# Patent

## Title
Liquid crystal display

## Abstract

A liquid crystal display (LCD) has a pixel matrix, a plurality of shift registers, a plurality of common voltage generators, a plurality of common voltage buffers, and a plurality of primary bidirectional switch circuits. The shift registers sequentially output gate signals to scan lines of the pixel matrix. The common voltage generators output initial common voltages according to the gate signals. The common voltage buffers are configured to buffer the initial common voltages to output a plurality of common voltages to a plurality of common voltage lines of the pixel matrix. Each of the primary bidirectional switch circuits is configured to control electrical connection between two of the common voltage lines according to one or more gate signals outputted from at least one of the shift registers.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a liquid crystal display, and more particularly to a liquid crystal display capable of sharing electric charge of common voltage lines thereof.
2. Description of the Prior Art
Liquid crystal displays (LCDs) are the most popular displays nowadays. Due to the properties of lightweight, low energy consumption, and free of radiation emission, LCDs have gradually replaced the cathode ray tube (CRT) monitors of conventional personal computers and have been widely-used in many portable information products, such as notebooks, personal digital assistants (PDAs), etc.
Due to the vigorous development of smart phones, customer demand for small-sized display panels with a narrow bezel and high resolution design is increasing. However, high resolution results in a greater load of the common voltage circuits of the display panel, such that it is required to increase the size of common voltage buffers of the display panel to improve the current driving capability thereof for feeding a greater load. Since large-sized common voltage buffers require a greater layout area, it is difficult to achieve a narrow bezel design of the display panel.
SUMMARY OF THE INVENTION
An embodiment of the present invention provides a liquid crystal display (LCD). The LCD comprises a pixel matrix, a plurality of shift registers, a plurality of common voltage generators and a plurality of primary bidirectional switch circuits. The pixel matrix comprises a plurality of pixels, a plurality of scan lines and a plurality of common voltage lines. The pixels are arranged in a plurality of rows. Each of the scan lines is coupled to pixels arranged in one of the rows. Each of the common voltage lines is coupled to the pixels arranged in one of the rows. The shift registers are coupled to the scan lines and configured to sequentially output gate signals to the scan lines. The common voltage generators are coupled between the shift registers and the common voltage lines and configured to output initial common voltages according to the gate signals. The primary bidirectional switch circuits are coupled to the shift registers and the common voltage lines. Each of the primary bidirectional switch circuits is configured to control electrical connection between two of the common voltage lines according to at least one of the gate signals output from the shift registers.
According to the embodiments of the present invention, with the help of primary bidirectional switch circuits, the LCD may control electrical connections of the common voltage lines according to timing of polarity inversion of each row of pixel. Accordingly, electric charge of each common voltage line may be shared to other common voltage lines, and an equivalent capacitance of pixels driven by the common voltage buffers may be not too great. Since the equivalent capacitance of pixels driven by the common voltage buffers may be not too great, the layout area of the common voltage buffers may be reduced to contribute to the achievement of a narrow bezel design of the display panel.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a liquid crystal display according to an embodiment of the present invention.
FIG. 2 is a schematic diagram of a pixel matrix in FIG. 1.
FIG. 3 is a circuit diagram of a pixel in FIG. 2.
FIG. 4 is a schematic diagram of the pixel matrix and a gate driver in FIG. 1.
FIG. 5 is a timing diagram of the liquid crystal display in FIG. 1.
FIG. 6 is a circuit diagram of a primary bidirectional switch circuit in FIG. 4.
FIG. 7 is a timing diagram of the primary bidirectional switch circuit in FIG. 6.
FIG. 8 is a circuit diagram of a common voltage generator in FIG. 4.
FIG. 9 is a circuit diagram of an inverting circuit in FIG. 8.
FIG. 10 is a schematic diagram of a liquid crystal display according to another embodiment of the present invention.
FIG. 11 is a schematic diagram of a pixel matrix and a first gate driver in FIG. 10.
FIG. 12 is a schematic diagram of the pixel matrix and a second gate driver in FIG. 10.
FIG. 13 is a schematic diagram of a liquid crystal display having a first gate driver and a second gate driver of the present invention.
FIGS. 14 and 15 are schematic diagrams of the pixel matrix, the first gate driver and the second gate driver in FIG. 13 according to another embodiment of the present invention.
FIGS. 16 and 17 are schematic diagrams of the pixel matrix, the first gate driver and the second gate driver in FIG. 13 according to another embodiment of the present invention.
FIG. 18 is a circuit diagram of a primary bidirectional switch circuit in FIGS. 16 and 17.
FIG. 19 is a timing diagram of the primary bidirectional switch in FIG. 18.
FIGS. 20 and 21 are schematic diagrams of the pixel matrix, the first gate driver and the second gate driver in FIG. 13 according to another embodiment of the present invention.
FIG. 22 is a circuit diagram of a secondary bidirectional switch circuit in FIGS. 20 and 21.



DETAILED DESCRIPTION
Please refer to FIGS. 1 to 3. FIG. 1 is a schematic diagram of a liquid crystal display 100 according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a pixel matrix 110 in FIG. 1. FIG. 3 is a circuit diagram of a pixel 112 in FIG. 2. The liquid crystal display 100 comprises the pixel matrix 110, a gate driver 120 and a source driver 130. The pixel matrix 110 comprises a plurality of pixels 112, a plurality of scan lines G1 to GN, a plurality of common voltage lines C1 to CN and a plurality of data lines D1 and DM. The pixels 112 are arranged in N rows and M columns, where M and N are positive integers. Each of the scan lines G1 to GN is coupled to the pixels 112 arranged in one of the rows, and each of the common voltage lines C1 to CN is coupled to the pixels 112 arranged in one of the rows. Each of the pixels 112 has a switch SW, a storage capacitor Cst and a liquid crystal capacitor Clc. The switch SW may be a thin film transistor (TFT). Each of the pixels 112 is coupled to a data line Dx, a scan line Gy and a common voltage line Cy, where x and y are positive integers, 1≦x≦M, and 1≦y≦N. The switch SW is turned on/off based on a voltage level of the scan line Gy. When the switch SW is turned on, the data line Dx charges the storage capacitor Cst and the liquid crystal capacitor Clc of the pixel 112 through the switch SW. A voltage level of the common voltage line Cy is switched once between a high voltage level and a low voltage level within a frame period. It should be noted that the circuit structure of the pixel 112 in FIG. 3 is merely an example used in the present invention, and the present invention is not limited thereto. In other words, the pixel 112 may have different circuit structures in other embodiments of the present invention.
Please refer to FIG. 4. FIG. 4 is a schematic diagram of the pixel matrix 110 and the gate driver 120 in FIG. 1. The gate driver 120 has a plurality of shift registers SRD1, SRD2 and SR1 to SRN, a plurality of common voltage generators A1 to AN, AD3 and AD4 and a plurality of primary bidirectional switch circuits E1 to EN. The shift registers SRD1, SRD2 and SR1 to SRN are coupled to the scan lines GD1, GD2 and G1 to GN and are configured to sequentially output gate signals VGD1, VGD2 and VG1 to VGN to the scan lines GD1, GD2 and G1 to GN. The first one shift register SRD1 and the second one shift register SRD2 are dummy shift registers, and the scan lines GD1 and GD2 are dummy scan lines and not directly coupled to any of the pixels 112. The common voltage generators A1 to AN, AD3 and AD4 are coupled between the shift registers SRD1, SRD2 and SR1 to SRN and the common voltage lines C1 to CN, CD3 and CD4. The common voltage generators A1 to AN, AD3 and AD4 are configured to output initial common voltages V1 to VN, VD3 and VD4 according to the gate signals VGD1, VGD2 and VG1 to VGN. The primary bidirectional switch circuits E1 to EN are coupled to the shift registers SRD1, SRD2 and SR1 to SRN and the common voltage lines C1 to CN, CD3 and CD4. The common voltage generators AD3 and AD4 are dummy common voltage generators, and the common voltage lines CD3 and CD4 are dummy common voltage lines.
In an embodiment of the present invention, the output ends of the common voltage generators AD3 and AD4 are electrically coupled to the common voltage lines C1 to CN, CD3 and CD4 so as to directly apply the initial common voltages V1 to VN, VD3 and VD4 to the common voltage lines C1 to CN, CD3 and CD4. In an embodiment of the present invention, the gate driver 120 further comprises a plurality of common voltage buffers B1 to BN, BD3 and BD4 coupled between the common voltage generators A1 to AN, AD3 and AD4 and the common voltage lines C1 to CN, CD3 and CD4. The common voltage buffers B1 to BN, BD3 and BD4 are configured to buffer the initial common voltages V1 to VN, VD3 and VD4 so as to output a plurality of common voltages VC1 to VCN, VCD3 and VCD4 to the common voltage lines C1 to CN, CD3 and CD4. The common voltage buffers BD3 and BD4 are dummy common voltage buffers.
In an embodiment of the present invention, the LCD 100 changes the polarities of the pixels 112 with row inversion. Please refer FIG. 5 with reference of FIG. 4. FIG. 5 is a timing diagram of the liquid crystal display 100 in FIG. 1. Within an Sth frame period, odd-numbered common voltages (e.g. VC1, VC3, VCD3) are pulled down from a high voltage level to a low voltage level, and even-numbered common voltages (e.g. VC2, VC4, VCD4) are pulled up from the low voltage level to the high voltage level. Within an S+1th frame period, odd-numbered common voltages (e.g. VC1, VC3, VCD3) are pulled up from the low voltage level to the high voltage level, and even-numbered common voltages (e.g. VC2, VC4, VCD4) are pulled down from the high voltage level to the low voltage level. The parameter S is a positive integer. Moreover, within each frame period of the LCD 100, the gate signals VGD1, VGD2 and VG1 to VGN are sequentially pulled up from the low voltage level to the high voltage level. Moreover, the gate driver 120 of the LCD 100 generates the common voltages VC1 to VCN, VCD3 and VCD4 according to the voltage levels of a clock signal FR and the gate signals VGD1, VGD2 and VG1 to VGN. The voltage level of each of the common voltages VC1 to VCN is switched two scan periods before the corresponding one of the gate signals VGD1 to VGN is pulled up from the low voltage level to the high voltage level. For example, two scan periods before the gate signal VG1 is pulled up from the low voltage level to the high voltage level (i.e. when the gate signal VGD1 is pulled up from the low voltage level to the high voltage level), the voltage level the common voltage VC1 is switched. Two scan periods before the gate signal VG2 is pulled up from the low voltage level to the high voltage level (i.e. when the gate signal VGD2 is pulled up from the low voltage level to the high voltage level), the voltage level the common voltage VC2 is switched. Two scan periods before the gate signal VG3 is pulled up from the low voltage level to the high voltage level (i.e. when the gate signal VG1 is pulled up from the low voltage level to the high voltage level), the voltage level the common voltage VC3 is switched. The rest may be deduced by analogy. Moreover, the voltage level the common voltage VCD3 is switched when the gate signal VGN−1 is pulled up from the low voltage level to the high voltage level. The voltage level the common voltage VCD4 is switched when the gate signal VGN is pulled up from the low voltage level to the high voltage level.
Please refer to FIG. 4 again. Each of the primary bidirectional switch circuits E1 to EN is configured to control electrical connection between two of the common voltage lines C1 to CN, CD3 and CD4 according to two of the gate signals output from two of the shift registers SRD1, SRD2 and SR1 to SRN. For example, in an embodiment of the present invention, the primary bidirectional switch circuit E1 is configured to control electrical connection between the common voltage lines C1 and C2 according to the gate signals VGD1 and VGD2 output from the shift registers SRD1 and SRD2. The primary bidirectional switch circuit E2 is configured to control electrical connection between the common voltage lines C2 and C3 according to the gate signals VGD2 and VG1 output from the shift registers SRD2 and SR1. The primary bidirectional switch circuit EN−1 is configured to control electrical connection between the common voltage lines CN−1 and CD3 according to the two gate signals which two of shift registers SRD1, SRD2, SR1 to SRN apply to the gate lines GN−2 and GN−3. The primary bidirectional switch circuit EN is configured to control electrical connection between the common voltage lines CN and CD4 according to the two gate signals which two of shift registers SRD1, SRD2, SR1 to SRN apply to the gate lines GN−1 and GN−2. Accordingly, electric charge may be shared among the common voltage lines C1 to CN, CD3 and CD4 via the primary bidirectional switch circuits E1 to EN.
Please refer to FIG. 6 and FIG. 4. FIG. 6 is a circuit diagram of a primary bidirectional switch circuit ET in FIG. 4, where T is a positive integer, and 1≦T≦N. The primary bidirectional switch circuit ET comprises a NOR gate 810, an inverter 820, a first switch 830 and a second switch 840. The NOR gate 810 has two input ends for receiving the two gate signals VGT−2 and VGT−1 output from the two shift registers SRT−2 and SRT−1. The NOR gate 810 is configured to perform a logic NOR operation on the two gate signals VGT−2 and VGT−1 so as to output a signal SWT. Regarding the primary bidirectional switch circuit E1 (i.e. T=1), the two shift registers SRT−2 and SRT−1 are SRD1 and SRD2, and the two gate signals VGT−2 and VGT−1 received by the NOR gate 810 are VGD1 and VGD2. Regarding the primary bidirectional switch circuit E2 (i.e. T=2), the two shift registers SRT−2 and SRT−1 are SR2 and SR1, and the two gate signals VGT−2 and VGT−1 received by the NOR gate 810 are VGD2 and VG1. Moreover, an input end of the inverter 820 is coupled to the output end of the NOR gate 810. A first end of the first switch 830 is coupled to a common voltage line CT, a second end of the first switch 830 is coupled to a common voltage line CT+2, and a control end of the first switch 830 is coupled to the output end of the inverter 820. A first end of the second switch 840 is coupled to the first end of the first switch 830 and the common voltage line CT, a second end of the second switch 840 is coupled to the second end of the first switch 830 and the common voltage line CT+2, and a control end of the second switch 840 is coupled to the output end of the NOR gate 810. Therefore, when one of the gate signals VGT−2 and VGT−1 is at the high voltage level, the first switch 830 and the second switch 840 are turned off, and the common voltage lines CT and CT+2 are electrically disconnected. When the gate signals VGT−2 and VGT−1 are at the low voltage level, the first switch 830 and the second switch 840 are turned on, and the common voltage lines CT and CT+2 are electrically connected. In other words, the Tth primary bidirectional switch circuit ET controls the electrical connection between the Tth common voltage line CT and the T+2th common voltage line CT+2 of the common voltage lines C1 to CN, CD3 and CD4 according to the two gate signals VGT−2 and VGT−1 output from the Tth shift register SRT−2 and the T+1th shift register SRT−1 of the shift registers SRD1, SRD2 and SR1 to SRN. Wherein, the first one of the shift registers is SRD1, the second one of the shift registers is SRD2, the third one of the shift registers is SR1, the fourth one of the shift registers is SR2, and so on. Therefore, the common voltage lines CT and CT+2 share electric charge through the primary bidirectional switch circuit ET. For example, the common voltage lines C1 and C3 share electric charge through the primary bidirectional switch circuit E1. The common voltage lines C2 and C4 share electric charge through the primary bidirectional switch circuit E2. Moreover, regarding the primary bidirectional switch circuit EN−1 (i.e. T=N−1), the two common voltage lines CT and CT+2 are the common voltage lines CN−1 and CD3. Regarding the primary bidirectional switch circuit EN (i.e. T=N), the two common voltage lines CT and CT+2 are the common voltage lines CN and CD4. Further, an equivalent capacitance of the pixels 112 driven by the common voltage lines CT and CT+2 when the common voltage lines CT and CT+2 are electrically connected is less than that when the common voltage lines CT and CT+2 are electrically disconnected. The primary bidirectional switch circuit ET may be any one of the primary bidirectional switch circuits E1 to EN, and the common voltage lines C1 to CN are driven by the common voltage buffers B1 to BN. Accordingly, due to the primary bidirectional switch circuits E1 to EN, an equivalent capacitance of the pixels 112 driven by the common voltage buffers B1 to BN in FIG. 4 may be not too great. Therefore, the layout area of the common voltage buffers B1 to BN may be reduced to contribute to the achievement of the narrow bezel design of the display panel.
Please refer to FIG. 7 and FIG. 6. FIG. 7 is a timing diagram of the primary bidirectional switch ET in FIG. 6. The voltage levels of the gate lines VGT−4 to VGT are sequentially at the high voltage level within the durations TA to TE, and the common voltages VCT−2, VCT and VCT+2 are pulled up from the low voltage level to the high voltage level respectively while the gates signals VGT−4, VGT−2 and VGT are pulled up to the high voltage level. Within the durations TC and TD, the common voltages VCT and VCT+2 are at different voltage levels, such that it is not proper to share electric charge between the common voltage lines CT and CT+2. Therefore, the primary bidirectional switch ET in FIG. 6 should electrically disconnect the common voltage line CT from the common voltage line CT+2 within the durations TC and TD. As shown in FIGS. 6 and 7, since the ate lines VGT−2 to VGT−1 are not both at the low voltage level within the durations TC and TD, the signal SWT is at the low voltage level, such that the common voltage lines CT and CT+2 are electrically disconnected within the durations TC and TD. Accordingly, when the common voltages VCT and VCT+2 are at different voltage levels, the sharing of electric charge between the common voltage lines CT and CT+2 is paused. In the same way, the signal SWT−2 is at the low voltage level within the durations TA and TB, such that the common voltage lines CT−2 and CT are electrically disconnected within the durations TA and TB. Therefore, when the common voltages VCT−2 and VCT are at different voltage levels, the sharing of electric charge between the common voltage lines CT−2 and CT is paused.
Please refer to FIGS. 8 and 9 with reference of FIG. 4. FIG. 8 is a circuit diagram of a common voltage generator AT in FIG. 4, and FIG. 9 is a circuit diagram of an inverting circuit 606 in FIG. 8, where T is a positive integer, and 1≦T≦N. The common voltage generator AT has two inverters 602 and 604 and two inverting circuits 606. The inverter 602 is configured to receive the gate signal VGT−2 output from the Tth shift register SRT−2, and the input end of the inverter 604 is coupled to the output ends of the two inverting circuits 606. In an embodiment of the present invention, each of the inverting circuits 606 may comprise two P-type metal-oxide-semiconductor field effect transistors (PMOSFETs) P1 and P2 and two N-type metal-oxide-semiconductor field effect transistors (NMOSFETs) N1 and N2. The source of the PMOSFET P1 is coupled to a gate-high voltage level VGH, the gate of the PMOSFET P1 is coupled to a first control end cp of the inverting circuit 606, and the drain of the PMOSFET P1 is coupled to the source of the PMOS P2. The gate of the PMOSFET P2 and the gate of the NMOSFET N1 are coupled to an input end SIN of the inverting circuit 606, and the drain of the PMOSFET P2 and the drain of the NMOSFET N1 are coupled to an output end SOUT of the inverting circuit 606. The drain of the NMOSFET N2 is coupled to the source of the NMOSFET N1, the gate of the NMOSFET N2 is coupled to a second control end cn of the inverting circuit 606, and the source of the NMOSFET N2 is coupled to a gate-low voltage level VGL. Therefore, the common voltage generator AT may latch the gate signal VGT−2 according to the clock signal FR so as to output the initial common voltage VT.
In the above embodiments, the LCD 100 uses the single gate driver 120 to perform single-sided scanning operations. However, the present invention may be also adopted in an LCD that uses two gate drivers to perform double-sided scanning operations. Please refer to FIGS. 10 to 12. FIG. 10 is a schematic diagram of a liquid crystal display 1000 according to another embodiment of the present invention. FIG. 11 is a schematic diagram of a pixel matrix 110 and a first gate driver 1020 of the LCD 1000 in FIG. 10. FIG. 12 is a schematic diagram of the pixel matrix 110 and a second gate driver 1030 of the LCD 1000 in FIG. 10. The LCD 1000 comprises the pixel matrix 110, a first gate driver 1020, a second gate driver 1030 and the source driver 130. The first gate driver 1020 and the second gate driver 1030 are positioned at two opposite sides of the LCD 1000. The functions of the pixel matrix 110 and the source driver 130 has explained in the previous descriptions, and the circuit structure of the first gate driver 1020 is completely the same as that of the gate driver 120, and will thus not be repeated herein. Moreover, the circuit structure of the second gate driver 1030 is completely symmetrical with that of the first gate driver 1020, and the components of the second gate driver 1030 has the same functions as the components of the first gate driver 1020, which are configured to generate and output the gate signals VG1 to VGN to the scan lines G1 to GN and are configured to output the common voltages VC1 to VCN, VCD3 and VCD4 to the common voltage lines C1 to CN, CD3 and CD4. Since each of the scan lines G1 to GN receives a corresponding one of the gate signals VG1 to VGN from the first fate driver 1020 and the second gate driver 1030 positioned at the two opposite sides of the LCD 1000, and each of the common voltage lines C1 to CN receives one of the common voltages VC1 to VCN from the first fate driver 1020 and the second gate driver 1030, the image quality at the rims of the LCD 1000 is better than that of the LCD 100.
The LCD 100 uses a single gate driver to perform single-sided scanning operations, and the LCD 1000 uses two gate drivers to perform double-sided scanning operations. However, the present invention may be also adopted in an LCD that uses two gate drivers to perform single-sided scanning operations. Please refer to FIGS. 13 to 15. FIG. 13 is a schematic diagram of a liquid crystal display 1300 having a first gate driver 1320 and a second gate driver 1330 of the present invention. FIGS. 14 and 15 are schematic diagrams of the pixel matrix 110, the first gate driver 1320 and the second gate driver 1330 in FIG. 13 according to another embodiment of the present invention. The LCD 1300 comprises the pixel matrix 110, the first gate driver 1320, the second gate driver 1330 and the source driver 130. The first gate driver 1320 and the second gate driver 1330 are positioned at two opposite sides of the LCD 1300. The functions of the pixel matrix 110 and the source driver 130 have explained in the previous descriptions, and will thus not be repeated herein. In the embodiment, the common voltage generators A1 to AN, AD3 and AD4, the common voltage buffers B1 to BN, BD3 and BD4 and the primary bidirectional switch circuits E1 to EN are divided into two parts, and each of the parts is integrated in one of the first gate driver 1320 and the second gate driver 1330 of the LCD 1300. In more detail, the odd-numbered common voltage generators A1, A3, . . . , AN−1 and AD3, the odd-numbered common voltage buffers B1, B3, . . . , BN−1 and BD3 and the odd-numbered common primary bidirectional switch circuits E1, E3, . . . and EN−1 are integrated in the first gate driver 1320. The even-numbered common voltage generators A2, A4, . . . , AN and AD4, the even-numbered common voltage buffers B2, B4, . . . , BN and BD4 and the even-numbered common primary bidirectional switch circuits E2, E4, . . . and EN are integrated in the second gate driver 1330. Therefore, the first gate driver 1320 transmits the odd-numbered common voltages VC1, VC3, . . . and VCN−1 to the pixel matrix 110 via the odd-numbered common voltage lines C1, C3, . . . and CN−1, and the second gate driver 1330 transmits the even-numbered common voltages VC2, VC4, . . . and VCN to the pixel matrix 110 via the even-numbered common voltage lines C2, C4, . . . and CN. Moreover, each of the first gate driver 1320 and the second gate driver 1330 has N+2 shift registers SRD1, SRD2 and SR1 to SRN that are configured to sequentially output the gate signals VGD1, VGD2 and VG1 to VGN to the scan lines GD1, GD2 and G1 to GN. The connections of the common voltage generators A1 to AN, AD3 and AD4, the common voltage buffers B1 to BN, BD3 and BD4, the primary bidirectional switch circuits E1 to EN, the scan lines GD1, GD2 and G1 to GN and the common voltage lines C1 to CN, CD3 and CD4 of the LCD 1300 are the same as those of the LCD 100, and will thus not be repeated herein.
In an embodiment of the present invention, the number of shift registers of the first gate driver 1320 in FIG. 14 and the second gate driver 1330 in FIG. 15 may be reduced. For example, the first gate driver 1320 in FIG. 14 may be replaced by a first gate driver 1320B in FIG. 16, and the second gate driver 1320 in FIG. 15 may be replaced by a second gate driver 1330B in FIG. 17. Moreover, the primary bidirectional switch circuits E1 to EN may be replaced by primary bidirectional switch circuits E′1 to E′N. Please refer to FIGS. 16 and 17. In the embodiment, the common voltage generators A1 to AN, AD3 and AD4, the common voltage buffers B1 to BN, BD3 and BD4 and the primary bidirectional switch circuits E′1 to E′N are divided into two parts, and each of the parts is integrated in one of the first gate driver 1320B and the second gate driver 1330B. In more detail, the odd-numbered common voltage generators A1, A3, . . . , AN−1 and AD3, the odd-numbered common voltage buffers B1, B3, . . . , BN−1 and BD3 and the odd-numbered common primary bidirectional switch circuits E′1, E′3, . . . and E′N−1 are integrated in the first gate driver 1320B. The even-numbered common voltage generators A2, A4, . . . , AN and AD4, the even-numbered common voltage buffers B2, B4, . . . , BN and BD4 and the even-numbered common primary bidirectional switch circuits E′2, E′4, . . . and E′N are integrated in the second gate driver 1330B.
Please refer to FIGS. 18 and 19 with reference of FIGS. 16 and 17. FIG. 18 is a circuit diagram of a primary bidirectional switch circuit E′T in FIGS. 16 and 17. T is a positive integer, and 1≦T≦N. FIG. 19 is a timing diagram of the primary bidirectional switch E′T in FIG. 18. The gate signal VGT−4 is at the high voltage level within the durations TA and TB, the gate signal VGT−3 is at the high voltage level within the durations TB and TC, the gate signal VGT−2 is at the high voltage level within the durations TC and TD, the gate signal VGT−1 is at the high voltage level within the durations TD and TE, and the gate signal VGT is at the high voltage level within the durations TE and TF. The primary bidirectional switch circuit E′T comprises an inverter 820, a first switch 830 and a second switch 840. The input end of the inverter 820 receives the gate signal VGT−2. A first end of the first switch 830 is coupled to the common voltage line CT, a second end of the first switch 830 is coupled to the common voltage line CT+2, and a control end of the first switch 830 receives the gate signal VGT−2. A first end of the second switch 840 is coupled to the first end of the first switch 830 and the common voltage line CT, a second end of the second switch 840 is coupled to the second end of the first switch 830 and the common voltage line CT+2, and a control end of the second switch 840 is coupled to an output end of the inverter 820. Therefore, when the gate signal VGT−2 is at the high voltage level, the first switch 830 and the second switch 840 are turned off, and the common voltage line CT is electrically disconnected from the common voltage line CT+2. When the gate signal VGT−2 is at the low voltage level, the first switch 830 and the second switch 840 are turned on, and the common voltage line CT is electrically connected to the common voltage line CT+2. In other words, the Tth primary bidirectional switch circuit E′T controls the electrical connection between the Tth common voltage line CT and the T+2th common voltage line CT+2 of the common voltage lines C1 to CN, CD3 and CD4 according to the gate signal VGT−2 output from the Tth shift register SRT−2 of the shift registers SRD1, SRD2 and SR1 to SRN. Therefore, the common voltage lines CT and CT+2 share electric charge through the primary bidirectional switch circuit E′T.
In an embodiment of the present invention, the first gate driver 1320B may be replaced by a first gate driver 1320C in FIG. 20, and the second gate driver 1320B may be replaced by a second gate driver 1330C in FIG. 21. As compared to the first gate driver 1320B and the second gate driver 1320B, the first gate driver 1320C and the second gate driver 1320C further comprise a plurality of secondary bidirectional switch circuits F1 to FN−1.
The even-numbered secondary bidirectional switch circuits F2, F4, . . . and FN−2 of the secondary bidirectional switch circuits F1 to FN−1 are integrated in the first gate driver 1320C, the odd-numbered secondary bidirectional switch circuits F1, F3, . . . and FN−1 of the secondary bidirectional switch circuits F1 to FN−1 are integrated in the second gate driver 1330C. The first gate driver 1320C and the second gate driver 1330C are positioned at two opposite sides of the liquid crystal display. The secondary bidirectional switch circuits F1 to FN−1 are coupled to the scan lines G1 to GN. Each of the secondary bidirectional switch circuits F1 to FN−1 controls the electric connection between two of the scan lines G1 to GN according to two of the gate signals VG1 to VGN. For example, the secondary bidirectional switch circuits F1 controls the electric connection between the scan lines G1 and G2 according to the gate signals VG1 and VG2; the secondary bidirectional switch circuits F2 controls the electric connection between the scan lines G2 and G3 according to the gate signals VG2 and VG3; the secondary bidirectional switch circuits F3 controls the electric connection between the scan lines G3 and G4 according to the gate signals VG3 and VG4; and so on.
Please refer to FIG. 22 with reference of FIGS. 20 and 21. FIG. 22 is a circuit diagram of a secondary bidirectional switch circuit FU in FIGS. 20 and 21. U is a positive integer, and 1≦U≦N−1. The secondary bidirectional switch circuit FU comprises an AND gate 1810, an inverter 1820, a first switch 1830 and a second switch 1840. The AND gate 1810 has two input ends for receiving the gate signals VGU and VGU+1 respectively from the shift registers SRU and SRU+1. The AND gate 1810 performs a logic AND operation on the two gate signals VGU and VGU+1. An input end of the inverter 1820 is coupled to an output end of the AND gate 1810. A first end of the first switch 1830 is coupled to the scan line GU, a second end of the first switch 1830 is coupled to the scan line GU+1, and a control end of the first switch 1830 is coupled to the output end of the inverter 1820. A first end of the second switch 1840 is coupled to the first end of the first switch 1830, a second end of the second switch 1840 is coupled to the second end of the first switch 1830 and the scan line GU+1, and a control end of the second switch 1840 is coupled to the output end of the AND gate 1810. Therefore, when the gate signals VGU and VGU+1 are at the high voltage level, the first switch 1830 and the second switch 1840 are turned on, such that the scan line GU is electrically connected to the scan line GU+1. When the gate signals VGU and VGU+1 are not both at the high voltage level, the first switch 1830 and the second switch 1840 are turned off, such that the scan line GU is electrically disconnected from the scan line GU+1. In other words, the Uth secondary bidirectional switch circuit FU of the secondary bidirectional switch circuits F1 to FN−1 controls the electric connection between the Uth scan line GU and the U+1th scan line GU+1 of the scan lines G1 to GN according to the gate signals VGU and VGU+1 output from the U+2th shift resister SRU and the U+3th shift resister SRU+1.
In the first gate driver 1320C, due to the even-numbered secondary bidirectional switch circuits F2, F4, . . . and FN−2, the gate signals VG1, VG3, . . . and VGN−1 generated by the first gate driver 1320C may compensate the gate signals VG2, VG4, . . . and VGN−2. Similarly, due to the odd-numbered secondary bidirectional switch circuits F1, F3, . . . and FN−1 of the second gate driver 1330C, the gate signals VG2, VG4, . . . and VGN generated by the second gate driver 1330C may compensate the gate signals VG1, VG3, . . . and VGN−1. Therefore, the signals at the ends of the scan lines G1 to GN−1 may be strengthened through the secondary bidirectional switch circuits F1 to FN−1, such that the image quality of the LCD may be ensured.
According to the embodiments of the present invention, with the help of primary bidirectional switch circuits, the LCD may control electrical connections of the common voltage lines according to timing of polarity inversion of each row of pixel. Accordingly, electric charge of each common voltage line may be shared to other common voltage lines, and an equivalent capacitance of pixels driven by the common voltage buffers may be not too great. Since the equivalent capacitance of pixels driven by the common voltage buffers may be not too great, the layout area of the common voltage buffers may be reduced to contribute to the achievement of the narrow bezel design of the display panel.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A liquid crystal display (LCD), comprising:
a pixel matrix, comprising:
a plurality of pixels, arranged in a plurality of rows;
a plurality of scan lines, each of the scan lines being coupled to pixels arranged in one of the rows; and
a plurality of common voltage lines, each of the common voltage lines being coupled to the pixels arranged in one of the rows;

a plurality of shift registers, coupled to the scan lines and configured to sequentially output gate signals to the scan lines;
a plurality of common voltage generators, coupled between the shift registers and the common voltage lines and configured to output initial common voltages according to the gate signals; and
a plurality of primary bidirectional switch circuits, coupled to the shift registers and the common voltage lines, wherein each of the primary bidirectional switch circuits is configured to control electrical connection between two of the common voltage lines according to at least one of the gate signals output from the shift registers;
wherein the pixels are arranged in N rows, the shift registers comprise N+2 first shift registers, and the primary bidirectional switch circuits comprise N first primary bidirectional switch circuits, wherein a Tth one of the first primary bidirectional switch circuits is configured to control electrical connection between a Tth one and T+2th one of the common voltage lines according to two gate signals output from a Tth one and T+1th one of the first shift registers, N is an integer greater than 1, T is an integer, and 1≦T≦N.

2. The liquid crystal display of claim 1, wherein a first one and a second one of the first shift registers are dummy first shift registers.

3. The liquid crystal display of claim 1, wherein odd-numbered primary bidirectional switch circuits of the primary bidirectional switch circuits are integrated in a first gate driver of the liquid crystal display, even-numbered primary bidirectional switch circuits of the primary bidirectional switch circuits are integrated in a second gate driver of the liquid crystal display, and the first gate driver and the second gate driver are positioned at two sides of the liquid crystal display.

4. The liquid crystal display of claim 1 further comprising a plurality of common voltage buffers, coupled between the common voltage generators and the common voltage lines, and configured to buffer the initial common voltages so as to output a plurality of common voltages to the common voltage lines.

5. The liquid crystal display of claim 1, wherein each of the primary bidirectional switch circuits is configured to control the electrical connection between two of the common voltage lines according a gate signal output from a single one of the shift registers.

6. The liquid crystal display of claim 1, wherein the shift registers further comprise N+2 second shift registers, and the primary bidirectional switch circuits further comprise N second primary bidirectional switch circuits, wherein a Tth one of the second primary bidirectional switch circuits is configured to control electrical connection between a Tth one and T+2th one of the common voltage lines according to two gate signals output from a Tth one and T+1th one of the second shift registers.

7. The liquid crystal display of claim 6, wherein a first one and a second one of the second shift registers are dummy first shift registers.

8. The liquid crystal display of claim 6, wherein the N+2 first shift registers and the N first primary bidirectional switch circuits are integrated in a first gate driver of the liquid crystal display, the N+2 second shift registers and the N second primary bidirectional switch circuits are integrated in a second gate driver of the liquid crystal display, and the first gate driver and the second gate driver are positioned at two sides of the liquid crystal display.

9. The liquid crystal display of claim 1, wherein each of the primary bidirectional switch circuits is configured to control the electrical connection between two of the common voltage lines according to two of the gate signals output from two of the shift registers.

10. The liquid crystal display of claim 9, wherein each of the primary bidirectional switch circuits comprises:
a NOR gate, having two input ends configured to receive the two gate signals output from the two shift registers;
an inverter, having an input end coupled to an output end of the NOR gate;
a first switch, a first end of the first switch being coupled to one of the two common voltage lines, a second end of the first switch being coupled to another of the two common voltage lines, and a control end of the first switch being coupled to an output end of the inverter; and
a second switch, a first end of the second switch being coupled to the first end of the first switch, a second end of the second switch being coupled to the second end of the first switch, and a control end of the second switch being coupled to the output end of the NOR gate.

11. The liquid crystal display of claim 9, wherein each of the primary bidirectional switch circuits comprises:
an inverter, having an input end for receiving the gate signal output from the single one of the shift registers;
a first switch, a first end of the first switch being coupled to one of the two common voltage lines, a second end of the first switch being coupled to another of the two common voltage lines, and a control end of the first switch receiving the gate signal output from the single one of the shift registers; and
a second switch, a first end of the second switch being coupled to the first end of the first switch, a second end of the second switch being coupled to the second end of the first switch, and a control end of the second switch being coupled to an output end of the inverter.

12. The liquid crystal display of claim 1 further comprising a plurality of secondary bidirectional switch circuits coupled to the scan lines, wherein each of the secondary bidirectional switch circuits is configured to control electrical connection between two of the scan lines according to two of the gate signals output from two neighboring shift registers of the shift registers.

13. The liquid crystal display of claim 12, wherein each of the secondary bidirectional switch circuits comprises:
an AND gate, having two input ends configured to receive the two gate signals output from the two neighboring shift registers;
an inverter, having an input end coupled to an output end of the AND gate;
a first switch, a first end of the first switch being coupled to one of the two scan lines, a second end of the first switch being coupled to another of the two scan lines, and a control end of the first switch being coupled to an output end of the inverter; and
a second switch, a first end of the second switch being coupled to the first end of the first switch, a second end of the second switch being coupled to the second end of the first switch, and a control end of the second switch being coupled to the output end of the AND gate.

14. The liquid crystal display of claim 12, wherein a number of the secondary bidirectional switch circuits is equal to N−1, wherein a Uth one of the second primary bidirectional switch circuits is configured to control electrical connection between a Uth one and U+1th one of the scan lines according to two gate signals output from a U+2th one and U+3th one of the shift registers, U is an integer, and 1≦U≦N−1.

15. The liquid crystal display of claim 14, wherein even-numbered secondary bidirectional switch circuits of the secondary bidirectional switch circuits are integrated in a first gate driver of the liquid crystal display, odd-numbered secondary bidirectional switch circuits of the secondary bidirectional switch circuits are integrated in a second gate driver of the liquid crystal display, and the first gate driver and the second gate driver are positioned at two sides of the liquid crystal display.