Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-01-21
2002-07-02
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S100000, C345S103000, C345S204000, C345S087000
Reexamination Certificate
active
06414668
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal device of a drive circuit built-in type in which a display pixel portion and a drive circuit portion are incorporated integrally on a common substrate.
A drive circuit built-in liquid crystal display device integrally incorporating its drive circuit on a glass substrate is under progressive researches and developments toward its practical use because it leads to a reduction of components, simplifies the process for packaging the drive circuit onto the liquid crystal display panel, and hence contributes to a reduction of the cost.
A drive circuit built-in liquid crystal device is typically made, in case of TFT-LCD, by enclosing a liquid crystal between an array substrate having thin-film transistors as switching elements arranged in a matrix in accordance with pixels and an opposite substrate having formed color filters, providing polarization plates to these substrates, respectively, and mounting an illumination back light behind them. The matrix-arrayed substrate includes a display pixel portion made up of scanning lines and signal lines which are aligned in form of a matrix on the glass substrate and liquid crystal pixels formed on their crossing points via thin-film transistors as switching elements, and a peripheral drive circuit which is made simultaneously with the thin-film transistors in a common manufacturing process to surround the display pixel portion. The peripheral drive circuit includes a scanning line drive circuit for controlling switching actions of the thin-film transistors connected to pixels and a signal line drive circuit for supplying video signals to the thin-film transistors via the signal lines.
The signal line drive circuit includes a group of analog switches responsive to a timing signal for selectively connecting video signal lines to signal electrodes to supply video signals, and must operate in a higher frequency than the scanning line drive circuit. Moreover, along with progressively increasing demands for high fidelity, large capacity display, and so on, in high vision using an increased number of pixels, there have been remarked problems such as insufficient transmission bands of the video bus lines for transmitting video signals within the signal line drive circuit, and insufficient writing capacity of the analog switches for sampling the video signals on the video bus lines and supplying them to pixel switching elements.
To cope with the problems, a conventional technique divides the signal line drive circuit into a plurality of blocks and effects sampling of analog switches simultaneously within the blocks to lower the operation frequency. That is, by dividing the video bus lines into some blocks to introduce video signals in parallel and by having analog switches connected to each block of the video bus lines via a connection wiring to operate collectively for sampling, the operation frequency can be lowered by the number of blocks of the video bus so as to compensate the insufficient writing capacity of the analog switches.
However, when the conventional drive circuit built-in liquid is crystal display device is configured to supply video signals to a plurality of divisional blocks of the video signal lines as explained above, there arises the problem that stripe-shaped imaging defects
2
(stripe-shaped defects) extending longitudinally (in the column direction) appear on the display screen
1
as shown in
FIG. 7
, and degrade the imaging quality.
The Inventors made researches to locate its reason, and found a strong relation between the positions of the imaging defects on the screen and the positions of connection between analog switches and the video buses.
More specifically, immediately after sapling by an analog switch, electric charges stored in the analog switch flow in toward a video bus and the signal line connected to the analog switch. The flow of the electric charge causes the potential on the signal line to shift, and hence causes the signal written in a liquid crystal pixel to slightly shift from the video signal on the video bus.
In an analog switch connected to a video bus located far from the display pixel portion, the connection wiring between the analog switch and the video bus is longer and results in increasing the resistance of the connection wiring. As a result, electric charges accumulated in the analog switch during sampling is difficult to flow toward the video bus, and the ratio of the charges flowing toward the signal line increases.
In contrast, in an analog switch connected to a video bus near the display pixel portion, the connection wiring is shorter, the wiring resistance is lower, and the ratio of electric charge accumulated in the analog switch and flowing toward the signal line decreases.
This results in the phenomenon that shift amounts of video signals are small in signal lines connected to analog switches with shorter connection wirings to video buses whereas shift amounts of video signals are large in signal lines connected to analog switches with longer connection wirings. Therefore, effective voltage values applied to liquid crystal pixels vary with positions of signal lines, and cause them to vary in transmittance.
The Inventors found that, since the arrangement of connection points of analog switches and video buses was repeated for every block of sampling circuits, the difference in transmittance of the liquid crystal pixels was produced on the screen periodically along the row direction, and was noticeable as imaging defects appearing in the column direction.
FIG. 6
shows a wiring pattern in a signal line drive circuit by a conventional approach.
In
FIG. 6
, video signals SV
1
to SV
6
are applied to video buses
101
to
106
in this order. These video buses
101
through
106
and analog switches SW are connected in this order by connection wirings
211
to
216
via contact holes. As a result, adjacent signal electrodes are supplied with signals from adjacent video buses. Since lengths in of connection wirings are different only by the distance S between their video buses, difference in capacities caused by the wiring resistance and crossing of wirings is small, and no image noise occurs there.
However, in case of this example for comparison, there is a large difference in length of the connection wiring at the position where the shift register is switched from a certain stage to another. That is, the last wiring to the first stage (SR
11
) of the shift register and the first wring to the second stage (SR
21
) are different in length as large as 5 pitches, and the difference is as large as five times the difference in length between other adjacent wirings. Thus, the difference in wiring resistance is large and causes the difference in shift amount of video signal mentioned above.
Therefore, in the conventional device, the load to the wiring changes largely at the position where the shift register is switched from a stage to another, and image noise such as imaging defects cannot be prevented.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a liquid crystal display device of having a built-in drive circuit, which alleviates imaging defects caused by changes in length of wirings and improves the imaging quality.
According to the first aspect of the present invention, there is provided a liquid crystal display device having:
a display pixel portion including a plurality of liquid crystal pixel cells arranged in a matrix on an insulating substrate and a plurality of signal lines each connected commonly to said liquid crystal pixels in a column; and
a signal line drive circuit including groups of positive-polarity video buses for transmitting positive-polarity video signals, groups of negative-polarity video buses disposed in parallel with said groups of the positive-polarity video buses to transmit negative-polarity video signals, and sampling circuit blocks made up of a plurality of positive-polarity switches connected individuall
Aoki Yoshiro
Hirai Hoko
Karube Masao
Miyatake Masaki
Nakamura Kazuo
Kabushiki Kaisha Toshiba
Nguyen Kevin N.
Pillsbury & Winthrop LLP
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