JPH09243996A - Liquid crystal display device, liquid crystal display system and computer system - Google Patents

Abstract

(57) Abstract: A liquid crystal display device using a TFT, a display system, and a device equipped with a liquid crystal display system such as a mobile terminal reduce power consumption. A latch circuit that stores an image data signal and a circuit that generates a drive signal for driving a liquid crystal from the stored image data signal are provided for each pixel forming a display screen. The peripheral circuits relating to the writing of the image data signal operate only during the operation period for writing the latch circuit, which is performed in a constant cycle, and stop during the other waiting period. Changeover switch 24 for manually changing the ratio of the operation period and the standby period of the peripheral circuit according to the contents of the displayed image (whether the moving image is a still image or a moving image, whether the moving image changes quickly or slowly), an instruction for automatic switching There is provided a computer section 23 for outputting and a control circuit 22 for giving a switching signal to the liquid crystal display device 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using a thin film transistor (hereinafter abbreviated as TFT), a liquid crystal display system using the liquid crystal display device, and a computer system such as a portable terminal equipped with the liquid crystal display system. The present invention relates to low power consumption of these devices or systems.

[0002]

2. Description of the Related Art A liquid crystal display device using a TFT is a flat panel display capable of displaying high quality images.
Nowadays, it is used in a wide range of fields such as laptop computers, car navigation systems, TV monitors, measuring instruments, and other information devices. FIG. 6 is a circuit diagram of a main part of a liquid crystal display device using a conventional TFT. Screen part 100
The pixels 101 are arranged in a matrix, and each pixel is connected to a scanning line (horizontal wiring) 102 and a data signal line (vertical wiring) 103 formed vertically and horizontally. A scan circuit 104 which is a peripheral driver circuit and a data signal circuit 105 are provided, and the scan line 102 is connected to the scan circuit 104 and the data signal line 103 is connected to the data signal circuit 105. Further, a common wiring 106 on the fixed potential side of the additional capacitor provided in each pixel 101 and a connection circuit 107 for the common wiring 106 are provided. However, there is also a configuration in which a common line for an additional capacitor and a connection circuit are not separately provided and the scanning line in the preceding stage is also used. Although the number of pixels varies depending on the application of the liquid crystal display device, it has been commercialized from a small number of tens of thousands of pixels to a large capacity of more than 3 million pixels.

FIG. 7 is a circuit diagram of one pixel 101.
Pixel transistor (n-channel TFT in this figure)
A gate of 110 is connected to the scanning line 102, and a drain or a source thereof is connected to the data signal line 103 or the pixel electrode 111. Between the pixel electrode 111 and the counter electrode 112, an alignment-treated TN liquid crystal or the like is formed. The liquid crystal layer 113 is sandwiched, and a liquid crystal display can be performed by combining it with a polarizing plate or the like. The capacitor 114 is an additional capacitor that earns the holding time of the image signal.

FIG. 8 is a schematic diagram of drive waveforms of a conventional liquid crystal display device using TFTs. The drive waveform is shown focusing on one pixel in the screen. Reference numeral 120 is a scanning signal applied to the scanning line 102, 121 is a drive signal applied to the data signal line 103, and 122.
Is the potential of the counter electrode 112 (counter electrode potential). At the timing when the scanning signal 120 rises to a positive pulse, the pixel transistor 110 is turned on, a drive signal is written in the pixel electrode, and the pixel electrode potential 123 appears in the pixel electrode. At this time, the voltage Vsig is applied to the liquid crystal layer 113.

When the pixel transistor 110 is turned off at the end of the positive pulse period of the scanning signal 120, Vsi
g gradually decreases with time due to leakage current and dielectric relaxation of the liquid crystal layer. In the case of FIG. 8, a bright state in the period a,
The driving is performed so that the period b is a dark state and the period c is a bright state (brightness and darkness can be reversed by using a polarizing plate). During this period, the peripheral drive circuit (scanning circuit 104, data signal circuit 105
Etc.) are always working. T described above
For details of the liquid crystal display device using the FT, for example, “Flat Panel Display '91” (Nikkei BP,
80-96 pages (issued on November 26, 1990) and many other magazines and journals.

[0006] Such a liquid crystal display device has one of the outstanding characteristics that it has a significantly lower power consumption than a CRT monitor, which is a major reason why it is widely used in notebook personal computers and the like. Further, further reduction of power consumption is desired in applications where it is difficult to use a commercial power source, such as a personal portable terminal (PDA) which is required to be portable.

[0007]

However, the conventional TF
A liquid crystal display device using T requires a power of several 100 mW to several W even if it is a reflection type device without a backlight, and therefore the usable time in the battery is short, and the liquid crystal display device and peripheral circuits are required for full-scale spread. It is necessary to drastically reduce power consumption.

In a liquid crystal display device using a TFT, it is conceivable to intermittently operate a drive circuit in a conventional drive method in order to significantly reduce power consumption. FIG. 9 shows that when the conventional driving method whose driving waveform is shown in FIG.
The drive waveforms when the peripheral drive circuit is operated for the period of time are shown. That is, in the case of FIG. 9, after writing the data of one screen, the data of seven screens are not processed and ignored. Also in this case, the drive signal 131 is written to the pixel electrode when a positive pulse is applied to the scanning signal 130.
The pixel electrode potential 132 is gradually attenuated thereafter, and the image signal is not updated for a long time until the peripheral drive circuit operates next time, that is, until a positive pulse is applied to the scan line 130.
The voltage applied to the liquid crystal will vary greatly.

In such a situation, the following side effects occur, and the display is actually very poor. That is, (a) Vsig fluctuates, and since the polarity inversion period is long, significant flicker is observed on the screen. (B) Vsig
The contrast decreases due to the attenuation of. (C) Accurate AC driving of the liquid crystal is impaired and the life is shortened.

Although it depends on the display capacity, when writing a drive signal to a pixel in the form of sequentially scanning the screen, in order to process a normal image signal, a transfer rate of the image signal source equal to or higher than several MHz. High speed operation at 100 MHz is required for the peripheral drive circuit. As for the load at the time of writing, the wiring capacitance of the data signal line, which is about 10 to 100 times the pixel capacitance, is actually charged and discharged. Even if the display does not change in the conventional driving method, it is necessary to constantly operate the peripheral driving circuit to prevent flicker.

Therefore, the present invention has an object of providing a liquid crystal display device, a liquid crystal display system, and a computer system, which have a novel configuration different from that of the above-mentioned conventional technique and which greatly contributes to the reduction of power consumption of the device or system. And

[0012]

In order to achieve the above object, a liquid crystal display device according to the present invention is provided with a drive circuit for driving a latch circuit for storing an image data signal and a liquid crystal for each pixel constituting a display screen. A drive signal generating circuit for generating a signal is provided, and the drive signal generating circuit is configured to generate a drive signal whose polarity is inverted in a cycle of 20 ms or less based on the image data signal stored in the latch circuit. Has been done.

The first feature of the liquid crystal display system of the present invention using the liquid crystal display device as described above is that the scanning position is shifted in the operation of selecting the scanning line every m lines and writing the image data signal. However, by repeating m times, the image data signal is written in all the pixels of the display screen.

For example, since it is possible to display an NTSC system TV image which is interlaced for every other scanning line as it is, it is possible to reduce power consumption by lowering the signal writing frequency and to use a double speed conversion circuit. The power consumption can be reduced by eliminating the need. Hereinafter, description will be added in comparison with the conventional technique.

When an interlaced image signal such as an NTSC TV image signal is displayed on a liquid crystal display device,
Conventionally, in order to prevent flicker from occurring, as shown in FIG. 10 (a), an interlaced image signal is converted to a non-interlaced image signal by a double speed conversion circuit and then applied to a liquid crystal display device. That is, 512 in the NTSC system
There are two scanning lines, and an even field image signal composed of only even numbered scanning lines and an odd field image signal composed of only odd numbered scanning lines are alternately sent every 1/60 seconds. However, the double speed conversion circuit temporarily stores this field image signal in the frame memory and converts it into a non-interlaced image signal for every 1/30 second in which no interlaced scanning is performed. Since the peripheral portion of the screen is cut, the number of scanning lines actually displayed on the liquid crystal is 48.
It is about 0. As shown in FIG. 10B, the number of scanning lines is reduced to 1/2 without using the double speed conversion circuit, that is,
There is also a method of displaying two lines by considering one scanning line, but the resolution deteriorates as the number of scanning lines decreases.

In the liquid crystal display device of the present invention, since the image data signal stored in the latch circuit is not a direct signal for driving the liquid crystal, it is not necessary to give periodic polarity inversion to the image data signal stored in the latch circuit. Therefore, unlike the conventional case, it is possible to easily write the image data signal according to the interlaced image signal. That is, one set of even field image signals and odd field image signals may be written without changing the order of the scanning lines to be written. Thus, the power consumption can be reduced without sacrificing the resolution and eliminating the need for the double speed conversion circuit.

The second feature of the liquid crystal display system of the present invention is that the peripheral circuit relating to the writing of the image data signal is operated only during the operation period necessary for storing the image data signal in the latch circuit, and the peripheral circuit is stored in the latch circuit. The standby period for generating the drive signal based on the image data signal is that the peripheral circuits are stopped.

In the conventional liquid crystal display system, it is necessary to constantly move the peripheral drive circuit in order to prevent flicker even when there is no change in the display contents. According to the above configuration of the present invention,
The image data signal is stored in the latch circuit in the pixel, then the peripheral drive circuit is stopped, and the drive signal of the liquid crystal layer whose polarity is inverted is generated in parallel at each pixel level. This parallel processing is sufficient at a processing speed corresponding to several kHz to 10 kHz (a normal moving image display rewrites the screen at a cycle of 20 to 10 ms, so about 1% thereof is sufficient). Further, the rewriting load is almost equal to the pixel capacity, and the capacity load at the time of rewriting due to the polarity inversion of the drive signal for one screen is reduced to about 1/10 to 1/100 as compared with the conventional case.
For this reason, the power consumption during the stop period of the peripheral drive circuit is significantly reduced, and moreover, accurate AC drive can be performed at intervals at which flicker is less likely to be recognized. Such side effects do not occur.

Preferably, a change-over switch for changing the ratio between the operating period and the standby period of the peripheral circuit, and a control circuit for changing the ratio between the operating period and the standby period based on a signal from the change-over switch. Is equipped with. Alternatively, instead of the changeover switch, the computer unit may be configured to output a command for switching the ratio between the operation period and the standby period. If you want to display a still image or a slow-moving video, increase the standby period,
When displaying a moving image that changes rapidly, it is preferable that the waiting period be short or zero.

In this way, by switching the ratio of the operation period and the standby period according to the content of the image to be displayed,
It is possible to provide the user with a comfortable use environment while achieving low power consumption.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described below with reference to Examples and the drawings. FIG. 1 shows a circuit configuration of one pixel of the liquid crystal display device according to the present invention. A latch (memory) circuit 3 is connected to the scanning line (horizontal wiring) 1 and the data signal line (vertical wiring) 2. This latch circuit 3
Has a function of storing the image data signal applied to the data signal line 2 when the selection pulse is input to the scanning line 1. A drive signal generation circuit 4 is connected to the latch circuit 3, and the drive signal generation circuit 4 operates on the basis of the data stored in the latch circuit and the control signal line 5 (in some cases, a plurality of liquid crystal drive signals). Is generated and applied to the pixel electrode 6. The alignment-treated TN liquid crystal layer 8 is driven by the potential difference between the counter electrode 7 and the pixel electrode 6. When manufacturing a liquid crystal display device having such a circuit configuration, a CM formed on a glass substrate by a low temperature process (substrate heating temperature of 650 ° C. or lower)
With the OS polysilicon thin film transistor, each circuit element was built together with the peripheral drive circuit.

The latch circuit 3 is composed of a digital memory circuit. The drive signal generation circuit 4 includes an image data signal stored in the latch circuit 3, timing from the control signal,
The drive signal is generated based on the reference voltage. A liquid crystal display device is constructed by arranging the pixels having such a circuit configuration in a matrix like the configuration of the conventional example shown in FIG.

FIG. 2 shows drive waveforms (focusing on one pixel) of the liquid crystal display device configured as described above. Similar to FIG. 8 of the conventional example, the waveforms in the case of driving so as to be in a bright state in the period a, a dark state in the period b, and a bright state in the period c are schematically shown. In FIG. 2, scan signal 1
The signal 16 of the data signal line when the selection pulse (positive pulse) appears at 0 is stored in the latch circuit 3. 11 is a stored signal. In this embodiment, the H level digital signal is held in the dark state of the period b, and the L level digital signal is held in the bright state of the periods a and c.

The drive signal generating circuit 4 is provided with the latch circuit 3
A drive signal for applying a voltage to the liquid crystal layer is generated based on the image data signal 11 stored in. This drive signal is set every 1/60 seconds according to the NTSC system.
The polarity is inverted with respect to the counter electrode potential 13 at each timing of marking. That is, the pixel electrode potential at this time has a waveform like 12.

A circuit corresponding to the conventional peripheral driving circuit (such as the scanning circuit 104 and the data signal circuit 105 in FIG. 6) operating at a high speed in the MHz class writes one screen as in the case of FIG. It operates only for a period. In FIG. 2, the peripheral drive circuit is operated only during the operation period 14 which is 1/8 of the entire period, and the operation is stopped during the other period 15.

In the period 15 for stopping the peripheral drive circuit (that is, the standby period), when polarity inversion is performed at the timing of ↑, the rewriting of the drive signal is collectively processed in parallel for all the pixels in the screen (Equation 100). The signal is rewritten in microseconds, and high-speed operation in the MHz class is not required). In this way, the voltage Vsig applied to the liquid crystal is accurately AC-driven even during the period in which the peripheral drive circuit is stopped. Although not shown in the drawing, a liquid crystal display system was constructed by adding a stop mechanism of peripheral circuits and a control signal system for driving the liquid crystal display device.

By constructing the liquid crystal display device and the liquid crystal display system as described above and stopping the peripheral drive circuit during the period of 7/8 in the driving method, the power consumption could be greatly reduced. In addition, there were no problems such as an increase in flicker, a decrease in contrast, and a shortened life due to the stop of the peripheral drive circuit. Of course, it is not necessary to limit the stop period of the peripheral drive circuit to 7/8, and it can be lengthened or shortened depending on the application. If a circuit corresponding to the stop period is manufactured at the same time as the peripheral drive circuit by using the polysilicon thin film transistor circuit on the glass substrate, the system becomes very efficient.

Further, as a computer system equipped with the liquid crystal display system as described above, a personal portable terminal (PDA) as described below was constructed. In FIG.
The configuration of a portion related to the display system of the mobile terminal is shown in a block diagram. In addition to the above-described liquid crystal display device 20 shown in FIGS. 1 and 2 (which is of a reflective type that does not use a backlight), a control device 22 that can freely set a stop period of a peripheral drive circuit, an image signal source 21, Computer section 2
3 and a changeover switch 24 are provided. Although not shown in the figure, a communication function such as a telephone and a fax, a TV receiving function, and the like are further provided.

The control circuit 22 determines the length of the standby period of the peripheral drive circuit, that is, the operating period, according to the position of the changeover switch 24 that is switched according to the user's intention or the command output by the computer unit 23 based on the display content. Switch the ratio with the waiting period. As described above, the period in which the peripheral circuit is operated to store the image data signal in the latch circuit is the operation period, and the peripheral circuit is controlled by generating the drive signal based on the image data signal stored in the latch circuit. The standby period is the period in which the system is stopped. In this way, it is possible to provide the user with a comfortable use environment while achieving low power consumption.

FIG. 4 shows an example of changes in the ratio of the operating period to the standby period of the peripheral drive circuit in a certain usage example of the portable terminal as described above. In the figure, a series of operation modes is shown in the frame of 30 to 37, and the transition of the ratio of the operation period of the peripheral drive circuit and the standby period in each operation mode is shown in the graph below. 38 represents the ratio of the operation period, and 3
9 represents the ratio of the waiting period. The power consumption associated with the display changes substantially in proportion to the operating period ratio 38.

The operation modes will be described in order. First, the reception waiting mode 30 is in a state where no key operation is performed and is waiting for an incoming call such as a telephone call. At this time, since it is not necessary to display, the peripheral drive circuit is completely stopped.
The next telephone mode 31 is a mode for making a call, and the display screen displays data such as the telephone number of the call destination and the call time, and waits for activation of software such as a database. Image data at this time Is sufficient once, and a large waiting period can be taken. The following spreadsheet mode 32 is a mode on the assumption that spreadsheet software is activated according to the contents of the telephone. At this time, the computer unit slightly reduces the ratio of the waiting period of the peripheral drive circuit according to the type of software. In order to match the input speed, the image data is updated, for example, about 3 times / second.

The next TV reception mode 33 is a mode in which the TV display is switched to watch TV news. Since the power consumption increases when the moving image display of the TV is fully performed, the image data is normally set to be updated about 10 times per second. The next TV reception mode 34 is a mode in which the moving image display is fully performed. For example, when the user wants to watch news of particular interest, the user operates the changeover switch 24 to enter this mode.

The next document creation mode 35 is a mode in which the software of the word processor is activated to create a document. Similar to the spreadsheet mode 32, the computer unit 23 determines the ratio of the waiting period of the peripheral drive circuit according to the type of software. Further, when the input speed of the document exceeds a predetermined speed, the computer unit 23 outputs to the control device 22 a command to decrease the ratio of the standby period of the peripheral drive circuit (increase the ratio of the operation period). In FIG. 4, this command is output at the timing indicated by the ↓ mark. The created document is faxed in the next fax mode 36. The ratio of the waiting period of the peripheral drive circuit in this mode is telephone mode 3
It is set to the same level as 1. When the series of operations is completed, the display disappears and the reception waiting mode 37 is entered again.

As described above, the control circuit 22 switches the ratio between the operating period and the standby period of the peripheral drive circuit in accordance with the position of the changeover switch 24 or the command output from the computer unit 23 based on the display contents. Efficient display with low power consumption can be performed according to the operating status of the terminal (computer system). By using another contact switch such as a rotary type switch as a changeover switch or by using a key switch and an appropriate signal processing circuit, the ratio of the operation period of the peripheral drive circuit to the standby period is changed stepwise. It is also possible to carry out with only the changeover switch. On the contrary, all may be switched by a command of the computer section without using the changeover switch together.

In the case of the TV display mode 34, since the moving image display is fully performed, the power consumption is not reduced by the waiting period of the peripheral drive circuit, but in the case of the liquid crystal display device of the present invention, the interlace drive is performed. Since the display can be performed as it is, the display can be performed with a power consumption of about 1/2 that of the conventional display without lowering the resolution. That is, FIG.
As shown in FIG. 10, an NTSC system TV video signal can be displayed as it is from an image signal source on a liquid crystal display device. Therefore, the conventional double-quick conversion circuit shown in FIG. This is different from the configuration in which the number of scanning lines is reduced to half in (b).

[0036]

According to the present invention, it is possible to significantly reduce power consumption in a liquid crystal display device and a liquid crystal display system using a TFT, and to display in a computer system such as a mobile terminal equipped with the liquid crystal display system. The power consumption can be effectively reduced according to the content.
As a result, it is possible to realize a high-performance product such as a mobile terminal which can be operated by one charge for a long time.

[Brief description of drawings]

FIG. 1 is a circuit diagram of one pixel of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a drive waveform focusing on one pixel of the liquid crystal display device of FIG.

3 is a block diagram of a portion related to display of a mobile terminal using the liquid crystal display device of FIG.

FIG. 4 is a diagram showing a transition example of a ratio between an operation period and a standby period of a peripheral drive circuit of a liquid crystal display device in a usage example of the mobile terminal of FIG.

5 is a block diagram showing a configuration for displaying an NTSC TV image in an interlaced manner on the mobile terminal of FIG.

FIG. 6 is a circuit diagram of a main part of a conventional liquid crystal display device using a TFT.

FIG. 7 is a circuit diagram of one pixel of a conventional liquid crystal display device.

FIG. 8 is a schematic diagram of a drive waveform focusing on one pixel of a conventional liquid crystal display device.

FIG. 9 is a schematic diagram of drive waveforms when a peripheral drive circuit is intermittently operated in a conventional liquid crystal display device.

FIG. 10 is a block diagram showing a configuration for displaying an NTSC TV image in an interlaced manner in a conventional liquid crystal display device.

[Explanation of symbols]

 1 Scan Line 2 Data Signal Line 3 Latch Circuit 4 Drive Circuit Generating Circuit 5 Control Signal Line 6 Pixel Electrode 7 Counter Electrode 8 Liquid Crystal Layer 9 Common Electrode 10 Scan Signal 11 Image Data Signal 12 Stored in Latch Circuit 12 Image Electrode Potential 13 Counter Electrode potential 14 Peripheral drive circuit operation period 15 Peripheral drive circuit stop period 16 Data signal line signal 20 Liquid crystal display device 21 Image signal source 22 Control circuit 23 Computer section 24 Changeover switch 38 Peripheral drive circuit standby period ratio 39 Peripheral Percentage of operating period of drive circuit

Claims (6)

[Claims] 1. A latch circuit for storing an image data signal and a drive signal generating circuit for generating a drive signal for driving a liquid crystal are provided for each pixel constituting a display screen, and the drive signal generating circuit comprises: A liquid crystal display device configured to generate a drive signal whose polarity is inverted in a cycle of 20 ms or less based on an image data signal stored in the latch circuit. 2. An operation of using the liquid crystal display device according to claim 1, selecting every m scanning lines forming a display screen, sequentially writing image data signals, and writing the image data signal every m lines. , A liquid crystal display system configured to write an image data signal to all pixels of a display screen by repeating scanning m times while shifting the scanning position. 3. A liquid crystal display system using the liquid crystal display device according to claim 1, wherein a peripheral circuit relating to the writing of the image data signal is operated only during an operation period necessary for storing the image data signal in a latch circuit. A liquid crystal display system in which the peripheral circuit is stopped during a standby period in which a drive signal is generated based on an image data signal stored in the latch circuit. 4. A computer system equipped with the liquid crystal display system according to claim 3, wherein a changeover switch for changing the ratio of the operating period and the standby period of the peripheral circuit and a signal from the changeover switch are used. And a control circuit for switching the ratio between the operating period and the standby period. 5. A computer system equipped with the liquid crystal display system according to claim 3, wherein a computer section for outputting a command for switching a ratio between an operation period and a standby period of the peripheral circuit, and a changeover switch thereof. A computer system comprising a control circuit for switching the ratio between the operating period and the standby period based on the signal of. 6. The computer unit lengthens the waiting period when displaying a still image or a moving image that changes slowly,
The computer system according to claim 5, which is configured to output a command for shortening or zeroing the standby period when displaying a moving image that changes rapidly.