JP7633898B2 - Liquid crystal display device - Google Patents

Description

The present invention relates to a liquid crystal display device.

Liquid crystal display devices have the advantage of being able to be made thin and compact, and can be operated with low power consumption, so in recent years they have been widely used as image display devices in the field of mobile devices and small electronic devices such as projectors, viewfinders, HMDs, and HUDs.

In a liquid crystal display device that displays images using field sequential driving, three light sources, red (R), green (G), and blue (B), are sequentially selected to emit light, and the driving of the liquid crystal panel is controlled in synchronization with the light, to display a color image. (See, for example, Patent Document 1.)

In liquid crystal display devices that display images using field sequential driving, for example, liquid crystal panels using ferroelectric liquid crystals (ferroelectric liquid crystal panels) are used. When a certain ON voltage is applied to a ferroelectric liquid crystal panel, it transmits the irradiated light, and when a certain OFF voltage (for example, a voltage that has the same amplitude as the ON voltage but the opposite polarity) is applied, it blocks the irradiated light, thereby displaying an image.

Ferroelectric liquid crystal panels are suitable for field sequential driving because their optical characteristics change rapidly depending on the applied voltage.

Figure 1 is a cross-sectional view showing an example of a liquid crystal panel. The liquid crystal panel shown in Figure 1 includes a transparent glass substrate 1, an ITO film 2, an organic alignment film 3 formed on the ITO film 2, a silicon substrate 4, a pixel electrode 5, an organic alignment film 6 and a SiO thin film 7 formed on the pixel electrode 5, and a liquid crystal 8 filled between the glass substrate 1 and the silicon substrate 4.

In this liquid crystal panel, before a voltage is applied to the liquid crystal 8, the orientation of the liquid crystal molecules is aligned in one direction by the orientation control force of the organic alignment film 3 and the organic alignment film 6. Normally, when a voltage is applied to the liquid crystal 8, the orientation of the liquid crystal molecules changes (switches), and when the voltage is no longer applied to the liquid crystal 8, the orientation of the liquid crystal molecules returns to the state where they were aligned in one direction.

JP 2014-182157 A

However, in cases where the alignment control force of the alignment film is weak, even when the liquid crystal display device is turned off (shutdown or sleep state) and no voltage is applied to the liquid crystal, the liquid crystal molecules may not align in one direction (sometimes called double domain). This causes a problem in that the image that was displayed before the power was turned off remains displayed on the liquid crystal panel as an afterimage (also called a ghost), even though no voltage is being applied to the liquid crystal.

The present invention aims to provide a liquid crystal display device that can suppress the occurrence of image retention.

A liquid crystal display device having a liquid crystal panel that displays one image by field sequential driving in one frame period unit including a blanking period for applying a reset voltage to a liquid crystal,
The liquid crystal display device extends the blanking period included in one frame period immediately before the power supply of the liquid crystal display device is turned off.

The one frame period during which the blanking period is extended may be only the one frame period immediately before the power supply to the liquid crystal display device is turned off.

The liquid crystal display device may include a plurality of blanking periods in one frame period during which the blanking period is extended, and the blanking period is extended in each of the plurality of blanking periods.

The liquid crystal display device may display a black image on the entire screen of the liquid crystal panel during the period from when the blanking period is extended to when the liquid crystal display device is turned off.

The liquid crystal display device may display the black image for a period of time equal to or longer than the length of one field period included in one frame period in which the blanking period is not extended.

The liquid crystal display device may be one in which the absolute value of the voltage applied to the liquid crystal panel when displaying the black image is greater than the absolute value of the voltage applied to the liquid crystal panel during one frame period in which the blanking period is not extended.

According to the present invention, it is possible to prevent the image displayed before the power is turned off from appearing as an afterimage after the power is turned off.

A cross-sectional view showing an example of a liquid crystal panel. 1 is a timing chart showing a driving waveform during an n-th frame period of a liquid crystal display device according to an embodiment of the present invention; A flowchart showing the operation steps of a liquid crystal display device according to an embodiment of the present invention. 1 is a timing chart showing a driving waveform for the n+1th frame of a liquid crystal display device according to an embodiment of the present invention;

The following describes an embodiment of the present invention.

The liquid crystal display device in the embodiment of the present invention includes a light source such as an LED, a liquid crystal panel such as a ferroelectric liquid crystal panel, and a drive circuit for driving the light source and the liquid crystal panel. The liquid crystal panel is driven using a field sequential method and is configured to display one image per frame period. When displaying a color image, three field periods are provided in which images corresponding to the respective colors of RGB are displayed, and one color image is displayed in these three field periods. Specifically, three light sources that emit light of each color of RGB are provided, and the liquid crystal panel is illuminated from the front or rear by the light of each color emitted from these three light sources. For example, the liquid crystal panel is irradiated with light of each color in the order of R (red), G (green), and B (blue), and this cycle is repeated in the same manner per frame period. The liquid crystal panel changes the optical modulation state of the liquid crystal in synchronization with this cycle.

Figure 2 is a timing chart showing the driving waveforms in the nth frame period of a liquid crystal display device in an embodiment of the present invention. In the field sequential driving shown in Figure 2, one frame period is made up of six field periods: an active period "R", a passive period "r", an active period "G", a passive period "g", an active period "B", and a passive period "b".

In FIG. 2, the period marked "R" represents an active period corresponding to the lighting period of the R light source, the period marked "G" represents an active period corresponding to the lighting period of the G light source, the period marked "B" represents an active period corresponding to the lighting period of the B light source, the period marked "r" represents a passive period corresponding to the extinguishing period of the R light source, the period marked "g" represents a passive period corresponding to the extinguishing period of the G light source, and the period marked "b" represents a passive period corresponding to the extinguishing period of the B light source. In each of the "R", "G", and "B" periods, a display drive is performed in which a predetermined voltage is applied to the liquid crystal panel to display an image, and in each of the "r", "g", and "b" periods, a non-display drive (inversion drive) is performed in which an inverted voltage of the voltage applied to the liquid crystal in the previous active period is applied to the liquid crystal panel.

At the beginning of each of the "R", "G", and "B" periods, there is a blanking period "Blanking (+)" in which a positive (+) reset voltage is applied to the liquid crystal with all light sources turned off, and at the beginning of each of the "r", "g", and "b" periods, there is a blanking period "Blanking (-)" in which a negative (-) reset voltage is applied to the liquid crystal with all light sources turned off. By providing such blanking periods in each field period, the liquid crystal molecules are able to move more easily during each field period, preventing image burn-in.

In particular, in the field sequential drive shown in Figure 2, within one frame period, there are three blanking periods "Blanking (+)" in which a positive (+) reset voltage is applied to the liquid crystal, while there are also three blanking periods "Blanking (-)" in which a negative (-) reset voltage is applied to the liquid crystal. This ensures a balance between the positive and negative energies applied to the liquid crystal during the blanking periods, making image burn-in less likely to occur.

Figure 3 is a flow chart showing the operation steps of a liquid crystal display device in an embodiment of the present invention. In the embodiment of the present invention, the generation of residual images after the power is turned off is suppressed by going through the operation steps (S1 to S6) shown in Figure 3 and turning off the power of the liquid crystal display device. First, when the power of the liquid crystal display device is turned on (S1), the liquid crystal display device repeats field sequential driving as shown in Figure 2 in response to a video signal input from an external device to sequentially display normal images on the liquid crystal panel (S2). Next, when a power-off command signal is input from the external device at any timing (S3), the liquid crystal display device displays the final image just before the power is turned off in the immediately following frame period (n+1th frame period) (S4). Next, the liquid crystal display device displays a black image on the liquid crystal panel (S5). Finally, the power of the liquid crystal display device is turned off (S6). S4 to S6 are described in more detail below.

<About S4>
4 is a timing chart showing drive waveforms in the n+1-th frame period of the liquid crystal display device according to the embodiment of the present invention. In the n+1-th frame period, field sequential driving is performed similarly to the n-th frame period, but the difference is that each blanking period is extended longer than the blanking period of the n-th frame period.

The length by which the blanking period is extended can be selected appropriately depending on the characteristics of the liquid crystal panel, etc. The length by which the blanking period is extended may be the same for all blanking periods within the n+1th frame period, or may be different for each blanking period. However, in this embodiment, in order to balance the positive and negative energies between the blanking periods, it is preferable that the blanking periods after extension have the same length. In any case, by extending the blanking period, the liquid crystal molecules are more likely to align in one direction, thereby suppressing the occurrence of afterimages.

To extend the blanking period, it is necessary to extend the entire length of the n+1th frame period by the amount of extension, or to allocate a part of the field period to the amount of extension without changing the entire length of the n+1th frame period. By doing this, the image quality of the image displayed in the n+1th frame period may change slightly, but since the n+1th frame period is only the last frame period before the power is turned off, even if the image quality changes slightly, it is not a particular problem. In addition to the n+1th frame period, it is possible to extend the blanking period in a frame period before the n+1th frame period (for example, the nth frame period). However, if the blanking period is extended for more frame periods than necessary, the overall image quality may change significantly, causing problems such as a sense of discomfort to the user, so it is preferable to extend the blanking period only in the n+1th frame period immediately before the power is turned off.

<About S5>
In a certain period immediately after the end of the n+1th frame period, a driving operation (similar driving operation to the "Blanking (-)" period) is performed to display a black image on the entire screen of the liquid crystal panel, thereby ensuring that the liquid crystal molecules are aligned in one direction. Image data for displaying a black image is read out, for example, from a memory built into the liquid crystal display device. In the period in which a black image is displayed, with all light sources turned off, for example, a negative voltage having the same amplitude (absolute value) as the voltage applied to the liquid crystal in the nth frame period is applied for a period of time equal to or longer than the length of one field period included in the nth frame period (for example, 1 ms or more). Note that a negative voltage having a larger amplitude (absolute value) may be applied for a longer period of time (for example, 50 ms or more). Here, it is more effective to apply a voltage with a larger amplitude (absolute value) for a longer period of time, but this is not limited to the above.

<About S6>
After the black image is displayed, the power supply to the liquid crystal display device is turned off to stop driving the liquid crystal panel. The above operation efficiently aligns the liquid crystal molecules in one direction, thereby effectively suppressing the occurrence of afterimages.

In this embodiment, a drive is performed to display a black image after the n+1th frame period has ended, but this drive is not essential and can be omitted. However, it is preferable to perform this drive in order to effectively suppress the occurrence of afterimages.

In the n+1th frame period, all blanking periods are extended, but this is not essential and only some of the blanking periods may be extended. However, in order to effectively suppress the occurrence of afterimages, it is preferable to extend all blanking periods.

The blanking period may be placed at the end of each field period instead of at the beginning of each field period.

The present invention can also be applied to liquid crystal display devices using field sequential driving in double speed modes such as 2 to 5 times speed.

The present invention can also be applied to liquid crystal display devices that only display black and white images.

The present invention is not limited to the above embodiments and may take various other forms.

REFERENCE SIGNS LIST 1 Glass substrate 2 ITO film 3 Organic alignment film 4 Silicon substrate 5 Pixel electrode 6 Organic alignment film 7 SiO thin film 8 Liquid crystal

Claims (4)

A liquid crystal display device having a liquid crystal panel that displays one image by field sequential driving in one frame period unit including a blanking period for applying a reset voltage to a liquid crystal,
extending the blanking period included in one frame period immediately before the power supply of the liquid crystal display device is turned off ;
The one frame period during which the blanking period is extended is only a one frame period immediately before the power supply of the liquid crystal display device is turned off.
A liquid crystal display device comprising: 2. The liquid crystal display device according to claim 1 , wherein one frame period during which the blanking period is extended includes a plurality of the blanking periods, and the blanking period is extended in each of the plurality of blanking periods. A liquid crystal display device having a liquid crystal panel that displays one image by field sequential driving in one frame period unit including a blanking period for applying a reset voltage to a liquid crystal,
extending the blanking period included in one frame period immediately before the power supply of the liquid crystal display device is turned off;
displaying a black image on the entire screen of the liquid crystal panel during a period from when the one frame period in which the blanking period is extended ends until when the power supply of the liquid crystal display device is turned off;
displaying the black image for a period of time equal to or longer than one field period included in one frame period in which the blanking period is not extended;
A liquid crystal display device comprising: A liquid crystal display device having a liquid crystal panel that displays one image by field sequential driving in one frame period unit including a blanking period for applying a reset voltage to a liquid crystal,
extending the blanking period included in one frame period immediately before the power supply of the liquid crystal display device is turned off;
displaying a black image on the entire screen of the liquid crystal panel during a period from when the one frame period in which the blanking period is extended ends until when the power supply of the liquid crystal display device is turned off;
an absolute value of a voltage applied to the liquid crystal panel when displaying the black image is greater than an absolute value of a voltage applied to the liquid crystal panel during one frame period in which the blanking period is not extended;
A liquid crystal display device characterized by the above-mentioned.

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