Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-02-04
2003-02-18
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S099000, C345S100000, C345S103000, C345S205000, C345S206000
Reexamination Certificate
active
06522317
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a drive circuit of a liquid-crystal display apparatus of the active-matrix system. More particularly, the present invention relates to a liquid-crystal display apparatus with a drive circuit thereof created on the same substrate as an active-matrix substrate of a display unit.
BACKGROUND OF THE INVENTION
A liquid-crystal display apparatus of the active-matrix system comprises a display unit and a drive circuit unit. The display unit comprises transistors created at cross points of a plurality of data lines and a plurality of scan lines which are perpendicular to the data lines. The drive circuit unit controls the voltages of the data lines and the scan lines. The transistors employed in the display unit can be a-Si TFTs (amorphous-Silicon thin-film transistors), p-Si (poly-Silicon) TFTs, single-crystal silicon MOS (metal-oxide semiconductor) transistors or transistors of another type. An a-Si TFT is created on a glass substrate with an externally attached single-crystal sillicon integrated circuit serving as a drive circuit thereof. A p-Si TFT can be a high-temperature p-Si TFT created on a quartz substrate or a low-temperature p-Si TFT created on a glass substrate. The drive circuits of a high-temperature p-Si TFT and a low-temperature p-Si TFT are created on the same substrate as the display unit along with the single-crystal silicon MOS transistors. In addition, an a-Si TFT and a low-temperature p-Si TFT created on a glass substrate can be implemented in a large size. On the other hand, a transistor using a quartz or single-crystal silicon substrate can be implemented only in a small size.
The configuration and the operation of a liquid-crystal display apparatus of such an active-matrix system are explained in more detail as follows.
The gate, the drain and the source of a transistor employed in the display unit are connected to a scan line, a data line and a display electrode respectively. A facing substrate having a transparent electrode on one of the surfaces thereof is provided to face the display electrode. A liquid-crystal is sandwiched by the display electrode and the facing substrate. Normally, a signal holding capacitor is connected to the display electrode. Thus, the signal holding capacitor and a liquid-crystal capacitor are connected to a source electrode in parallel. When a gate electrode is selected, the transistor including the gate electrode is put in a conductive state, allowing a picture signal on the data line to be written into the liquid-crystal capacitor and the signal holding capacitor. As the gate electrode is deselected, the transistor including the gate electrode is put in a high-impedance state in which the picture signal written into the signal holding capacitor is sustained.
The drive circuit unit comprises a vertical drive circuit for controlling the voltages of the scan lines and a horizontal drive circuit for controlling the voltages of the data lines. The vertical drive circuit applies a scanning pulse to each of the scan lines in a frame time. Normally, the timings of the pulses are shifted from each other as the scanning moves from the top of the panel to the bottom. Generally, a frame time is {fraction (1/60)} seconds. In a panel having a representative pixel configuration of 1,024×768 dots, 768 scanning operations are carried out during a frame time so that the width of a scanning pulse is about 20 &mgr;s. The vertical drive circuit employs an ordinary shift resister with an operating speed corresponding to a frequency of about 50 kHz.
On the other hand, the horizontal drive circuit applies a liquid-crystal driving voltage corresponding to pixels on a line driven by a scanning pulse to each data line. In a pixel to which a scanning pulse is applied, the voltage of the gate electrode of the transistor connected to the scan line applying the scanning pulse increases, putting the transistor in a turned-on state. In this state, a liquid-crystal driving voltage on the data line is applied to the liquid-crystal through the drain and the source of the transistor, electrically charging a pixel capacitor which comprises the liquid-crystal capacitor and the signal holding capacitor connected in parallel. By repeating this operation, a voltage corresponding to a picture signal repeated for each frame time is applied to the liquid-crystal on the entire surface of the panel, electrically charging the pixel capacitors on the entire surface of the panel.
The horizontal drive circuit can be of an analog system or a digital system in dependence on the input picture signal. In the case of the analog system, the horizontal drive circuit for driving a data line comprises a shift register and a sample-and-hold circuit. The shift register determines timing of the sample-and-hold circuit for each pixel. With this timing, the sample-and-hold circuit samples a picture signal corresponding to each pixel and applies a liquid-crystal driving voltage to each data line. This driving method allows the shift register for determining timing and the sample-and-hold circuit for sampling a picture signal to be implemented by a simple circuit. Thus, this method is mainly adopted in a liquid-crystal display panel incorporating a drive circuit in a single integrated assembly.
In the case of the pixel configuration described above, the shift register employed in the horizontal drive circuit generates a timing signal 1,024 times in a period of time corresponding to the width of a scanning pulse output by the vertical drive circuit. Thus, the interval between 2 consecutive timings is shorter than 20 ns. That is to say, the shift register needs to operate at a speed corresponding to a frequency of at least 50 MH. Thus, the sample-and-hold circuit is required to sample a picture signal with timing corresponding to such a short interval. A liquid-crystal display panel incorporating a drive circuit in a single integrated assembly adopts a technique whereby a picture signal is divided into a plurality of input signals to increase the sampling interval. With a high-speed picture signal split into a plurality of sampled picture signals in this way, however, it is necessary to provide a signal conversion circuit for amplifying and the split signals and converting the signals into alternating-current signals.
In the case of the digital system, on the other hand, the horizontal drive circuit for driving a data line comprises a shift register, latch circuits at 2 stages and a digital-to-analog conversion circuit. A digital signal supplied sequentially to the horizontal drive circuit is stored in the latch circuits at the 2 stages through the shift register. On the other hand, the digital-to-analog conversion circuit converts the digital data into an analog voltage applied to each of the data lines as a liquid-crystal driving voltage.
The bit counts of the latch circuits and the digital-to-analog conversion circuit in this system are determined by a display tone. In the case of a full color display requiring 256 tones, the number of bits is 8. In the case of the pixel configuration described above, a 16384-bit (=8×2×1024 bits) latch circuit and 1,024 8 bit digital-to-analog conversion circuits are required. There is adopted a method for selecting a reference voltage by means of a switch in order to reduce the number of variations among digital-to-analog conversion circuits of the data lines. Since the picture signal is a digital signal in this digital system, it is possible to prevent the S/N ratio from deteriorating during transmission of the signal.
In addition, in the case of the digital system, there is provided a method whereby, after a digital picture signal is converted into an analog signal by a digital-to-analog conversion circuit operating at a high speed, a voltage to be applied to each of the data lines is generated by using the same technique as the analog system.
The method wherein a digital-to-analog conversion circuit is provided for each of the data lines is disclosed in documents such as Japa
Aono Yoshinori
Kageyama Hiroshi
Mikami Yoshiro
Miyazawa Toshio
Nagano Takahiro
Antonelli Terry Stout & Kraus LLP
Awad Amr
Hitachi , Ltd.
Saras Steven
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