Liquid Crystal Device and Method of Driving Same

1. Field of the Invention

The present invention relates to a liquid crystal device and a method of driving the same, and more particularly to a liquid crystal device and a method of driving the same capable of suppressing high intensity lines produced while driving the device in units of blocks by using TFT's (thin film transistors) as switching elements.

2 Related Background Art

As shown in FIG. 3, in a conventional method of driving a liquid crystal panel having a TFT active matrix circuit, internal video signal lines of a display panel 1 are divided into a plurality of blocks. A matrix circuit 2 is provided for matrix-connection between the internal video signal lines of each block and external video signal lines having the same number of lines as the former lines. Sample/hold switching elements constructed of a B-TFT (block dividing TFT) array 3 are interposed on the respective internal video lines between the matrix circuit 2 and the display panel 1. Control signals are supplied to the switching elements of each block to drive the display panel in time division using one horizontal period (lH) as a reversal period.

FIG. 4 shows a detailed connection diagram of FIG. 3, wherein external video signal lines Dl, D2, . . . , Dm are divided into m internal video signal lines Sl, S2, . . . , Sm per one block by the matrix circuit 2. In case of k blocks, the total number of video signal lines is m.times.k. Each of the internal video signal lines Sl, S2, . . . , Sm is grounded via a hold capacitor 10. Switching elements 11 interposed between the capacitor and the matrix circuit are driven in time division by respective block division gate drivers Bl, B2, . . . , Bk to output video signals to pixels.

When a liquid crystal panel constructed as above is driven using one horizontal period (lH) as a reversal period, a charge shift phenomenon of a so-called charge sharing effect occurs at the intersection between divided blocks, e.g., between lines Sm and Sl of FIG. 4, due to the capacitance between source lines of B-TFT's. As a result, .DELTA.V is superposed on the video signal of line Sm so that a video signal having a larger voltage amplitude than the original video signal is outputted (with opposing electrode 12 being grounded).

FIG. 5 illustrates the principle of the charge sharing effect, and FIG. 6 is a timing chart showing the charge sharing effect. In FIG. 5, a central broken line indicates the intersection between blocks, the block at the left of the line being called block 1 and that at the right being called block 2. The last signal line Sm of block 1 is driven by the output signal from the last source line Dm and the drive voltage Bl for the block division TFT's of block 1. The first signal line Sl of block 2 is driven by the output signal from the first source line Dl and the drive voltage B2 for the block division TFT's of block 2, source line capacitance Cm and Cl as seen from source terminal side of the block division TFT's, correspond to the video signal hold capacitor C. Interline capacitance Css producing .DELTA.V appears between the source lines. Referring now to FIG. 6, when a gate pulse is applied to line Bl, a video signal on line Dm is transferred to line Sm via the B-TFT to charge the source line capacitor Cm. After charging the source lines of block 1 to which the capacitor Cm belongs is completed, another gate pulse is applied to line B2 to thereby charge the source lines including line Sl of block 2. In this case, the charging waveforms on lines Sm and Sl at the intersection between the two blocks change as shown in FIG. 6. Particularly, .DELTA.V shown by oblique lines is superposed on line Sm and its video signal becomes larger in amplitude than its original, while the video signal on line Sl changes at the start of reversal as shown by oblique lines. Such phenomenon results from the charge sharing effect of the source interline capacitance Css between the capacitors Cm and Cl. The relationship between .DELTA.V and V is approximately defined by the following formula:

.DELTA.V .apprxeq.Css/(C+Css).multidot.V(v)

(C=Cm.apprxeq.Cl)

If a liquid crystal display panel as above is driven without any correction, the last lines Sm of the blocks are highly brightened so that it is quite unsuitable for a display device.

The present invention seeks to solve the above problems and provide a liquid crystal device and method of driving the same wherein high intensity lines of blocks produced by the charge sharing effect during lH reversal driving are suppressed thereby realizing a high quality image.

In order to solve the above problems, the present invention provides a method of driving a liquid crystal device wherein internal video signal lines of a TFT active matrix panel are divided into a plurality of blocks, a matrix circuit is provided for matrix-connection between the internal video signal lines of each block and external video signal line having the same number of lines as the former lines, sample/hold switching elements are interposed on the (respective internal video lines between the matrix circuit and the panel, and control signals are supplied to the switching elements of each block to drive the panel in time division using one horizontal period as a reversal period; a method of driving a liquid crystal device wherein the switching elements of each block are further divided into two half-blocks, a switching signal line is provided for each of the half-blocks, the phase of a control signal applied to the switching elements for each of the half-blocks is shifted between adjacent half-blocks to output video signals onto the internal video signal lines at superposed timings.

FIG. 1 is a connection diagram showing the main part of the liquid crystal device according to an embodiment of the present invention;

FIG. 2 is a timing chart showing the operation of the device of FIG. 1;

FIG. 3 is a schematic block diagram showing the liquid crystal device according to the prior art;

FIG. 4 is a timing chart showing the operation of the device of FIG. 3;

FIG. 5 is an equivalent circuit illustrating the charge sharing effect;

FIG. 6 is a timing chart showing the operation of the equivalent circuit; and

FIG. 7 is an equivalent circuit during time-division driving.

The charge sharing effect occurs during the time from when a pulse is applied to turn on the B-TFT's of one block and to when another pulse is applied to turn on the B-TFT's of the next block. In FIG. 5, during turning-off of the B-TFT at line Bl, line Sm keeps a potential charged in the capacitor Cm and maintains an open state relative to the signal source such as the source line driver of FIG. 3. When a signal is applied during this open state to the B-TFT's of the next block 2 to turn them, line Sl is enabled to receive the signal from the signal source so that the capacitor Cl is charged. Simultaneously, the signal on line Sl charges the capacitor Css and its charge is transferred to and stored in the capacitor Cm. As a result, the waveform of line Sm changes by .DELTA.V as shown in FIG. 6. FIG. 7 is an equivalent circuit wherein line Sm maintains an open state and the signal source 13 is coupled to line Sl.

In order to eliminate or decrease .DELTA.V, it can be considered that isolation from the signal source and connection to the signal source should not be conducted simultaneously between the lines of adjacent blocks and that Css and V be made small. Since Css is determined from the panel configuration, the remaining factors to solve the problem of .DELTA.V are switching timings and the value V.

Paying attention to the switching timings, the present invention enables one to make a potential difference between signals on Sm and Sl very small. Particularly, the B-TFT arra of switching elements of each block is further divided into two half-blocks. A switching signal line is provided for each of the half-blocks, the phase of a control signal applied to the switching elements for each of the half-blocks is shifted between adjacent half-blocks to output video signals onto the internal video signal lines at superposed timings.

The embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is a connection diagram showing the B-TFT's and matrix circuit embodying the present invention. In this embodiment, the same display panel as the conventional one shown in FIG. 3 is used. A TFT active matrix circuit constituting a display portion, B-TFT array and matrix circuit are fabricated on a single substrate. In FIG. 1, the total number of matrix wirings is 240 which are here identified as first half 120 wirings and latter half 120 wirings. Video signal lines of one block, i.e., panel source lines are connected to 240 bit B-TFT's. The panel source lines and B-TFT's are similarly identified as first half 120 ones and latter half ones.

A control signal line for turning on and off the B-TFT's of the first 120 bits of block 1 is identified as "Bl-first", while a control signal line for turning on and off the B-TFT's of the latter 120 bits of block 1 as "Bl-latter". Similar identification is made up to "B8-latter". Thus, the total number of panel source lines is 8.times.240. The number of gate lines (scanning lines) is 480 and the panel corresponds to a TV screen of about 7 inches.

FIG. 2 is a timing chart showing the operation of the liquid crystal device of FIG. 1 wherein an NTSC television signal is used as a video signal source. The television video signal is divided into eight portions which are assigned to blocks 1 to 8 as a video signal source of the display panel, each of the blocks being divided into the first half and the latter half. In the present invention, the divided video signal is processed by controlling the output timings of the source line driver as in the following.

When image data of the first 120 bits of block 1 is inputted, this data is outputted on source lines Dl to Dl20. Simultaneously therewith, a pulse for turning on the B-TFT's of the first 120 bits is applied to control line "Bl-first" to charge the first 120 source lines of block 1. Next, when image data of the latter 120 bits of block 1 is prepared, this data is outputted on source lines Dl2l to D240. Simultaneously therewith, a pulse for turning on the B-TFT's of the latter 120 bits is applied to control line "Bl-latter" to charge the latter 120 source lines of block 1. The phase of the on/off control signals on "Bl-first" and "Bl-latter" and so on is phase shifted 90 degrees so as to be superposed between two adjacent half-blocks. Similar timings of the control signals are repeated up to block 8 to write a lH television signal on 1920 source lines. In this case, the liquid crystal (TN liquid crystal, ferroelectric liquid crystal) is ac-driven by grounding the opposing electrode or by reversing every lH in synchro with the television signal.

In the above liquid crystal drive, the source line waveforms in the panel shown in FIG. 2, particularly the charge/discharge waveforms of the source lines Sl20 and Sl2l at the intersection of blocks, are observed. The potential difference V on the source line Sl2l is very small at the time when a pulse on "Bl-first" for the source line Sl20 of block 1 turns off. This V corresponds to the V of the above-described approximate formula. Therefore, .DELTA.V in the formula becomes considerably small. The potential difference .DELTA.V at the leading edge of the source line waveform shown in FIG. 2 becomes extremely small.

One block of the above embodiment may be divided into three or more.

As seen from the foregoing description of the present invention,eeven if the charge sharing effect occurs during lH reversal drive, the resultant potential difference can be reduced to a minimum by driving the finely divided blocks at superposed timings. Further, in case the charge speed with a B-TFT is high, the potential difference can theoretically be made zero. Thus, it is possible to eliminate high intensity lines at the intersection of blocks and provide a liquid crystal device and method of driving the same capable of obtaining a high image quality.