EP1528534B1

EP1528534B1 - Driving circuit of a liquid crystal display and driving method thereof

Info

Publication number: EP1528534B1
Application number: EP03025009A
Authority: EP
Inventors: Cheng-Jung Chen; Yuh-Ren Shen; Liang-Chen Chien
Original assignee: VastView Technology Inc
Current assignee: VastView Technology Inc
Priority date: 2003-10-30
Filing date: 2003-10-30
Publication date: 2012-04-18
Anticipated expiration: 2023-10-30
Also published as: EP1528534A1

Description

The invention relates to a driving circuit of a liquid crystal display and a driving method thereof according to the pre-characterizing clauses of claims 1 and 6. Such a method and circuit are disclosed in US 2002/0196218 A1 . Further, EP 1 443 486 A1 discloses a liquid crystal display and driving method having features corresponding to features (h) and (i) defined in claim 2.
A liquid crystal display (LCD) has advantages of light weight, low power consumption, and low divergence, and is applied to various portable equipment, such as notebook computers and personal digital assistants (PDA). In addition, LCD monitors and LCD televisions are gaining in popularity as a substitute for traditional cathode ray tube (CRT) monitors and televisions. However, an LCD still has some disadvantages. Because of the limitations of physical characteristics, the liquid crystal molecules should be twisted and rearranged when changing input data, and the images will be delayed. For satisfying the rapid switching requirements of multimedia equipment, improving the response speed of liquid crystal is desired.
With these problems in mind, the present invention aims at providing a driving circuit of a liquid crystal display and a driving method thereof to solve the above-mentioned problem.
This is achieved by the present invention by a driving method of a liquid crystal display as claimed in claim 1 and a driving circuit for driving a liquid crystal display as claimed in claim 6. The dependent claims pertain to preferred embodiments thereof.
The invention is illustrated by way of example with reference to the accompanying drawings, in which

Fig.1 is a timing diagram of pixel voltage and transmission rate according to prior art,
Fig.2 is another timing diagram of pixel voltage and transmission rate according to prior art using an over-driving method,
Fig.3 is a diagram of liquid crystal display,
Fig. 4 is a block diagram of one embodiment of the present invention,
Fig.5 is a reference table used for the lookup table in Fig.4,
Fig.6 is a block diagram of another application of the present invention,
Fig.7 is a reference table used for the lookup table in Fig. 6,
Fig.8 is a block diagram of another application of the present invention, and
Fig.9 is a reference table used for the lookup table in Fig. 8.

The prior art is disclosed in U.S. published application No. 2002/0050965 . The U.S. published application No. 2002/0050965 discloses an over-driving method using a brief table to store the over-driving image data. The brief table only includes part of the over-driving image data for driving the pixels switched from one gray level to another. When the driving circuit receives the image data from the input terminal, a processor is used to perform an interpolation operation to expand the brief table. Hence, an extra algorithm is needed in the conventional over-driving method and the algorithm will slow down the response speed.
Please refer to Fig.1, which is a timing diagram of the pixel voltage and the transmission rate V1 according to a prior art LCD. In Fig. 1, the pixel voltage is shown with the straight lines, and the transmission rate V1 is shown with a dotted line. In Fig.1, frame N means a frame period, and frame N+1, N+2... mean the following frame periods. When the pixel voltage is switched from a data voltage C1 to a data voltage C2, due to the physical characteristics of liquid crystal molecules, the liquid crystal molecules cannot be twisted to a predetermined angle within a frame period and fail to perform a predetermined transmission rate. As the curve of the transmission rate V1 shows, the transmission rate V1 cannot reach a predetermined transmission rate until the frame period of frame N+2. The delayed switch will cause blurring on the LCD.
An over-driving method is utilized to improve the delayed switch. Please refer to Fig.2, which is a timing diagram of the pixel voltage and the transmission rate V2 according to a prior art LCD using an over-driving method. When the pixel voltage is switched from the data voltage C1 to the data voltage C2, an over-driving data voltage C3 is added to accelerate the response speed of the liquid crystal molecules. Since a higher data voltage can obtain a faster response speed of the liquid crystal molecules, the data voltage C3 higher than the data voltage C2 can improve the delayed switch to reach the predetermined transmission rate in a frame period. As Fig.2 shows, the curve of the transmission rate V2 can reach the predetermined transmission rate in frame N.
Please refer to Fig.3, which is a diagram of a general LCD 30. The LCD 30 comprises a liquid crystal panel 31, and the liquid crystal panel 31 comprises a plurality of scan lines 32, a plurality of data lines 34, and a plurality of pixels 36. Each pixel 36 is connected to a corresponding scan line 32 and a corresponding data line 34, and each pixel 36 has a switching device 38 and a pixel electrode 39. The switching device 38 is connected to the corresponding scan line 32 and the corresponding data line 34.
The driving method of the LCD 30 provides scan voltages to the scan lines 32 to open the switching devices 38, and data voltages are provided to the data lines 34 and transferred to the pixel electrodes 30 through the switching devices 38. When scan voltages are provided to the scan lines 32 to open the switching devices 38, data voltages on the data lines 34 will charge the pixel electrodes 39 through the switch devices 38, and twist the liquid crystal molecules. When scan voltages on the scan lines 32 are removed to close the switching devices 38, the electrical connections between the data lines 34 and the pixel 36 will be cut and the pixel electrodes 39 will remain charged. The scan lines 32 control the switching devices 38 to repeatedly open and close, and thus the pixel electrodes 39 can be repeatedly charged. Different data voltages will cause different twisting angles and show different transmission rates. Hence, the LCD 30 displays different images.
Please refer to Fig. 4, which is a block diagram of the first embodiment. A driving circuit 40 is utilized for driving the LCD 30 in Fig.3. The driving circuit 40 comprises an image data input terminal 41, a bit processor 42, an image memory 43, a comparison circuit 44, a lookup table (LUT) 45, a multiplexer 46, a data line driving circuit 47, a memory 48, a table selector 49, and a temperature detector 51. In this embodiment, the image memory 43 is a 16-bit (5,6,5 or 5,5,5) memory having the necessary circuitry to read/write the memory cells. The image data input terminal 41 transfers 3 image data (RGB) to the bit processor 42, and each image data is 8 bits for controlling the gray levels of the pixel 30. Each color has 256 (2⁸) gray levels, so the 3 image data need 24 bits (8X3) to determine a RGB image. For using the 16-bit image memory 43 in this embodiment, the bit processor 42 is used to extract most significant bits (MSB) of the 3 RGB image data. For example, extracting 5 MSB of the R image data, 6 MSB of the G image data, and 5 MSB of the B image data, and storing the extracted data in the image memory 43. It is of course possible that 5 or other quantities MSB can be extracted from 3 RGB image data as long as the total extracted bits are not more than 16.
In this embodiment, one of the 3 RGB image data is representative to explain the present invention. The image data input terminal 41 transfers an 8-bit image data D8 to the bit processor 42. The bit processor 42 processes the 8-bit image data D8 and outputs a 6-bit second extracted image data D6 and a current 8-bit image data D8. The second extracted image data D6 is the 6 MSB extracted from the current 8-bit image data D8 by the bit processor 42, and the second extracted image data D6 is stored in the image memory 43 to delay a frame period. After delayed a frame period, the second extracted image data D6 is outputted as a first extracted image data D6' . In Fig. 4, the first extracted image data D6' and the second extracted image data D6 received by the comparison circuit 44 belong to different frame cycles as they differ one frame period.
The bit processor 42 transfers the second extracted image data D6 to the comparison circuit 44 and transfers the current 8-bit image data D8 to the multiplexer 46. The image memory 43 transfers the first extracted image data D6' to the comparison circuit 44. The first extracted image data D6' and the second extracted image data D6 are compared in the comparison circuit 44. A result value of 0 or 1 is determined after comparing the first extracted image data D6' and the second extracted image data D6. The result value 0 means that the first extracted image data D6' and the second extracted image data D6 are the same, and the result value 1 means that they are different. Since the first extracted image data D6' and the second extracted image data D6 are extracted from two different 8-bit image data D8, the result value 0 means that the differences between these two 8-bit image data D8 is less than 4.
For example, if the values of the first extracted image data D6' and the second extracted image data D6 are both 2 (000010), the result value of the comparison circuit 44 is 0, and the two corresponding 8-bit image data D8 are 8~11 (00001000~00001011). When the result value is 0, the pixel 36 does not need the over-driving control. On the other hand, if the result value is 1, the difference between these two 8-bit image data D8 is at least 4 and the pixel 36 needs the over-driving control. For example, if the value of the first extracted image data D6' is 2 (000010) and the value of the second extracted image data D6 is 5 (000101), the two corresponding 8-bit image data D8 are 8~11 (00001000~00001011) and 20~23 (00010100~00010111). In this situation, the pixel 36 needs the over-driving control.
The lookup table 45 comprises a reference table, and the lookup table 45 is operated in accordance with the reference table. Please refer to Fig.5, which illustrates a reference table 50 of the lookup table 45 in Fig.4. The reference table 50 is recorded with (2⁶ x 2⁶) or (2⁵ x 2⁵) 8-bit image data values 52, and each image data value 52 corresponds to different first extracted image data D6' and second extracted image data D6. When the result value is 1, meaning the first extracted image data D6' and the second extracted image data D6 are different, the first extracted image data D6' and the second extracted image data D6 are transferred to the lookup table 45. Then the lookup table 45 selects a corresponding 8-bit image data value 52 from the reference table 50 as a first image value D8' according to the first extracted image data D6' and the second extracted image data D6, and transfers the first image value D8' to the multiplexer 46.
For example, when the value of the first extracted image data D6' is 2 (000010) and the value of the second extracted image data D6 is 3 (000011), the lookup table 45 selects 25 (00011001) from the reference table 50 as the first image value D8' , and transfers the first image value D8' to the multiplexer 46.
In addition, the result value of the comparison circuit 44 is transferred to the multiplexer 46 to control the operation of the multiplexer 46. If the result value is 0, the multiplexer 46 will output the current 8-bit image data D8. If the result value is 1, the multiplexer 46 will output the over-driving image data D8'. The output Dout of the multiplexer 46 is transferred to the data line driving circuit 47, and the data line driving circuit 47 produces a corresponding data voltage in accordance with the output Dout (D8 or D8') of the multiplexer 46. The data voltage is applied to the corresponding data line 34 to control the pixel 36.
For example, if the values of the first extracted image data D6' and the second extracted image data D6 are both 2 (000010) and the value of the current 8-bit image data D8 is 10 (00001010), the output Dout of the multiplexer 46 will be 10 (00001010) and the data line driving circuit 47 will produce a first data voltage corresponding to the output Dout. If the value of the first extracted image data D6' is 2 (000010) and the value of the second extracted image data D6 is 63 (111111) , the over-driving image data D8' outputted by the lookup table 45 will be 255 (11111111), the output Dout will be 255, and the data line driving circuit 47 will produce a second data voltage corresponding to the output Dout.
Fig. 6 shows a similar embodiment of the present invention. In this situation, the bit processor 42 extracts different MSBs of the 8-bit image data D8. For example, 5 and 6 MSBs of the 8-bit image data D8 are extracted to be the first extracted image data D5' and the second extracted image data D6 respectively. As with the previous embodiment, the comparison circuit 44 compares the first extracted image data D5' and the second extracted image data D6 and determines the result value. When comparing the first extracted image data D5' and the second extracted image data D6, the comparison circuit fills the least significant bits (LSB) of the first extracted image data D5' with 0 and compares the filled first extracted image data D5' with the second extracted image data D6. For example, if the first extracted image data D5' is 7 (00111) and the second extracted image data D6 is 10 (001010), the LSB of the first extracted image data D5' is filled with 0 so that the filled first extracted image data D5' becomes 14 (001110). Then, 14 (001110) is compared with 10 (001010). Again, if the result value is 0, the pixel 36 does not need the over-driving control. If the result value is 1, the pixel 36 needs the over-driving control.
In addition, when comparing the first extracted image data D5' and the second extracted image data D6, the comparison circuit 44 can delete the LSB of the second extracted image data D6 and compare the first extracted image data D5' with the modified second extracted image data D6. For example, if the first extracted image data D5' is 7 (00111) and the second extracted image data D6 is 10 (001010), the LSB of the second extracted image data D6 is deleted, and the modified second extracted image data D6 is 5 (00101). Then, 7 (00111) is compared with 5 (00101). Similarly, if the result value is 0, the pixel 36 does not need the over-driving control. If the result value is 1, the pixel 36 needs the over-driving control.
In this embodiment, the reference table used in the lookup table 45 is different. Please refer to Fig.7, which is a reference table 70 used for the lookup table 45 in this situation. The reference table 70 is recorded with (2⁵ x 2⁶) 8-bit image data values 72. When the result value is 1, meaning that the first extracted image data D5' and the second extracted image data D6 are different, the first extracted image data D5' and the second extracted image data D6 are transferred to the lookup table 45. Then the lookup table 45 selects a corresponding 8-bit image data value 72 from the reference table 70 as a first image value D8' according to the first extracted image data D5' and the second extracted image data D6, and transfers the first image value D8' to the multiplexer 46.
For saving power, the comparison.circuit 44 can further output a LUT enable signal to the lookup table 45. When the result value is 1, the LUT enable signal will turn on the lookup table 45. When the result value is 0, the LUT enable signal will turn off the lookup table 45.
In this embodiment, the bit processor 42 extracts N and P MSBs of the 8-bit image data D8 to form the first extracted image data and the second extracted image data. As described above, the combination of (N,P) is (6,6) or (5,6), and can be other suitable values such as (5, 5). Please refer to Fig. 8 and Fig.9. Fig.8 is a block diagram of an embodiment where (N,P) is (5,5), and Fig.9 is a reference table 90 used for the lookup table 45 in Fig.8. The operation where (N,P) is (5, 5) is similar to that where (N,P) is (6,6), and the only difference is whether 5 or 6 MSBs of the 8-bit image data D8 is extracted. When (N,P) is (5, 5), the first extracted image data D5' and the second extracted image data D5 are both 5-bit image data, and the reference table 90 is stored with (2⁵ x 2⁵) 8-bit image data 92. The lookup table 45 selects a corresponding 8-bit image data value 92 from the reference table 90 according to the first extracted image data D5' and the second extracted image data D5 to control the followed operation of the data line driving circuit 47.
In contrast to the prior art, the reference tables in the present invention are built by actually measuring the over-driving voltages needed for properly driving the liquid crystal panel in a frame period. The reference tables include all of the over-driving image data that drives the pixels from any gray level to another, so the processor used to expand the brief table is not needed, and the efficiency can be improved. Additionally, the driving circuit and the driving method of the present invention extract LSB or MSB of a general bit length, so the management of the image memory can be more convenient and efficient.

Claims

A driving method of a liquid crystal display (30), the liquid crystal display (30) comprising:
a liquid crystal panel (31), the liquid crystal panel (31) comprising:
a plurality of scan lines (32);

a plurality of data lines (34); and

a plurality of pixels (36), each pixel (36) is connected to a corresponding scan line (32) and a corresponding data line (34), and each pixel (36) has a switching device (38) connected to the corresponding scan line (32) and the corresponding data line (34);

the driving method comprises:
(a) continuously providing scan voltages to the scan lines (32);

(b) receiving an M-bit image data from an image data input terminal (41);

(c) extracting N most significant bits of the M-bit image data to form an N-bit image data, N being smaller than M;

(d) delaying the N-bit image data by a frame period to form an N-bit delayed image data;

(e) comparing P most significant bits of a current M-bit image data with the N-bit delayed image data to determine a result value;

(f) if the result value equals a first result value, selecting a first image value (52, 72, 92) from a reference table (50, 70, 90) used in a look up table (45) in accordance with the P MSB and the N-bit delayed image data and forming a first data voltage according to the first image value (52, 72, 92), and providing the first data voltage to the corresponding data line (34) ; and

(g) if the result value equals a second result value, forming a second data voltage in accordance with the current M-bit image data and providing the second data voltage to the corresponding data line (34);

characterized by:
(f1) if the result value equals the first result value, output a look up table enable signal to turn on the look up table (45); and

(g1) if the result value equals the second result value, output a look up table enable signal to turn off the look up table (45).
The driving method of claim 1 characterized in that the driving method further comprises:
(h) producing a temperature compensation signal in accordance with temperature of the liquid crystal panel (31); and

(i) selecting the reference table (50, 70, 90) to be used in the look up table (45) in step (f) from a plurality of tables (54) in accordance with the temperature compensation signal.
The driving method of claim 1 characterized in that the reference table (50, 70, 90) is recorded with 2^N x 2^P image values (52, 72, 92).
The driving method of claim 1 characterized in that P is greater than N.
The driving method of claim 1 characterized in that P equals N.
A driving circuit (40) for driving a liquid crystal display (30), the liquid crystal display (30) comprising:
a liquid crystal panel (31), the liquid crystal panel (31) comprising:
a plurality of scan lines (32);

a plurality, of data lines (34); and

a plurality of pixels (36), each pixel (36) is connected to a corresponding scan line (32) and a corresponding data line (34), and each pixel (36) has a switching device (38) connected to the corresponding scan line (32) and the corresponding data line (34);

the driving circuit (40) comprises:
a scan line driving circuit for continuously providing scan voltages to the scan lines (32);

an image data input terminal (41) for receiving an M-bit image data;

a bit processor (42) for extracting N most significant bits from the M-bit image data to form an N-bit image data, N is smaller than M;

an image memory (43) for storing the N-bit image data and delaying the N-bit image data by a frame period;

a comparison circuit (44) for comparing P most significant bits of a current M-bit image data with the N-bit delayed image data to determine a result value;

a lookup table (45) for outputting an image value (52, 72, 92) from a reference table (50, 70, 90) in accordance with the P MSB and the N-bit delayed image data;

a multiplexer (46) for outputting the image value (52, 72, 92) or outputting the M-bit image data in accordance with the result value; and

a data line driving circuit (47) for forming a data voltage in accordance with output of the multiplexer (46), and providing the data voltage to the corresponding data line (34),

characterized in that:
the comparison circuit (44) further outputs a lookup table enable signal to the lookup table (45) to turn on the lookup table (45) when the multiplexer (46) should output the image value (52, 72, 92) and to turn off the lookup table (45) when the multiplexer (46) should output the M-bit image data.
The driving circuit (40) of claim 6 characterized in that the driving circuit (40) further comprises:
a temperature detector (51) for detecting temperature of the liquid crystal panel (31), and producing a temperature compensation signal in accordance with temperature of the liquid crystal panel (31);

a memory (48) for storing a plurality of tables (54); and

a selector (49) for selecting a reference table (50, 70, 90) from the plurality of tables (54) stored in the memory (48) in accordance with the temperature compensation signal, and transferring the selected reference table (50, 70, 90) to the look up table (45) to make the look up table (45) output the image value (52, 72, 92) in accordance with the selected reference table (50, 70, 90).
The driving circuit (40) of claim 6 characterized in that the reference table (50, 70, 90) is recorded with 2^N x 2^P image values (52, 72, 92).
The driving circuit (40) of claim 6 characterized in that P is greater than N.
The driving circuit (40) of claim 6 characterized in that P equals N.