JP2005114786A - Method and circuit for driving liquid crystal display unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the size of a transistor and power consumption in display unit operation by using a reference-voltage inversion type driving system for a self-positioning type manufacturing system by supplying the common reference potential of a liquid crystal capacitor and a storage capacitor in a pixel to different AC signals. <P>SOLUTION: 1. A scanning signal, a video signal, a 1st AC signal, and a 2nd AC signal are supplied to a pixel. 2. The scanning signal is supplied to a transistor to perform ON/OFF control over the transistor. 3. When the transistor is ON, the video signal is supplied to a 1st terminal of the liquid crystal capacitor and a 1st terminal of the storage capacitor and the 1st terminals of the liquid crystal capacitor and storage capacitor are connected to each other. 4. The 1st AC signal and 2nd AC signal are supplied to a 2nd terminal of the liquid crystal capacitor and a 2nd terminal of the storage capacitor respectively, the 1st AC signal and 2nd AC signal are synchronously with each other, and the 2nd AC signal has a DC offset voltage that the 1st AC signal does not have. 5. When the transistor is ON, the video signal is introduced in the pixel to charge the storage capacitor and liquid crystal capacitor, and liquid crystal having an internal voltage is driven. When the transistor is OFF, on the other hand, the internal voltage is continuously supplied from the storage capacitor to maintain a potential difference for liquid crystal driving, and the liquid crystal of the pixel is continuously driven. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a driving method and a driving circuit for a liquid crystal display, and more particularly to a driving method and a driving circuit for liquid crystal polarity inversion of a liquid crystal display.

  Conventional driving methods of an active liquid crystal display include frame polarity inversion, column polarity inversion, column polarity inversion, point polarity inversion, and the like. . Which method is selected depends on the image quality, power consumption, and driving complexity. Of these methods, frame polarity reversal is the simplest, but it is not usually used because its video image quality is the worst. On the other hand, point polarity inversion produces the best video quality, but both power consumption and complexity are high. Therefore, when the video image quality is not required to be high, the row polarity inversion and column polarity inversion methods are used as an eclectic method among the methods described above.

FIG. 1 shows an equivalent circuit used in a pixel driving method of a conventional active liquid crystal display. A scanning signal V _s 102 in the figure is connected to the gate of the transistor 110 to control on / off thereof. The video signal V _D 104 is connected to the source of the transistor 110, and when the transistor 110 is turned on, the video signal 104 obtains an internal voltage via the transistor 110 (the voltage value in the figure). _V1c ). The internal voltage 112 is stored in a storage capacitor 130 (capacitor C _st shown) and a liquid crystal capacitor 120 (capacitor C _1c shown). The liquid crystal angle in the liquid crystal capacitor 120 is driven by the potential difference (the DC voltage value V _com shown) between the internal voltage 112 and the DC signal 106. On the other hand, when the transistor 110 is off, the potential difference for driving the liquid crystal capacitor 120 is continuously provided from the storage capacitor 130.

FIG. 2 shows a driving method based on various polarity inversions, and is a time chart of each driving signal in accordance with the equivalent circuit of FIG. In FIG. 2, the DC signal (V _com ) 206 is a reference voltage, and the scanning signal (V _s ) 202 is a high potential. When the video signal (V _D ) 204 is introduced into the pixel, The storage capacitor 130 and the liquid crystal capacitor 120 are charged. Since the internal voltage 212 is stably maintained by charging the capacitor, the potential difference 210 required by the liquid crystal capacitor 120 is provided.

In the polarity inversion driving method described above, since the polarity must be inverted after the video signal is written to the pixel every time, the power consumption is significantly increased due to the high voltage amplitude and the inversion frequency. On the other hand, in order to reduce such power consumption, the driving method is slightly changed, and a driving method of reference potential inversion (referred to as V _com inversion or common toggle) is used. With such a driving method, the voltage amplitude when the video signal is inverted can be reduced, so that the power consumption when the display is activated is reduced.

  FIG. 3 shows a pixel equivalent circuit using a reference potential inversion driving method of a conventional active liquid crystal display. A scanning signal 302 in the figure is connected to the gate of the transistor 310 to control on / off thereof, and a video signal 304 is connected to the source of the transistor 310. When the transistor 310 is on, the video signal 304 passes and the internal voltage 312 is obtained. At this time, the internal voltage 312 is stored in the storage capacitor 330 and the liquid crystal capacitor 320. The first ends of the storage capacitor 330 and the liquid crystal capacitor 320 are connected to the drain of the transistor 310, and the second ends (common electrodes) of the storage capacitor 330 and the liquid crystal capacitor 320 are both connected to the AC signal 306. Actually, the liquid crystal angle in the liquid crystal capacitor 320 is driven by the voltage difference between the internal voltage 312 and the AC signal 306. Thus, when the transistor 310 is off, the potential difference required by the liquid crystal capacitor 320 is continuously supplied from the storage capacitor 330.

FIG. 4 shows the inversion driving method of the reference potential described above, and is a time chart of each driving signal in accordance with the equivalent circuit of FIG. In FIG. 4, the reference voltage necessary for driving the liquid crystal is changed to an AC signal (V _com ) 406. When the scanning signal (V _s ) 402 is at a high potential, the video signal (V _D ) 404 is introduced into the pixel, whereby the storage capacitor 330 and the liquid crystal capacitor 320 are charged to generate an internal voltage. 412 is obtained. Due to the internal voltage 412 and the AC signal 406, a voltage difference 410 for driving the liquid crystal is obtained and acts on the liquid crystal capacitor 320.
JP 2000-04-1003 A JP 2000-202760 A

  The amplitude of the video signal 404 can be reduced by the above-described driving method of reference potential inversion, so that power consumption can be reduced. On the other hand, a common electrode between the liquid crystal capacitor 320 and the storage capacitor 330 is connected to the same potential. When the storage capacitor 330 is an asymmetric capacitor having polarity, the above-described reference potential inversion driving method cannot be used.

  In addition, with the progress of manufacturing technology, the size of thin film transistors (TFTs) has been reduced, but the demand for precision has also been relatively increased. Therefore, the conventional optical alignment method is not required, and a self-aligned manufacturing method is required. Note that when a self-aligned manufacturing method is used, the performance of the thin film transistor can be improved. However, a storage capacitor formed between a gate electrode and a polycrystalline silicon electrode has a polarity. This is an asymmetric capacitor (shown in FIG. 8). After all, the conventional reference potential inversion driving method cannot be used, and the purpose of reducing power consumption cannot be achieved.

  An object of the present invention is to provide a driving method for a liquid crystal display, using a reference potential inversion driving method for a self-alignment manufacturing method, and reducing the size of the transistor and the power consumption when the display is operated. Eliminates the disadvantages of conventional driving methods.

  According to the present invention, a method for driving a liquid crystal display is provided. This liquid crystal display is composed of a plurality of pixels, and each pixel includes a transistor, a liquid crystal capacitor, and a storage capacitor. According to this method, the common reference potential of the liquid crystal capacitor and the storage capacitor is supplied to two different types of AC signals in the conventional pixel. These types of AC signals are synchronous to each other, the latter having a DC offset voltage (DC offset) not found in the former. That is, the method of the present invention includes the following. 1. A scanning signal, a video signal, a first AC signal, and a second AC signal are supplied to the pixel. 2. A scanning signal is supplied to the transistor to control on / off of the transistor. 3. When the transistor is on, the video signal is supplied to the first end of the liquid crystal capacitor and the first end of the storage capacitor, and the first end of the liquid crystal capacitor and the storage capacitor are connected to each other. 4). A first AC signal and a second AC signal are respectively supplied to the second end of the liquid crystal capacitor and the second end of the storage capacitor, and the first AC signal and the second AC signal are synchronized with each other, The two AC signals have a DC offset voltage that is not present in the first AC signal. 5). When the transistor is on, a video signal is introduced into the pixel and the storage capacitor and the liquid crystal capacitor are charged to obtain an internal voltage to drive the liquid crystal. On the other hand, when the transistor is off, the internal voltage is continuously supplied from the storage capacitor, so that the potential difference for driving the liquid crystal is maintained and the liquid crystal of the pixel is continuously driven.

  In the driving method of the liquid crystal display according to another preferred embodiment of the present invention, the DC offset voltage value is at least larger than the sum of the critical voltage value of the storage capacitor and the maximum value of the liquid crystal driving potential difference. The critical voltage value of the equivalent thin film transistor of the storage capacitor can be changed by adjusting the doping amount, and the DC offset voltage can be lowered. In other alternative embodiments, the critical voltage value of the equivalent thin film transistor of the storage capacitor may be different from the critical voltage value of the thin film transistor that controls the on / off of the pixel.

  The driving circuit of another liquid crystal display provided by the present invention is configured in accordance with the driving method described above. This liquid crystal display is composed of a plurality of pixels, and each pixel has a liquid crystal driving circuit, and this liquid crystal driving circuit includes a transistor, a liquid crystal capacitor, and a storage capacitor. The drain of the transistor is connected to the scanning signal, and the source of the transistor is connected to the video signal. The first end of the liquid crystal capacitor is connected to the drain of the transistor, and the second end of the liquid crystal capacitor is connected to the first AC signal. Liquid crystal is filled in the liquid crystal capacitor, and the transmittance of the liquid crystal is changed by the voltage difference between the first end and the second end. The first end of the storage capacitor is connected to the drain of the transistor, and the second end of the liquid crystal capacitor is connected to the second AC signal.

  In the driving circuit of the liquid crystal display according to another preferred embodiment of the present invention, the transistor is formed of a thin film transistor (TFT). The storage capacitor formed between the gate electrode and the polycrystalline silicon electrode is an asymmetric capacitor having a polarity, which is completed by a self-aligned manufacturing technique (self-aligned).

  The driving method of the liquid crystal display of the present invention maintains the potential difference between the storage capacitor and the liquid crystal capacitor by using different AC signals and providing different reference voltages to the liquid crystal capacitor and the storage capacitor in the pixel. Both AC signals are synchronized with each other and have the same amplitude. The AC signal connected to the storage capacitor has a DC offset voltage that is not present in the AC signal of the liquid crystal capacitor, and maintains the storage capacitor at the maximum value. Therefore, the method of the present invention can eliminate the drawbacks of the conventional driving method and is applied to the self-alignment manufacturing process, thereby reducing power consumption.

  In order to better understand the above and other objects, features and advantages of the present invention, a preferred embodiment will now be described in detail with reference to the drawings.

FIG. 5 shows a preferred embodiment of the present invention. The driving method of the illustrated liquid crystal display of the present invention uses an equivalent circuit of pixels. A scanning signal (signal V _s shown) 502 in the figure is connected to the gate of the transistor 510 to control on / off thereof. A video signal (signal V _D shown in the figure) 504 is connected to the source of the transistor 510. When the transistor 510 is on, the video signal 504 passes and the internal voltage 512 (the voltage value V _1c shown) is obtained. The internal voltage 512 is stored in a storage capacitor (capacitor C _st shown) 530 and a liquid crystal capacitor (capacitor C _1c shown) 520. The first ends of the storage capacitor 530 and the liquid crystal capacitor 520 are connected to the drain of the transistor 510, the second end of the storage capacitor 530 is connected to the second AC signal (V ₂ ) 508, and The second end of the liquid crystal capacitor 520 is connected to the first AC signal (V ₁ ) 506. The waveforms shown in the lower half of the figure are time charts of the first AC signal (V ₁ ) and the second AC signal (V ₂ ). In the present embodiment, the first AC signal (V ₁ ) and the second AC signal (V ₂ ) have the same amplitude (V _b ), frequency and phase, but the second AC signal (V ₂ ) It has _a DC offset voltage (V _a ) that is not present in the first AC signal (V ₁ ). This DC offset voltage value (V _a ) needs to be at least larger than the sum of the critical voltage value of the equivalent thin film transistor of the storage capacitor 530 and the maximum value of the liquid crystal driving potential difference. The liquid crystal angle in the liquid crystal capacitor 520 is driven by the potential difference between the internal voltage 512 and the first AC signal 506. When the transistor 510 is off, the potential difference required by the liquid crystal capacitor 520 is continuously provided from the storage capacitor 530.

FIG. 6 shows the driving method of the present invention, and is a time chart of each driving signal in accordance with the equivalent circuit shown in FIG. FIG. 7 shows an embodiment of the driving circuit of the liquid crystal display of the present invention based on the equivalent circuit of FIG. 5 and the driving method of FIG. As shown in FIG. 6, the potential difference 610 for driving the liquid crystal is provided by the potential difference between the internal voltage 612 and the first AC signal 606. When the scanning signal (V _s ) 602 is at a high potential, the video signal (V _D ) 604 is introduced into the pixel and charges the storage capacitor 530 and the liquid crystal capacitor 520. Since the internal voltage 612 is obtained after the storage capacitor 530 and the liquid crystal capacitor 520 are charged, the potential difference 610 required by the liquid crystal capacitor 520 described above is provided. Also, the second AC signal (V ₂ ) 608 has a DC offset voltage 630 that is not present in the first AC signal (V ₁ ) 606. In the preferred embodiment, by selecting the value of the DC offset voltage 630, the DC offset voltage 630 is determined by at least the sum of the critical voltage value of the equivalent thin film transistor of the storage capacitor 530 and the maximum voltage value of the liquid crystal driving voltage difference 610. It is getting bigger. Therefore, the second signal 608 becomes larger than the difference 620 between the internal voltage 612 and the storage capacitor potential, and the storage capacitor 530 is held in the same polarity direction to maintain the maximum capacitor value. The first AC signal 606 and the second AC signal 608 must be synchronized with the timing of potential inversion and have the same amplitude. Thereby, the voltage across both ends of the liquid crystal capacitor can be maintained at a stable value. Therefore, the driving method of the present invention is applied to the self-alignment manufacturing process.

  According to a preferred embodiment of the present invention, the critical voltage value of the equivalent thin film transistor of the storage capacitor can be changed by adjusting the doping amount to lower the DC offset voltage. FIG. 8 is a diagram showing the relationship between the voltage difference across the storage capacitor and the dielectric constant. This figure shows that the capacitance of the storage capacitor varies nonlinearly with the end voltage. For example, as curve 82 shows, the voltage can only be stored after the end voltage exceeds critical voltage 81. Therefore, in order to reduce the power consumption by lowering the voltage of the driving circuit, the power consumption is reduced by changing the doping amount. A curve 82 shows a characteristic curve of a storage capacitor having a certain doping amount, and a curve 83 on the left after changing the doping amount. At the same time, the initial critical voltage 81 has dropped to the new critical voltage 84. In one selected embodiment, the critical voltage value of the equivalent thin film transistor of the storage capacitor can be set to a negative value according to the design needs. As another alternative embodiment, the critical voltage value of the equivalent thin film transistor of this storage capacitor may be set to be different from the critical voltage value of the thin film transistor that controls pixel on / off.

  The method of the present invention maintains charge / discharge of the storage capacitor at a predetermined polarity by using different AC signals and providing different reference voltages to the liquid crystal capacitor and the storage capacitor in the pixel. These AC signals are synchronized with each other and have the same amplitude. Further, the AC signal connected to the storage capacitor has a DC offset voltage that is not present in the AC signal of the liquid crystal capacitor. Therefore, the method of the present invention can eliminate the drawbacks of the conventional driving method and is applied to the self-alignment manufacturing process, thereby reducing power consumption.

  Although the present invention has been described above by means of a preferred embodiment, it is not limited to this embodiment. Those skilled in the art can make slight changes and modifications without departing from the spirit and scope of the present invention. Accordingly, the protection scope of the present invention should be determined by the appended claims.

It is a figure which shows the equivalent circuit for a pixel drive based on a prior art. FIG. 2 is a time chart illustrating various polarity inversion driving methods according to the related art and illustrating various driving signals in accordance with FIG. 1. It is a figure which shows the pixel equivalent circuit using the drive system of the conventional reference potential inversion. 4 is a time chart showing a driving method of reference potential inversion according to the prior art and showing various driving signals in accordance with FIG. 1 shows a driving method and a driving circuit of a liquid crystal display of the present invention, and shows an equivalent circuit of a pixel in a driving circuit of a liquid crystal display of a preferred embodiment. FIG. 6 is a time chart showing a driving method of a preferred embodiment of the present invention and showing each driving signal in accordance with FIG. 5. 6 shows a driving method and a driving circuit of a liquid crystal display according to the present invention, and also shows a pixel driving circuit in the liquid crystal display, and is an actual circuit diagram completed in accordance with FIG. The driving method and driving circuit of the liquid crystal display of the present invention are shown, the relationship between the storage capacitor end voltage and the dielectric constant is shown, and the influence of different doping amounts on the characteristics of the storage capacitor in the manufacturing process is shown.

Explanation of symbols

102 Scan Signal 104 Video Signal 106 DC Signal 110 Transistor 112 Internal Voltage 120 Liquid Crystal Capacitor 130 Storage Capacitor 202 Scan Signal 204 Video Signal 206 DC Signal 210 Liquid Crystal Driving Potential 212 Internal Voltage 302 Scan Signal 304 Video Signal 306 AC Signal 310 Transistor 312 Internal voltage 320 Liquid crystal capacitor 330 Storage capacitor 402 Scan signal 404 Video signal 406 AC signal 410 Liquid crystal drive potential difference 412 Internal voltage 502 Scan signal 504 Video signal 506 First AC signal 508 Second AC signal 510 Transistor 512 Internal voltage 520 Liquid crystal capacitor 530 Storage capacitor 602 Scan signal 604 Video signal 606 First AC signal 608 Second AC signal 610 Liquid crystal driving potential difference 61 Internal voltage 620 storage capacitor potential difference 630 DC offset voltage 81 doping amount before the change threshold voltage 82 doping amount before the change of the storage capacitor of the characteristic curve 83 characteristic curve 84 doping amount after change of threshold voltage of the storage capacitor after the doping amount change

Claims (8)

  In a liquid crystal display composed of a plurality of pixels, each pixel having a transistor, a storage capacitor, and a liquid crystal capacitor, supplying a scanning signal to the transistor to control on / off of the transistor; When the transistor is on, supplying a video signal to the first end of the liquid crystal capacitor and the first end of the storage capacitor connected to each other; and a second end corresponding to the first end of the liquid crystal capacitor And supplying a first AC signal and a second AC signal to the second end corresponding to the first end of the storage capacitor, respectively, and the first AC signal and the second AC signal are synchronized with each other, The method of driving a liquid crystal display, wherein the second AC signal has a DC offset voltage that is not present in the first AC signal.   The method of claim 1, wherein the first AC signal and the second AC signal have the same amplitude.   3. The liquid crystal display driving method according to claim 2, wherein the DC offset voltage value is at least larger than a sum of a critical voltage value of an equivalent thin film transistor of the storage capacitor and a maximum value of a liquid crystal driving potential difference.   4. The method according to claim 3, wherein the critical voltage value of the equivalent thin film transistor of the storage capacitor is changed by adjusting a doping amount to lower the DC offset voltage.   4. The method of driving a liquid crystal display according to claim 3, wherein a critical voltage value of the storage capacitor is different from a critical voltage value of the transistor. In a liquid crystal display composed of a plurality of pixels, each pixel having a liquid crystal driving circuit, a transistor having a gate connected to a scanning signal and a source connected to a video signal;
A liquid crystal capacitor, a first end of the liquid crystal capacitor is connected to the drain of the transistor, a second end corresponding to the first end of the liquid crystal capacitor is connected to the first AC signal, and the liquid crystal capacitor Liquid crystal is filled, and the transmittance of the liquid crystal is changed by a voltage difference between the first end and the second end, and further includes a storage capacitor, the first end of the storage capacitor Is connected to the drain of the transistor, and the second end corresponding to the first end of the liquid crystal capacitor is connected to the second AC signal.   The liquid crystal display driving circuit according to claim 6, wherein the transistor comprises a thin film transistor (TFT). 8. The driving circuit of a liquid crystal display according to claim 7, wherein the storage capacitor is an asymmetric capacitor having a polarity, which is completed by a self-aligned manufacturing technique.

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