JPS6126074B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a matrix type liquid crystal image display drive circuit.

More specifically, the present invention relates to a circuit for driving a liquid crystal at a low voltage AC when one electrode for driving the liquid crystal is a common electrode over the entire matrix screen.

In display devices that apply the electro-optical effect of various liquid crystals, liquid crystals are generally driven with alternating current voltage because the durability time for driving with direct current voltage is short, or from the viewpoint of display quality such as homogeneity of displayed images. Therefore,
Simply put, AC drive is performed with an operating voltage approximately twice the threshold voltage during DC drive. In order to drive with an operating voltage width approximately equal to the operating voltage during DC driving, the signal potentials applied to both electrodes sandwiching the liquid crystal must be inverted for each. These issues are already well known.

Here, FIG. 1 shows a conventional example of a circuit configuration when a matrix type image display device according to the present invention has a pixel selection transistor.

The part surrounded by the broken line 1 is a liquid crystal display part having a matrix structure. 2, 3, and 4 are peripheral circuits. 2 is a row electrode driving circuit; 3 is a column electrode, ie, data electrode driving circuit; and 4 is a circuit for supplying timing signals and data signals to 2 and 3. Display body 1
When displaying a television image on the display body 1, the block 4 is a television receiver, and when displaying a character or graphic image on the display body 1, the block 4 is a television receiver.
The main CPU that processes the data to be displayed,
This is an interface between the CPU and a display device.
In the display section 1, a liquid crystal display pixel 5, a pixel selection transistor 6, and a capacitor 7 are provided at each intersection of a row electrode group and a column electrode group. The liquid crystal pixel electrode is connected to the pixel selection transistor and capacitor. Further, the electrodes facing the pixel electrodes via the liquid crystal are all connected to the common potential GND shown in 8, that is, they serve as a common electrode over the entire display screen. In FIG. 1, the transistor is a MOSFET, and the common electrode potential is GND, which matches the substrate potential of the transistor 6. Here, in order to make it easier to understand, the operating potential at each point during driving will be explained using a video signal as an example.

FIG. 2 shows an example of DC driving when the common electrode potential of the liquid crystal is made to match GND8 as shown in FIG.

10 is a video signal waveform to be applied to the pixel electrode of the liquid crystal. 11 is a timing pulse train for sequentially sampling and holding video signals for each column electrode in the column electrode drive circuit 3; for example, 12 signals are sampled corresponding to one column electrode. 13 is a timing pulse train in which the row electrode drive circuit 2 turns on the pixel selection transistor of each row and takes in the image signal sampled on the column electrode to each pixel in the corresponding row; Therefore, it is taken into the pixel electrode. V _th in FIG. 2 is the display threshold voltage of the liquid crystal, and Vmax2 is the maximum value of the image signal voltage applied to the liquid crystal. At this time, the voltage required to drive the liquid crystal display is at least the voltage _G2 applied to the gate for switching the pixel selection transistor.
only required. V _G 2 becomes larger than V max2 by the threshold voltage of the transistor. In the above case, the liquid crystal is driven by direct current.

FIG. 3 shows an example of potential levels when AC driving the liquid crystal. In FIG. 3, the common electrode potential of the liquid crystal is not at the GND potential, but at the potential indicated by the dashed line 20. Therefore, the wiring diagram of the display body circuit is somewhat different from that in FIG. Here, the common electrode potential 20 is approximately equal to the value of Vmax2 in FIG. In Figure 3, the video signal requires AC driving of the liquid crystal, so the common electrode potential is 2.
One of the waveforms is selectively supplied to each pixel at regular intervals as a waveform that is substantially vertically symmetrical with respect to 0. Therefore the maximum amplitude of the video signal output
Vmax3 is approximately twice that of Figure 2, and for the same reason as Figure 2, the voltage required to drive the display is shown as V _G 3 in Figure 3, which is twice the voltage required for DC drive. It turns out that

As described above, when the operating voltage is doubled, not only the withstand voltage of each circuit element but also the circuit conditions such as leakage current become more severe, and at the same time, the current consumption of the device also increases. The power consumption can be easily calculated to be four times as much.

The present invention realizes a matrix type liquid crystal image display body which drives a liquid crystal in alternating current using the operating voltage required for direct current driving, in contrast to the conventional circuit having such drawbacks.

In a liquid crystal display in which a pixel electrode is individually provided at each intersection of a matrix, the polarity of the signal applied to the electrode is inverted, and in order to drive AC with the same operating voltage as DC drive, it is necessary to connect each pixel electrode via the liquid crystal. Similarly, opposing electrodes had to be provided individually for each pixel. In other words, the counter electrode must not be a common electrode for the entire screen. Conventionally, when one of the electrodes sandwiching the liquid crystal was a common electrode for the entire surface, there was no choice but to drive with direct current or with twice the voltage.

In the present invention, by changing the circuit system without separating the common electrode for each pixel or row, AC drive is realized with a voltage width that was conventionally used for DC drive, and the drawbacks of the conventional method are overcome. This is an excellent display AC drive circuit that eliminates

FIG. 4 shows an example of a matrix drive circuit configuration of a liquid crystal image display device according to the present invention.

One electrode of the liquid crystal display pixel 5 is connected to the drain of a pixel selection transistor 6 provided for each pixel and a capacitor 30. The other electrode of the liquid crystal pixel consists of a common electrode over the entire screen. Here, among the electrodes of the capacitor 30, the electrode 31 that is not connected to the liquid crystal pixel electrode is separated from the substrate of the transistor 6, unlike in the case of FIG. And they are connected to each other in rows. Row unit capacitor common electrode 3 that connects one electrode of the capacitor in each row unit
2 and 33 are connected to the output of a capacitor common electrode AC drive circuit 34 which sets the common electrode potential to a required level in accordance with the AC drive of the liquid crystal.

Next, a method of actually AC driving the liquid crystal in the circuit configuration shown in FIG. 4 will be explained.

FIG. 5 is an explanatory diagram of the potential level of a liquid crystal drive signal drawn in correspondence with the conventional system shown in FIGS. 2 and 3. Potential 20 of liquid crystal display common electrode 35
is the same value as 20 in Fig. 3, and Vmax2 in Fig. 2
is equal to the value of As the gate applied signal voltage of the pixel selection transistor 6, V _G 5 is required. Here, V _G 5 is the sum of the video signal amplitude and the gate threshold voltage of transistor 6, so V _G in FIG.
It may be smaller than the value of 2 by the liquid crystal threshold voltage V _Th . The capacitor common electrode AC drive circuit 34 is
The potential supplied to the capacitor common electrode 32, 33, etc. has two values, GND and _VCAP in FIG.
The value of V _CAP is the sum of the liquid crystal display common electrode potential 20 and the threshold voltage V _Th of the liquid crystal itself. That is, Figure 2
It is equal to the sum of the value of Vmax2 and V _Th . As mentioned above, the maximum value of the voltage required for the circuit of FIG. 4 is V _CAP . Suppose that the threshold voltage of transistor 6 |Vmax2−V _G 2
When | and V _Th are the same value (for example, 1 volt), the maximum value V _CaP of the voltage required to drive the display device according to the present invention is the same as the voltage V _G 2 required for the conventional DC drive device shown in FIG. This means that the voltage will be 1/2 compared to that shown in Figure 3. Now, in FIG. 5, 41 and 42 are video signals with inverted polarities, which are applied as periodically alternating video signals to each liquid crystal pixel via a sample circuit in the column electrode drive circuit 3. When applying a positive video signal to the pixel electrode with respect to the liquid crystal common electrode, the row unit common electrode 31 of the capacitor 30 is held at GND or V _CaP , and the video signal 41 is applied to the pixel electrode via the pixel selection transistor 6. be done. Conversely, when applying a negative video signal to the pixel electrode, the common side electrode 31 of the capacitor 30 is held at the GND level while the video signal 42 is written to the pixel electrode via the pixel selection transistor 6, and when the writing is completed. At the same time, it is lowered to the level of -V _CAP . As a result, the potential level of the video signal applied to each pixel electrode in one row is represented by waveform 43. If the capacitor capacitance is sufficiently large compared to the liquid crystal pixel capacitance, the value of V _CAP is the sum of the liquid crystal display common electrode potential 20 and V _Th as described above, and the negative video signal 4 applied to the liquid crystal pixel electrode
3 and the positive video signal 41 are symmetrical about the liquid crystal display common electrode potential 20. Therefore, the liquid crystal pixels are driven by alternating current.

FIG. 6 explains the change in potential level in more detail.

(a) shows the potential at each point on the capacitor 30 and pixel 5 when the video signal 42 is taken into the pixel, and (b) shows the potential at each point on the capacitor 30 and pixel 5 when the potential on the common electrode side is lowered to V _CAP in each row of the capacitor 30. The potential at each point is shown. The potential of the signal taken into the pixel electrode in (a) is V _s
t , (b), the pixel electrode potential after the potential change is V _st
+1 .

Q _c = V _st・C _c Q _L = (V _COM - V _st )・C _L Q _t+1 = Q _C - Q _L = C _C・ (V _st+1 - V _CAP ) + C _L・
(Since V _st+1 - V _COM , converting V _st+1 becomes V _st+1 = V _st + V _CAP - _{C L} /C _C + C _L · V _CAP .

Here, if C _C ≫ C _L , the third term on the right side is ignored, and the pixel electrode potential moves by the same potential as when the capacitor common electrode potential is lowered, and the fifth term
It can be seen that the waveform shown in FIG. 43 is obtained. If C _C ≫ _{C L}
If this does not hold true, since the third term is a constant value, it is sufficient to superimpose a voltage corresponding to the third term on the value of V _CAP .

FIG. 7 is a timing chart of each signal when a television video signal is applied to the pixels of a liquid crystal display by the AC drive circuit according to the present invention.

60 is a vertical synchronization signal, and 61 is a horizontal synchronization signal. Reference numeral 62 indicates a timing pulse train for sampling a video signal for each column electrode in the column electrode drive circuit 3. There are as many pulses in one horizontal scan line as there are column electrodes of the matrix display. Although the explanation of the row electrode drive signal was omitted at the beginning, it conventionally corresponds to the horizontal synchronization signal 61. In each row, the sample pulse train 62 samples the video signal to the corresponding column electrode, and then the pulse 61 drives each pixel. A sampled video signal was taken in through a transistor 6 at the electrode.

In the liquid crystal AC driving according to the present invention, row electrode driving can be performed using timing pulses similar to the conventional method. However, in order to correctly capture the video signal into the pixel electrode, in the present invention, the video signal corresponding to each pixel in the corresponding row is transferred to the column electrode drive circuit 3.
While sampling, the pixel selection transistor shorts the column electrode and the pixel electrode. 63 indicates the timing to short-circuit each pixel selection transistor in a specific row. In other words, in the conventional circuit, the column electrode drive circuit 3 sequentially samples the video signal, and then simultaneously takes in the sampled signal for one row of pixel electrodes, thereby writing a two-stage video signal. However, in the present invention, when the column electrode drive circuit samples the video signal, the video signal is written to the pixel electrode at the same time, and the signal is directly transmitted to the pixel electrode, so accurate signals are transferred to the pixel electrode. You can get immersed in it. 64 is
This is a time chart showing potential reversal of capacitor electrodes commonly connected in rows. 64 is a pixel selection transistor that is a P-channel MOSFET.
Alternatively, this is a time chart for a transistor similar to this, and when the pixel selection transistor is an N-channel MOSFET or a transistor similar to this, the polarity of the signal is reversed.

The polarity of the video signal 65 is reversed in the vertical synchronization signal (or within the vertical retrace time), and the signal levels have amplitudes in the same range for the video signals of opposite polarities. 66 is a video signal applied to the pixel liquid crystal. The liquid crystal drive video signal has an alternating current waveform centered around the potential 20 of the liquid crystal display common electrode. The potentials at points 44 and 44' on the waveform correspond to 44 and 44' in FIG.
67 is another example in which the potentials of capacitor electrodes commonly connected in rows are inverted. 67, the capacitor electrode is usually at a low potential (-V _CA
P ), and the high potential is applied every other pulse of the row electrode drive pulse 63 and only during the period when the video signal corresponding to all pixels of the row is taken into the pixels for the same width as the row electrode drive pulse or for the period when the video signal corresponding to all pixels of the corresponding row I'll lift it up. As a result, the signal applied to the liquid crystal pixels has a waveform of 68, and the liquid crystal is driven with alternating current.

When capturing an image signal, the signal applied to the pixel liquid crystal has a time different from the video signal to be applied during the time corresponding to one horizontal scanning line of the video signal, but it is sufficiently short compared to the time of one frame. This signal is ignored because it is time and the liquid crystal display cannot follow it. 67 is a case where the pixel transistor is a P channel, and when the transistor is an N type, the polarity is opposite.

FIG. 8 shows a specific implementation example of the capacitor electrode drive circuit 34 shown as a block in FIG. Terminals 70 and 71 are capacitor common electrode drive outputs. The output waveform corresponds to FIG. 764, and the outputs of successive stages are delayed by one horizontal scanning line time. 73 is a clock input terminal for shifting data every horizontal scanning line;
is the output data. Output signal waveform 6 of terminal 70
4 is inverted in frame period units, and during inversion, the potential is shifted from high potential to low potential immediately after capturing the video signal of the corresponding row, and from low potential to high potential immediately before capturing the video signal of the corresponding row. do.
As mentioned above, this is because the pixel selection transistor is P
This is a channel type case, and if the inversion position changes, the image signal will flow from the PN junction of the transistor's drain portion to the transistor substrate, making it impossible to display a correct image.

FIG. 9 shows still another example of the capacitor electrode drive circuit according to the present invention. The circuit in Figure 9 is
This circuit serves both as the row electrode drive circuit 2 and the capacitor electrode drive circuit 34 in FIG. The output of each delay flip-flop has two output terminals, a direct output terminal and a gated output terminal.
0, 81, and 82 become row electrode drive signals, and outputs 83, 84, and 85 through the gates become capacitor electrode drive signals. Reference numeral 86 is an alternating signal that is inverted every frame, and a NAND gate causes a capacity electrode drive output pulse to be output every other frame. The output signal of 83 is shown in FIG. 7, 67. In the case of FIG. 9, since the shift register circuit is shared by the row electrode drive circuit 2 and the capacitor electrode drive circuit 34, the number of circuit elements can be saved significantly, and the AC drive circuit according to the present invention can be easily realized. It shows that it is something.

[Brief explanation of the drawing]

FIG. 1 shows a conventional circuit configuration of a matrix type liquid crystal display, and FIGS. 2 and 3 show potentials when driving the liquid crystal with direct current or alternating current, respectively, according to FIG. 1. FIG. 4 shows an AC drive circuit for a liquid crystal display according to the present invention, and FIG. 5 shows its operating potential. FIG. 6 shows potential fluctuations of liquid crystal pixel electrodes due to AC driving according to the present invention. FIG. 7 is a time chart of each point signal in the present invention. FIGS. 8 and 9 are examples of capacitor common electrode drive circuits. DESCRIPTION OF SYMBOLS 1... Matrix type liquid crystal display body part, 2... Row electrode drive circuit, 3... Image signal sample, column electrode drive circuit, 5... Liquid crystal pixel, 6... Pixel selection transistor,
7... Capacitor, 10, 21, 22, 41, 4
2... Video signal, 32, 33... Capacitor electrode common in each row, 34... Capacitor electrode drive circuit, 35... Liquid crystal display common electrode, 66, 68... Liquid crystal pixel drive AC image signal, 70, 83... Capacitor electrode drive Signal output terminal.

Claims (1)

[Claims] 1 Each pixel arranged in a matrix has a pixel selection transistor and a capacitor, one electrode of the capacitor is commonly connected to each other in each row of the matrix, and the image signal taken into the pixel electrode is A liquid crystal display alternating current drive circuit characterized in that the polarity of brightness and darkness is reversed at a constant cycle, and the capacitor electrode common to each row is reversed sequentially at a constant cycle for each row in response to the polarity reversal of the image signal. . 2. The AC driving circuit for a liquid crystal display according to claim 1, wherein the potential ranges of image signals having different polarities of brightness and darkness taken into the pixel electrodes overlap with each other. 3 The image signal sample circuit is located at one part of the matrix.
2. The AC drive circuit for a liquid crystal display according to claim 1, wherein while sampling signals corresponding to pixels in a row, pixel selection transistors in the corresponding row are in a short-circuited state. 4. The liquid crystal display AC according to claim 1, wherein the time during which the image signal sampling circuit maintains the circuit in a short-circuited state in order to sample the image signal of the corresponding pixel is longer than the sampling period of the image signal. drive circuit. 5 The pixel selection transistor is a P-channel type
The potential of the capacitor electrode, which is a MOSFET and is commonly connected in each row, is inverted between two potentials, and the inversion from a high potential to a low potential is performed immediately after each pixel in the corresponding row captures an image signal. Claim 1, characterized in that the inversion from a low potential to a high potential is performed immediately before each pixel captures an image signal.
The liquid crystal display AC drive circuit described in 2. 6 Pixel selection transistor is N-channel type
The potential of the capacitor electrode, which consists of MOSFETs and is commonly connected in each row, is inverted between two potentials, and the inversion from a low potential to a high potential is performed immediately after each pixel in the corresponding row captures an image signal. 2. The AC drive circuit for a liquid crystal display according to claim 1, wherein the inversion from a high potential to a low potential is performed immediately before capturing an image signal. 7. The liquid crystal display according to claim 1, wherein the capacitor electrodes commonly connected in rows are invertedly driven by output signals from each stage of a shift register connected in series. AC drive circuit. 8. Claim 7, characterized in that the circuit that drives the capacitor electrodes that are commonly connected on a row-by-row basis and the circuit that drives each row electrode of the matrix display section share the same shift register. LCD display AC drive circuit.