystal capacitance Clc, and a wiring capacitance Cst. The liquid crystal capacitance Clc includes a pixel electrode, a common electrode, and liquid crystal interposed between the pixel electrode and the common electrode. The pixel transistor TR is an n-channel type thin-film transistor operating as a switch. A gate electrode of the pixel transistor TR is connected to the scanning line GL corresponding to the pixel circuit PC including the same pixel transistor TR. A source electrode of the pixel transistor TR is connected to the data line DL corresponding to the pixel circuit PC, whereas a drain electrode thereof is connected to the pixel electrode. The thin-film transistor has no polarity and therefore, the distinction between the source electrode and the drain electrode is made based on the potential applied thereto, for the sake of convenience. Although the destinations of connection of the source electrode and the drain electrode are described above for the sake of convenience, the destinations of connection may be interchanged with each other. The common electrode is electrically connected to the common line CL. Here, the wiring capacitance Cst other than the liquid-crystal capacitance Clc is formed between a node to which the pixel electrode is connected and the common line CL, and a parasitic capacitance Cgs of the pixel transistor TR is formed between a node to which the pixel electrode is connected and the scanning line GL.
The common-potential providing circuit VCG provides the common potential to the common line CL, and the reference-potential providing circuit VGR provides a plurality of reference potentials to be used by the data-line driving circuit XDV. The precharge circuit PRC outputs output data DO based on display grayscale data DI and an input synchronous signal SS input thereto. The timing control circuit TC inputs the output data DO output from the precharge circuit PRC to the data-line driving circuit XDV, and provides a horizontal synchronous signal SX to the data-line driving circuit XDV and a vertical synchronous signal SY to the scanning-line driving circuits YDV at timing in accordance with the input synchronous signal SS. A positive-polarity signal is a signal for setting a potential of the pixel electrode higher than a common potential, whereas a negative-polarity signal is a signal for setting the potential of the pixel electrode lower than the common potential. In the example of this embodiment, the positive polarity means that the potential of a signal or the like is higher than the common potential, whereas the negative polarity means that the potential of the signal or the like is lower than the common potential.
In the liquid crystal display device, even when the potential is applied to the data line DL, it takes time for a potential of a source electrode of the pixel transistor TR included in the pixel circuit PC to reach the applied potential due to the parasitic capacitance formed between the data line DL and the scanning line GL. In order to bring the potential of the source electrode closer to a target potential within a horizontal interval 1H, the data-line driving circuit XDV applies, to the data line DL, a potential Vp of a precharge signal in a first half of the horizontal interval 1H and a potential Vd of an image signal in a second half of the horizontal interval 1H.
The data-line driving circuit XDV outputs the potential indicated by the value of the precharge data PD as the precharge signal in the first half of one horizontal interval and outputs the potential indicated by the value of the display grayscale data DI as the image signal in the second half.
For each of the lookup tables, information for obtaining the correction amount for each combination of the grayscale value of the display grayscale data DI and the grayscale value of the previous display grayscale data LDI is set. The different lookup tables are prepared depending on T types of temperature condition, M types of column condition, N types of row condition, and conditions of the polarity of the image signal. The number of the lookup tables which are present is equal to the number of combinations of the aforementioned conditions. Therefore, a total number of lookup tables is (T×M×N×2). The M columns corresponding to the M types of the column condition are a part of all the columns of the pixel circuits PC and are referred to as representative columns. The N rows corresponding to the N types of the row condition are a part of all the rows of the pixel circuits PC and are referred to as representative rows.
More specifically, each of the lookup tables is a set of correction-amount data for each of the combinations of some representative values of the grayscale value of the display grayscale data DI and some representative values of the grayscale value of the previous display grayscale data LDI.
The positional-information acquiring section LG generates positional information in accordance with the input display grayscale data DI based on the input synchronous signal SS. The positional information indicates the position of the pixel circuit PC which is fed with the image signal The positional-information acquiring section LG also outputs polarity information indicating which of positive polarity or negative polarity the signal fed to the pixel circuit PC has.
The lookup table selecting section LTS selects a lookup table to be used for calculating the correction amount based on the positional information, the polarity information, and temperature information. The lookup table selecting section LTS first acquires a temperature condition which is the closest to the temperature indicated by the temperature signal TMP. Next, the lookup table selecting section LTS acquires one representative column on the same column coordinate x indicated by the positional information or two representative columns which are the closest thereto, and acquires one representative row on the same row coordinate y indicated by the positional information or two representative rows which are the closest thereto. Next, the lookup table selecting section LTS selects a lookup table(s) corresponding to the combination of the representative column(s) and the representative row(s) described above from the lookup tables satisfying the acquired polarity information and temperature condition. The number of lookup tables selected by the lookup table selecting section LTS is 1 to 4.
The representative correction-amount calculating section DRG uses the lookup table(s) selected by the lookup table selecting section LTS to calculate the correction amount for each of the selected lookup table(s). The interpolation processing section IPC performs interpolation processing based on the correction amount obtained for each of the selected lookup table(s), the representative column (s) and the representative row (s) corresponding to the lookup table(s), and the positional information in order to obtain the correction amount on the column coordinate x and the row coordinate y indicated by the positional information. The interpolation processing section IPC outputs the obtained correction amount as the correction-amount data PDD.
The contents which are set in the lookup tables are now described.
Here, in this embodiment, for realizing display with the grayscale closest to that perceived by a human, the amount of change in potential when the grayscale value is changed by one differs in accordance with the grayscale value before being changed. Therefore, the magnitude relation between the values in the cells in the tables of
First, as can be seen from the case where the grayscale value of the previous display grayscale data LDI is the smallest (0), at least in the case where the grayscale value of the display grayscale data DI is the smallest, the positive-polarity precharge correction amount V and the negative-polarity precharge correction amount V are the same. This fact shows that the potential of the positive-polarity precharge signal and the potential of the negative-polarity precharge signal are symmetric with respect to the common potential. Therefore, in this case, an average of the potential of the positive-polarity precharge signal and the potential of the negative-polarity precharge signal, which are corrected with the precharge correction amounts, becomes equal to the common potential. Even if the grayscale value of the display grayscale data DI increases from the smallest grayscale value, the positive-polarity precharge correction amount and the negative-polarity precharge correction amount are the same when the grayscale value increases from the smallest grayscale value described above to any one of the values within the range of the grayscale value.
When the positive-polarity precharge correction amount and the negative-polarity precharge correction amount are different from each other, the positive-polarity precharge correction amount is larger than the negative-polarity precharge correction amount. This fact shows that the average of the potential of the positive-polarity precharge signal and the potential of the negative-polarity precharge signal is higher than the common potential. Each of the pixel transistors TR is an n-channel type thin-film transistor. For turning ON the pixel transistor TR, a potential higher than the common potential and the largest potential of the positive-polarity image signal is fed to the scanning line GL. Comparing the case where the positive-polarity signal is applied to the source electrode of the pixel transistor TR and the case where the negative-polarity signal is applied thereto, a current more easily flows with the negative-polarity signal than with the positive-polarity signal because of a different potential difference between the source and a gate. Therefore, if the average of the positive-polarity signal and the negative-polarity signal becomes equal to the common potential, the average of the potential of the positive-polarity signal and the potential of the negative-polarity signal, which are applied to the pixel electrode, deviates from the common potential. Moreover, as the difference between the potential of the positive-polarity signal and the potential of the negative-polarity signal becomes larger, the amount of deviation becomes larger. Except for the case where, for example, the grayscale value of the previous display grayscale data LDI is 0 and a limit described below is provided, the correction amount is set in the lookup table so that the difference between the positive-polarity precharge correction amount and the negative-polarity precharge correction amount increases monotonously with an increase in the grayscale value of the display grayscale data DI. In this embodiment, the average of the potential of the positive-polarity precharge signal and the negative-polarity precharge signal is adjusted in accordance with the display grayscale data DI indicating the potential of the image signal. Therefore, the generation of a ghost image due to a change in potential of the signal can be suppressed.
Further, assuming that the grayscale value of the previous display grayscale data LDI is constant, the positive-polarity precharge correction amount increases monotonously as the grayscale value of the grayscale data DI increases from the smallest value. However, instead of increasing in a simple manner, the precharge correction amount V increases as the grayscale value increases until the grayscale value becomes equal to the grayscale value (change limit grayscale value) at which it is determined that the precharge correction amount V becomes equal to a predetermined amount (1 V in the case of
According to an experiment conducted by the inventors of the present invention, when the positive-polarity precharge correction amount becomes larger than the predetermined amount, a variation occurs in the potential that the potential of the pixel electrode reaches due to the positive-polarity image signal. Therefore, the average of the potential of the pixel electrode which is reached due to the positive-polarity image signal and the potential of the pixel electrode which is reached due to the negative-polarity image signal deviates from the common potential, sometimes resulting in the generation of a ghost image. In this embodiment, by providing the limit as described above, the generation of the ghost image due to the variation can be suppressed.
The pixel transistor TR may be a p-channel type thin-film transistor. In this case, the polarity of the scanning signal fed to the scanning line GL is inverted. Therefore, the current more easily flows with the positive-polarity signal than with the negative-polarity signal. A direction in which the common potential deviates becomes opposite. Therefore, the direction of correction also becomes opposite. For example, when the grayscale value of the display grayscale data DI increases from the smallest value to any one of the values within the range of the grayscale value, the difference between the positive-polarity precharge correction amount and the negative-polarity precharge correction amount decreases monotonously. As the temperature decreases, the difference decreases monotonously. Moreover, the positive-polarity precharge correction amount and the negative-polarity precharge correction amount are set in accordance with the grayscale value, the temperature, and the position of the pixel circuit PC so that the negative-polarity precharge correction amount does not exceed the predetermined amount.
While there have been described what are at present considered to be certain embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-052649 | Mar 2011 | JP | national |
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| 20120229525 A1 | Sep 2012 | US |
