JPS6129893A - Active matrix type liquid crystal display - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to an active matrix liquid crystal display device having an independent drive circuit for each pixel.

(Prior Art) FIG. 1 is a block diagram of a conventional active matrix liquid crystal display device, in which 1 is a control signal generator, 2 is a source line drive circuit, 3 is a gate line drive circuit, 4 is a MOS transistor,
5 is a capacitor, 6 is a pixel electrode, 7 is a counter electrode, 8 is a liquid crystal, 9 is an alternating current signal generator, 81 to 5 ITl are source lines, G□ to Gn are gate lines, U□, □ to U n
, m is a pixel circuit.

Here, each of the pixel circuits U1,1 to U n , m is a MOS transistor 4. It is composed of a capacitor 59 and a pixel electrode 6, and a counter electrode 7 is connected to a DC voltage source Vc.

Next, its operation will be explained. In the control signal generator 1, a control signal for controlling the operation of the source line drive circuit 2 and the gate line drive circuit 3 is generated from the vertical synchronization signal Vv, horizontal synchronization signal 12, and basic clock φ input from the outside. be done. The source line drive circuit 2 operates according to this control signal, and drives the source lines S, to Sm to a predetermined potential based on the first row data signal inputted from the outside.

Next, the gate line drive circuit 3 operates according to the control signal, a voltage is applied to the gate line G1 in the first row, and the potential of the source lines S~Sm changes to the pixel circuits U□2, ~Un,
m is input. Each pixel liquid crystal is driven by the input potential, that is, the potential difference between the potential of each pixel electrode 6 and the potential Vc of the counter electrode 7. Here, to simplify the explanation, it is assumed that the input data signal is a digital signal, and that the liquid crystal display device displays bright "1" and dark "0'". That is, the potential difference is If it is above the threshold voltage of the liquid crystal, it will display brightness, and if it is below, it will display darkness.Hereinafter,
By sequentially repeating the above-mentioned operation for the second row G2, a desired image is displayed on the entire display surface.

By the way, it is known that if a DC voltage is continuously applied to a liquid crystal, its life will be shortened and it will become impractical. For this reason, it is essential to apply an alternating current voltage to the liquid crystal. The AC signal generator 9 in FIG. 1 is a circuit provided for AC driving the liquid crystal, and its output VM alternately goes to a high level "H" in synchronization with the vertical synchronizing signal Vv.
, becomes low level "L".

FIG. 2 is a block diagram showing a part of an AC converting circuit in a conventional source line drive circuit, in which 21 is an inverter, 22 is an AND circuit, and 23 is a switch.

As is clear from the figure, for example, the potential of the source line Sj is
The data signal Dj is LL I I+ and the alternating current signal vM
If Dj is “H”, it becomes 2vc, and if Dj is “1” and VM is l (L), it becomes zero (ground potential), and Dj is “O”.
If so, it becomes Vc regardless of vl. Therefore, in synchronization with the vertical synchronization signal Vv, that is, for each frame period,
The liquid crystal of the pixel in the bright state is driven by AC voltage.

One of the major problems with the conventional active matrix liquid crystal display device shown in FIG. It will be done. In addition to the leakage current mentioned above, the leakage current through the resistance of the liquid crystal is also a factor in the fluctuation, but since the resistance of the liquid crystal is usually very high, the main cause is the leakage current through the OFF resistance of the MOS transistor 4. be.

FIG. 3 is a diagram showing temporal changes in the pixel electrode potential in a conventional active matrix liquid crystal display device, and shows the temporal changes in the pixel electrode potential, which is considered to be most affected by the leakage current mentioned above. It is something.

In FIG. 3, Vo (j, tj) and Vs (j) respectively put the pixel in column j and row 1 into a dark "0" state. j
These are the potentials of the pixel electrode and source line Sj in the first row of the jth column when the pixels in the second to nth rows of the column are set to the bright state. Vs(k) is the potential of the pixel electrode and the source line Sk in the nth row of the column when all the pixels in each column are brought into the bright "1" state. TP is a frame period, and for each T, the polarity of the potential input to the pixel electrode in the bright state with respect to the potential Vc of the counter electrode is reversed. T1. T2. ..., Tn are the first row G1. 2nd line G2. ..., first row G1. This shows the timing at which information is input to the pixel circuit.

As is clear from FIG. 3, the potential VD (1, j) of the pixel electrode in column j and row 1 is Vc immediately after writing, but the potential Vs (j) of source line Sj is then 2V c. (
Therefore, it is charged (discharged) over time due to leakage current through the off resistance of the MOS transistor, and gradually approaches 2Vc (zero). As a result, a voltage is applied to the liquid crystal of the pixel in column j and row 1, which should be in a dark state.

If the effective value of the applied voltage is not higher than the threshold voltage of the liquid crystal, the pixel that should be in the dark state will be in the bright state.

On the other hand, the potential V of the pixel electrode of the k-th column and n-th row. (n,k) is.

Immediately after writing, it is 2vc (zero), but since the potential Vs (k) of the source line Sk becomes zero (2Vc) after that,
The leakage current through the off resistance of the MO8I transistor is discharged (charged) over time, and gradually decreases to zero (2
approach Vc). As a result, the effective value of the voltage applied to the liquid crystal of the pixel, which should be in a bright state, decreases.

As shown above, due to the leakage current through the off resistance of the MOS transistor, voltage is applied to the liquid crystal of the pixel that should be in the dark state on the one hand, and voltage is applied to the liquid crystal of the pixel that should be in the bright state on the other hand. The effective voltage applied to the output voltage decreases. For this reason, conventional active matrix liquid crystal display devices have had problems such as deterioration of visibility and erroneous display due to a decrease in contrast.

(Object of the Invention) In order to solve these drawbacks, the present invention inputs a potential higher than or equal to Vc (or lower) to the pixel circuits in the odd rows, and inputs a potential lower than or equal to Vc (or lower) to the pixel circuits in the even rows. or more)
By inputting the potential of each source line, the time average value of the potential of each source line becomes approximately vc, and therefore
The present invention is characterized by reducing fluctuations in pixel electrode potential due to leakage current through the FF resistor, and its purpose is to provide an active matrix liquid crystal display device with a large operating margin and excellent display quality.

(Structure and operation of the invention) FIG. 4 is a block diagram of an embodiment of an active matrix type liquid crystal display device according to the present invention, the characteristic structure of which is an alternating current signal generator 41. As shown in FIG. That is, conventionally, It H71 and tl L are alternately synchronized with the vertical synchronizing signal Vv.
11, whereas in the present invention, the horizontal synchronizing signal V) 1. Alternatively, an alternating current signal V synchronized with both the horizontal synchronizing signal VH and the vertical synchronizing signal Vv is generated. The configuration and operation of the device other than the alternating current signal generator 41 are completely the same as the conventional active matrix liquid crystal display device shown in FIG.

FIG. 5 shows the configuration of the AC signal generator 41 according to the present invention, in which 51 is a T-type F/F (T-type flip-flop circuit) and 52 is an exclusive OR circuit.

FIG. 5(a) shows the configuration when the number of gate lines n is an odd number,
The alternating current signal VMM alternately becomes "H" and "L" in synchronization with the horizontal synchronizing signal VH.

FIG. 5(b) shows the configuration when the number of gate lines is an even number,
The alternating current signal V□ alternately changes to 1 in synchronization with the horizontal synchronization signal VH.
'H' and L, and their states are inverted in synchronization with the vertical synchronizing signal Vv.

When the alternating current signal VMM is inputted to the conventional alternating current converting circuit shown in FIG.
The potential of Vc) is input. Therefore, unless the data signal of II I 11 is a display button that is unevenly distributed in either odd-numbered rows or even-numbered rows, the time average value of the potential of the source line in the (1) frame period will be approximately equal to vc. As a result, the potential Vc of the pixel electrode of the pixel in the dark KIO31 state hardly changes over time because the leakage current through the off resistance of the MOS transistor is canceled out by both charging and discharging. Further, since the leakage current is averaged, the fluctuation width of the potential (2Vc or zero) of the pixel electrode of the pixel in the bright "1" state is reduced to about 172 compared to the conventional case even in the worst state.

FIG. 6 is a diagram showing temporal changes in the pixel electrode potential according to the present invention, and shows a case where a display button similar to that in FIG. 3 is input.

In FIG. 6, Vo (1, u) and Vs (u) are the U
These are the potential of the pixel electrode in the first row of the column and the potential of the source line Su. Further, Vn (n, v) and Vs (v) are the potential of the pixel electrode of the V column and nth row and the potential of the source line Sv, respectively, when all the pixels in the seven columns are in a bright state.

As is clear from FIG. 6, the potential V n (], u ) of the pixel electrode in the first row of column U in the dark state hardly changes. In addition, the potential V D (n, v) of the pixel electrode of the Vth column and nth row
The fluctuation width has been reduced to about 172 compared to the conventional case (■. (n, k) in FIG. 3).

In FIG. 6, the potential VD (1
, u) hardly fluctuates, which is a great advantage for the following reasons. In a conventional active matrix liquid crystal display device, in order to compensate for a drop in effective voltage due to fluctuations in the potential of a pixel electrode of a pixel in a bright state, it is sufficient to increase the driving voltage, that is, Vc. However, this also increases the fluctuation width of the potential of the pixel electrode of the pixel in the dark state, so increasing the drive voltage has little effect on increasing the operating margin.

In contrast, in the present invention, since the potential of the pixel electrode of the pixel in the dark state hardly changes regardless of the driving voltage, it is possible to increase only the effective voltage applied to the liquid crystal of the pixel in the bright state. It becomes possible to significantly expand the operating margin.

Further, as is clear from the above explanation, each pixel circuit is inputted with either the potential of vc or 2vC1 zero, and the voltage of 2V
The number of pixel circuits to which the potential of c is input.

The number of pixel circuits to which zero potential is input is almost equal, and the potential of the pixel electrode set to the initial potential of Vc hardly changes, and the rate of decrease in the potential of the pixel electrode set to the initial potential of 2Vc and zero The rate of increase in the potential of the pixel electrode set to the initial potential is approximately equal if the off resistance of the MoS transistor is linear. Therefore, the potential of the counter electrode is fixed to approximately the potential of Vc via the resistance of the liquid crystal, regardless of whether or not it is connected to the DC voltage source Vc. In other words, when the counter electrode is composed of one electrode common to the entire display surface,
There is no need to connect the electrode to any voltage source. As a result, it becomes unnecessary to take out the electrodes, and it becomes possible to reduce the manufacturing cost of the active matrix liquid crystal display device.

The embodiment shown in FIG. 4 shows the case where the capacitor 5 is applied to each pixel circuit, but if the capacitor 5 is not present, the influence of leakage current via the off resistance of the Mo8t transistor becomes large. , the effects of the present invention are further enhanced.

In the present invention, the case where a digital signal is input as the data signal is shown, but the same effect can be obtained even if the data signal is a video signal. Further, in the above explanation, the case where the potential of the counter electrode is Vc is shown, but the potential may be set to zero and positive and negative potentials are inputted to each pixel circuit. Although we have shown the case where a common potential is applied to the entire display surface, this is an active matrix liquid crystal display in which counter electrodes are provided in stripes for each row, and a predetermined voltage is applied between the counter electrodes and the corresponding pixel electrodes. It can also be applied to equipment.

Further, in the present invention, the case where the polarity of the voltage applied to the liquid crystal is reversed every frame period has been described, but it is also applicable to the case where the polarity is reversed at a period that is an even multiple of the frame period. In the present invention, a case has been shown in which the vertical synchronization signal □□ is input from the outside, but these signals can also be generated within one display device by using a counter or the like.

(Effects) As explained above, according to the present invention, a potential higher than (or lower than) a predetermined potential is input to the pixel circuits in the odd rows, and ゛ is input to the pixel circuits in the even rows. By inputting a potential lower than (or higher than) the potential, the time average value of the potential of the source line can be set to approximately the predetermined potential.
o8) - There is an advantage that the fluctuation width of the pixel electrode potential due to the leakage current flowing through the OFF resistance of the transistor can be reduced. As a result, it becomes possible to suppress a decrease in operating margin and a decrease in contrast due to the leakage current.

In addition, since the average value of the potential of each pixel electrode is approximately constant regardless of the display pattern, it is possible to automatically set the potential of the opposing electrode via the capacitance or resistance of the liquid crystal. It becomes unnecessary to pull out the electrode from the electrode,
This has the advantage of reducing manufacturing costs of active matrix liquid crystal display devices.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional active matrix liquid crystal display device, Fig. 2 is a block diagram showing a part of an AC converter circuit in a conventional source line drive circuit, and Fig. 3 is a block diagram of a conventional active matrix liquid crystal display. 4 is a diagram showing temporal changes in pixel electrode potential in the device; FIG. 4 is a configuration diagram of an embodiment of an active matrix liquid crystal display device according to the present invention; FIG.
This figure shows the configuration of the alternating current signal generator 41 according to the present invention, and FIG. 6 is a diagram showing temporal changes in the pixel electrode potential according to the present invention. DESCRIPTION OF SYMBOLS 1...Control signal generator, 2...Source line drive circuit, 3...Gate line drive circuit, 5...Capacitor, 6...Pixel electrode, 7...Counter electrode, 8...・Liquid crystal, 9.41... AC signal generator, 21... Inverter, 22... AND circuit, 23.
...Switch, 51...T-F/F, 52...Exclusive OR circuit, 81-5L11...Source line, G□
~Gn...Gate line, Gll□~Gn, m...Pixel circuit, 9,■□...AC conversion signal, Vv...Vertical synchronization signal. vH...Horizontal synchronization signal, φ...Basic clock. Patent applicant Nippon Telegraph and Telephone Public Corporation Figure 4 Figure 5 (a) (b)

Claims (5)

[Claims] (1) A substrate having n gate lines and m source lines that intersect with each other and pixel circuits that are arranged in n rows and m columns at the intersections and hold input information for a desired period, and a transparent electrode. In an active matrix liquid crystal display device consisting of a counter substrate and a liquid crystal sandwiched between these substrates, a potential higher than (or lower than) a predetermined potential is input to the pixel circuits in the odd rows, and the pixel circuits in the even rows An active matrix liquid crystal display device, wherein a potential lower than (or higher than) the predetermined potential is input to the pixel circuit. (2) The active matrix according to claim (1), characterized in that a potential above a predetermined potential and below a predetermined potential is alternately input to a given pixel circuit every predetermined frame period. type liquid crystal display device. (3) Claims (1) or (2) characterized in that the transparent electrode is composed of one electrode that is common to the entire display surface, and that electrode is connected to a predetermined DC voltage source. The active matrix type liquid crystal display device described in . (4) Claim (1) or Claim 1 characterized in that the transparent electrode is composed of one electrode that is common to the entire display surface, and that electrode is not connected to any voltage source including ground potential. The active matrix liquid crystal display device described in (2). (5) Claims (1) or (2) characterized in that the transparent electrodes are arranged in stripes in a direction parallel to the gate line, and voltages with different waveforms are applied every predetermined frame period. The active matrix type liquid crystal display device described in .