Dot-inversion data driver for liquid crystal display device

In a data driver 10A of a dot-inversion driving type, the outputs of voltage buffer amplifiers B1 to B12 are connected to respective data bus lines D1 to D12 of a LCD panel, short-circuiting switches S1, S3, S5, S7, S9 and S11 are connected between ones of every other adjacent data bus lines concerned with the same display color, and interconnecting lines on first and second rows are arranged in a staggered configuration. These short-circuiting switches are formed at one sides of every other data bus lines, and turned on by a control circuit 13 when the outputs of the voltage buffer amplifier are in a high impedance state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a data driver for a liquid crystal display device, comprising voltage buffer amplifiers each outputting an analog gradation voltage, applying the analog gradation voltages to data bus lines such that voltage polarities of adjacent data bus lines concerned with a same display color are inverse to each other, and more particularly, to a data driver for driving the data bus lines of a liquid crystal display device in a dot inversion fashion regarding time and space.

2. Description of the Related Art

FIG. 8 shows the output stage of a prior art data driver 10 X connected to the data bus lines of a liquid crystal display (LCD) panel.

The voltage buffer amplifiers B 1 to B 12 of the data driver 10 X are respective voltage followers, and the outputs thereof are connected to the respective data bus lines D 1 to D 12 of the LCD panel. The data driver 10 X drives the data bus lines in a dot inversion fashion regarding time and space. That is, voltages applied to adjacent data bus lines at the same time have inverse polarities to each other, and analog gradation voltages corresponding to display data are outputted from the respective voltage buffer amplifiers B 1 to B 12 such that voltage polarity of each data bus line is inverted every horizontal period. According to the dot-inversion driving technique, potential variations of a pixel electrode caused by cross capacitance between a data bus line and a scan bus line can be effectively canceled and further, a common potential of the opposite electrode can be stabilized, resulting in reducing a flicker.

However, charge and discharge currents of each of the voltage buffer amplifiers B 1 to B 12 are relatively large, leading to higher power consumption.

Facing such a disadvantage, in order to effectively utilize electric charge accumulated on the data bus lines and decrease power consumption, short-circuiting switches S 1 to S 12 are connected between a common line CL and the respective data bus lines D 1 to D 12 . When the outputs of the voltage buffer amplifiers B 1 to B 12 are rendered to be in a high impedance state during a horizontal blanking period, the short-circuiting switches S 1 to S 12 are simultaneously turned on. Thereby, potentials of the data bus lines D 1 to D 12 are rendered to be nearly equal to the common potential of the opposite plane electrode of the liquid crystal display panel, enabling a current to be consumed in the voltage buffer amplifiers B 1 to B 12 to reduce up to a half.

However, since a necessity arises that the short-circuiting switches are provided to the respective voltage buffer amplifiers, an occupied area of the data driver 10 X increases, thereby disturbing higher density of data bus lines in arrangement.

FIG. 9 shows a data driver 10 Y of a dot inversion driving type disclosed in JP 10-282940 A.

In this circuit, short-circuiting switches S 1 to S 9 are connected between every other adjacent data bus lines. With this circuit, since the number of the short-circuiting switches is reduced to a half that of FIG. 8 , the above described problem can be solved.

However, since different color signals are provided onto adjacent bus lines, there is no correlation therebetween and an efficiency of utilization of electric charge accumulated on the data bus lines is not so satisfactory. For example, potentials of the data bus lines D 1 to D 6 are distributed in a horizontal period as shown in FIG. 10 , and when the short-circuiting switches S 1 , S 3 and S 5 turns on in the next horizontal blanking period, the potentials are distributed as shown in FIG. 11 to produce differences between each potential of the data bus lines and the common potential VCOM of the opposite electrode, which increases power consumption of the data driver 10 Y compared with the case of FIG. 8 . Further, the differences become a cause for variations in the common potential VCOM, resulting in generation of a flicker.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a data driver for a liquid crystal display device, capable of not only suppressing increase in circuit area but also reducing power consumption together with alleviating a flicker.

In a first aspect of a data driver for a liquid crystal display device according to the present invention, short-circuiting switches are intermittently connected between adjacent data bus lines concerned with a same display color, and the short-circuiting switches are turned on when the outputs of the voltage buffer amplifiers or locations between the voltage buffer amplifiers and the respective data bus lines are in a high impedance state.

Pixel data signals in the adjacent same color have inverse polarities, and it is a high probability that absolute values thereof are nearly equal. Particularly, this probability is higher in a region of a background image. Hence, with this data driver for a liquid crystal display device, by turning on the short-circuiting switches, the potentials of the data bus lines become nearly equal to a common potential of the opposite electrode of a LCD panel, whereby a current to be consumed in the voltage buffer amplifiers can be reduced more than in a case where short-circuiting switches are intermittently connected between adjacent data bus lines.

Further, since the common potential is stabilized, a flicker is alleviated, and thereby an image quality is improved compared with a case where short-circuiting switches are intermittently connected between adjacent data bus lines.

In addition, since the number of the short-circuiting switches is smaller than a case where a short-circuiting switch is connected between each adjacent data bus lines, the circuit area of the data driver can be reduced.

In a second aspect of a data driver for a liquid crystal display device according to the present invention, the short-circuiting switches are connected through interconnecting lines arranged in first and second rows in a staggered configuration in the above described first aspect.

With this data driver for a liquid crystal display device, since the short-circuiting switches and the interconnecting lines for them are arranged such that the densities thereof are nearly uniform, the circuit area of the data driver can be narrower, and the higher density of the data bus lines can be realized.

In a third aspect of a data driver for a liquid crystal display device according to the present invention, the short-circuiting switches are formed at one side of every other data bus lines in the above described second aspect.

With this configuration, the above-described effect is further enhanced.

Other aspects, objects, and the advantages of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout several views, preferred embodiments of the present invention are described below.

First Embodiment

FIG. 1 schematically shows a liquid crystal display device of a first embodiment according to the present invention. In FIG. 1 , there is shown a LCD panel 11 having a pixel matrix in 4 rows and 6 columns for simplification.

In the LCD panel 11 , a pair of opposed glass substrates, not shown, are disposed, and a gap therebetween is filled with a liquid crystal and sealed. Pixel electrodes are arranged in a matrix on one of the glass substrates, thin film transistors are formed for the respective pixels, scan bus lines (gate lines) G 1 to G 4 are formed for respective first to fourth rows of the thin film transistors, and data bus lines D 1 to D 6 are formed for first to sixth columns of the thin film transistors, wherein the scan bus lines G 1 to G 4 and the data bus lines D 1 to D 6 cross each other with an insulating film interposing therebetween. On the other glass substrate, a transparent plane electrode in common with all the pixels is formed and a common potential VCOM is applied thereto. For example, in regard to a liquid crystal pixel C 11 of the first row and the first column, a thin film transistor T 11 is connected between the pixel electrode and the data bus line D 1 , the gate of the thin film transistor T 11 is connected to the scan bus line G 1 , and the common potential VCOM is applied to the opposite electrode of the liquid crystal pixel C 11 .

The data bus lines D 1 to D 6 of the LCD panel 11 are connected to the outputs of the data driver 10 and the scan lines G 1 to G 4 of the LCD panel 11 are connected to the outputs of a scan driver 12 .

A control circuit 13 receives a video signal VS, a pixel clock CLK, a horizontal sync signal HSYNC, and a vertical sync signal VSYNC, and generates timing signals to provide to the data driver 10 and the scan driver 12 , and provides a video signal to the data driver 10 .

The scan bus lines G 1 to G 4 are line-sequentially activated by the scan driver 12 , while signal charges for pixels on a selected row are renewed by the data driver 10 . The data driver 10 simultaneously provides display data signals of a row onto the data bus lines D 1 to D 6 , and renews the signals in each horizontal period.

The data driver 10 drives in a dot inversion fashion. That is, the data driver 10 provides analog gradation voltages according to display data such that voltage polarities of adjacent data bus lines are inverse to each other and a voltage polarity of each data bus line is inverted every horizontal period. FIGS. 2 (A) and 2 (B) show pixel voltage polarity distributions of odd and even frames, respectively.

FIG. 3 shows the output stage of the data driver 10 . The number of data bus lines is actually, for example, 1024 3 3072, and FIG. 3 shows only data bus lines D 1 to D 12 as part thereof.

The data bus lines D 1 to D 12 on the LCD panel 11 are respectively connected to outputs of voltage buffer amplifiers B 1 to B 12 of the data driver 10 , and each voltage buffer amplifier is constituted of a voltage follower. Data bus lines of each of red (R), green (G), and blue (B) color signals are arranged every three lines.

Short-circuiting switches are connected between ones of every other adjacent data bus lines concerned with the same display color. That is, the short-circuiting switch S 1 is connected between adjacent R data bus lines D 1 and D 4 , no short-circuiting switch is connected between the next adjacent R data bus lines D 4 and D 7 , and a short-circuiting switch S 7 is connected between the still next adjacent R data bus lines D 7 and D 10 . Likewise, a short-circuiting switch S 2 is connected between adjacent G data bus lines D 2 and D 5 , and a short-circuiting switch S 8 is connected between adjacent G data bus lines D 8 and D 11 . Further, a short-circuiting switch S 3 is connected between adjacent B data bus lines D 3 and D 6 , and a short-circuiting switch S 9 is connected between adjacent B data bus lines D 9 and D 12 .

A control circuit 13 puts the outputs of the voltage buffer amplifiers B 1 to B 12 into a high impedance state during each of successive horizontal blanking periods, and during each period, turns on all the short-circuiting switches S 1 to S 3 and S 7 to S 9 .

Adjacent pixel data signals of the same color have inverse polarities to each other, and the absolute values thereof are almost the same as each other with a high probability. Particularly, this probability is higher in the region of a background image. Therefore, the potentials of the data bus lines D 1 to D 12 are rendered to be almost equal to the common potential VCOM when short-circuited, and currents consumed in the voltage buffer amplifiers B 1 to B 12 can be reduced to almost a half that of a case where no short-circuiting switch is connected. Further, the common potential VCOM of the opposite electrode is prevented from varying by capacitive coupling, and thereby a flicker is reduced compared with the case of FIG. 9 . Furthermore, since the number of the short-circuiting switches is a half that of the case of FIG. 8 , a circuit area of the data driver 10 can be reduced, enabling higher data bus line density to be achieved.

Second Embodiment

FIG. 4 shows an output stage of a data driver 10 A of a second embodiment according to the present invention.

In this circuit, interconnecting lines L 1 to L 3 for connecting short-circuiting switches S 1 , S 5 and S 9 on a first row and interconnecting lines L 4 to L 6 for connecting short-circuiting switches S 3 , S 7 and S 11 on a second row are arranged in a staggered configuration.

In each of these first and second rows, one ends of adjacent short-circuiting switches are connected to respective adjacent data bus lines: that is, one ends of the short-circuiting switches S 1 and S 5 are connected to the respective data bus lines D 4 and D 5 , one ends of the short-circuiting switches S 5 and S 9 are connected to the respective data bus lines D 8 and D 9 , one ends of the short-circuiting switches S 3 and S 7 are connected to the respective data bus lines D 6 and D 7 , and one ends of the short-circuiting switches S 7 and S 11 are connected to the respective data bus lines D 10 and D 11 .

The short-circuiting switches S 1 , S 3 , S 5 , S 7 , S 9 and S 11 are controlled by the control circuit 13 in a similar manner to the above-described first embodiment.

According to the second embodiment, a similar effect to that of the first embodiment is obtained. Furthermore, since interconnecting lines for short-circuiting switches are arranged only in the first and second rows such that the density of interconnecting lines is roughly uniform, and the arrangement density of short-circuiting switches is also roughly uniform, the area of the data driver 10 A can be smaller than that of the case of FIG. 3 with placing data bus lines in higher density.

Third Embodiment

FIG. 5 shows part of a data driver 10 B of a third embodiment according to the present invention.

Positive-polarity voltage buffer amplifiers PB 1 to PB 3 each are for providing higher ( H side) voltages than the common potential VCOM (for example, 5V), while negative-polarity voltage buffer amplifiers NB 1 to NB 3 each are for providing lower ( L side) voltages than the common voltage VCOM. The reason why the two types of the voltage buffer amplifiers are employed, one being for use in the H side and the other being for use in L side, is to realize a narrower output amplitude so as to simplify the configuration thereof.

In order to provide the outputs of the positive-polarity voltage buffer amplifier PB 1 and the negative-polarity voltage buffer amplifier NB 1 to each of the output terminals T 1 and T 2 alternately in each successive horizontal period (1 H), transfer gates P 1 and P 2 are connected between the output of the positive-polarity voltage buffer amplifier PB 1 and the respective output terminals T 1 and T 2 , and transfer gates N 1 and N 2 are connected between the output of the negative-polarity voltage buffer amplifier NB 1 and the respective output terminals T 1 and T 2 . Transfer gates P 1 , P 2 , N 1 , and N 2 constitute one set of changeover switches. This applies to changeover switches between other voltage buffer amplifiers and corresponding output terminals in a similar way. Between these changeover switches and the output terminals T 1 to T 6 , the short-circuiting switches S 1 , S 3 and S 5 are connected in a similar manner to the case of FIG. 4 .

FIG. 6 shows a circuit layout of part 20 in FIG. 5 lower than a short dashed line. In FIG. 6 , electrodes A to F, I to T, and U to W correspond to respective locations indicated by the same reference characters in FIG. 5 .

Each of the transfer gates of FIG. 5 has a PMOS transistor and an NMOS transistor connected in parallel to each other, and the PMOS transistors are formed in a region 21 and the NMOS transistors are formed in a region 22 .

For example, the PMOS transistor of the transfer gate P 1 has the electrodes A and I, and a gate drawn by a thick black line therebetween, and the PMOS transistor of the transfer gate N 1 has the electrodes A and J, and a gate drawn by a thick black line therebetween. The NMOS transistors of the transfer gates P 1 and N 1 have portions corresponding to those, in the NMOS transistor region 22 .

The PMOS transistor of the short-circuiting switch S 1 has the electrodes A and U, and a gate drawn by a thick black line therebetween, the PMOS transistor of the short-circuiting switch S 3 has the electrodes C and V, and a gate drawn by a thick black line therebetween, and the PMOS transistor of the short-circuiting switch S 5 has the electrodes E and W, and a gate drawn by a thick black line therebetween. Likewise, the NMOS transistors of the short-circuiting switches S 1 , S 3 and S 5 have portions corresponding to those, in the NMOS transistor region 22 . The electrode U is connected to the electrode D through the interconnecting line L 1 on a first row, the electrode V is connected to the electrode F through an interconnecting line L 4 on a second row, and the electrode W is connected to an interconnecting line L 5 on the first row. In FIG. 6 , these interconnecting lines L 1 , L 4 and L 2 in an upper wiring layer not shown are simply drawn.

Since the short-circuiting switches are formed at one sides of every other data bus lines, and the interconnecting lines L 1 , L 4 and L 5 for connecting the short-circuiting switches are arranged only on the first and second rows between the PMOS transistor region 21 and the NMOS transistor region 22 such that the density of interconnecting lines is nearly uniform, the area of the circuit 20 can be narrowed and the output terminals T 1 to T 6 , which are considered to be part of the respective data bus lines, can be arranged in higher density.

Referring back to FIG. 5 , each of positive-polarity voltage selectors PS 1 to PS 3 selects one of positive-polarity gradation voltages VP 31 to VP 0 according to the corresponding output value of respective registers R 1 , R 3 and R 5 to provide it to corresponding one of the respective positive-polarity voltage buffer amplifiers PB 1 to PB 3 . Likewise, each of the negative-polarity voltage selectors NS 1 to NS 3 selects one of negative-polarity gradation voltages VN 31 to VNO according to the corresponding output values of respective registers R 2 , R 4 and R 6 to provide it to corresponding one of the respective negative-polarity voltage buffer amplifiers NB 1 to NB 3 . To the clock inputs of the registers R 1 to R 6 , a latch signal LT is provided.

FIG. 7 is a waveform diagram showing operation of the output stage of FIG. 5 .

The latch signal LT is a pulse issued in each cycle of 1 H , and pixel data are latched into the registers R 1 to R 6 on the rise of each pulse. During each pulse period of the latch signal LT, the transfer gates P 1 to P 6 , and N 1 to N 6 stay off, and a high impedance state arises between the voltage buffer amplifiers and the output terminals. In this period, the short-circuiting switches S 1 , S 3 and S 5 are turned on, and thereby the voltages of the terminals connected by the short-circuiting switches are averaged.

Although preferred embodiments of the present invention has been described, it is to be understood that the invention is not limited thereto and that various changes and modifications may be made without departing from the spirit and scope of the invention.

For example, voltage buffer amplifiers may be respective source follower circuits. Further, a data driver may be formed in one piece with a LCD panel by employing thin film transistors.