JP4659944B2 - Driving method of liquid crystal driving device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a driving method for driving a matrix type liquid crystal display device having scan electrodes and signal electrodes, and in particular, to improve non-uniformity of contrast in a liquid crystal display device having a duty number larger than the number of scan electrodes. The present invention relates to a method for driving a liquid crystal display device.
[0002]
[Prior art]
As a driving method of the liquid crystal display device, there are a static driving method and a time division driving method (matrix driving method). Even in a liquid crystal display device used in a small device such as a cellular phone, driving by a full dot matrix driving method has become mainstream as the display capacity of a display screen increases. The matrix driving method sequentially drives each scanning electrode in a liquid crystal panel provided with a plurality of linear scanning electrodes extending in the row direction and a plurality of signal electrodes extending in the column direction so as to intersect the scanning electrode group. In addition, a signal corresponding to a display image is applied to the signal electrode group.
[0003]
In general, each scanning electrode is selected line-sequentially, and a signal corresponding to a display image is applied to the signal electrode group in synchronization with the selection period of each scanning electrode. The ratio of the time during which one scan electrode is selected to the time (one frame period) from when the first scan electrode is scanned (selected) to when the first scan electrode is selected next is the duty ratio. (Hereinafter referred to as drive duty). Hereinafter, the reciprocal of the drive duty is referred to as the duty number. Usually, the number of scanning electrodes, that is, the number of scanning lines and the number of duties coincide.
[0004]
However, the number of duties may be greater than the number of scanning lines due to restrictions on the driving IC used and restrictions on the liquid crystal display screen or display control software. For example, when the drive duty of the drive IC is 1/128 but the number of scanning lines is 122, the duty number becomes larger than the number of scanning lines. In this case, during a period in which the actually existing scan electrode is not scanned (hereinafter referred to as a dummy scan period), a dummy signal corresponding to all on or all off is applied to each signal electrode.
[0005]
Incidentally, in recent years, the display capacity of liquid crystal display devices tends to increase further. As a result, the number of scan electrodes and the number of duties increase, and so-called high duty drive tends to be used. Further, from the viewpoint of low power consumption, low voltage liquid crystal tends to be adopted.
[0006]
[Problems to be solved by the invention]
However, when high-duty driving or low-voltage liquid crystal is used, non-uniform contrast, so-called display unevenness, is likely to occur. One cause of display unevenness is uneven current supply. When a voltage is applied to the liquid crystal display element, ionic impurities are biased in the liquid crystal. A phenomenon occurs in which the threshold voltage changes due to the bias of ionic impurities, which is uneven conduction. And the nonuniformity of contrast will be caused by the unevenness of energization. The unevenness of energization is more likely to occur at higher duty, lower voltage liquid crystal, and higher temperature.
[0007]
The present invention is an invention for solving the above-described problems, and in particular, in a liquid crystal display device in which the reciprocal of the drive duty is larger than the number of scanning lines, the non-uniformity of contrast generated during energization of the liquid crystal display element. It is an object to provide a driving method of a liquid crystal driving device capable of improving the above.
[0008]
[Means for Solving the Problems]
The driving method of the liquid crystal driving device according to the present invention is such that each scanning electrode of a liquid crystal panel having a plurality of scanning electrodes and a plurality of signal electrodes is selected and driven in a line-sequential manner, and the first scanning electrode is selected. Next, a simple matrix type liquid crystal display device in which the duty number which is the reciprocal of the ratio of the time for selecting one scan electrode to the time until the first scan electrode is selected is larger than the number of scan electrodes. In order to suppress the movement and uneven distribution of ionic impurities in the liquid crystal, it is necessary to always apply an ON signal and an OFF signal alternately to the signal electrode in the dummy scanning period. Features. According to such a driving method, the high-frequency component increases in the driving waveform applied to the liquid crystal panel, and the movement and uneven distribution of ionic impurities in the liquid crystal are suppressed. As a result, the occurrence of display unevenness due to this is suppressed. It is suppressed.
[0010]
The driving method of the liquid crystal drive device is such that an ON signal and an OFF signal are alternately applied to the signal electrodes, and an ON signal and an OFF signal are alternately applied between adjacent signal electrodes. You may drive so that. According to such a driving method, the movement of ionic impurities in the liquid crystal can be prevented two-dimensionally, and the display unevenness suppressing effect is further increased.
[0011]
The driving method of the liquid crystal driving device may be a method of alternating current with a predetermined cycle. In a drive system with a duty number of 64 or more, AC is generally performed for each frame, but in a drive system with a higher duty, AC is generally performed in a cycle shorter than the frame period. is there. Therefore, in particular, by performing alternating current at a predetermined cycle shorter than the frame period in the high duty driving method, the synergistic effect of the increase in driving frequency due to alternating current and the increase in frequency in the dummy scanning period is displayed. The unevenness suppressing effect is further increased.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1, the liquid crystal display device used in this embodiment has a pair of transparent substrates 2 and 3 each having transparent electrode groups 4 and 5 and alignment films 6 and 7 and arranged substantially in parallel. A nematic liquid crystal layer 8 containing an optical rotatory substance and having a positive dielectric anisotropy is sandwiched in a space formed by the peripheral sealing material 9, and at least one (two in this example) outside the liquid crystal layer 8. A liquid crystal panel 1 provided with polarizing plates 10 and 12 and a phase difference plate 11, and a driving circuit for displaying a display pattern by applying a voltage higher than the liquid crystal layer 8 to the transparent electrode groups 4 and 5. (Not shown).
[0013]
As shown in FIG. 2, a transparent substrate 2 on which a transparent electrode group 4 serving as a scanning electrode group (hereinafter simply referred to as scanning electrode group 4) is formed, and a transparent electrode group 5 serving as a signal electrode group (hereinafter simply referred to as signal). The transparent substrate 3 on which the electrode group 5 is formed) is overlaid so that the scanning electrode and the signal electrode are almost orthogonal to each other.
[0014]
Each scanning electrode is line-sequentially driven by the row driver 21, and each signal electrode is driven by the column driver 22. The row driver 21 sequentially drives each scan electrode in accordance with the latch pulse (LP) from the control unit 23, but the actual duty number is larger than the scan electrode number. The column driver 22 has a shift register that inputs image data for one row via the control unit 23, and the data in the shift register is applied to each signal electrode by a latch pulse from the control unit 23. . The column driver 22 applies a predetermined dummy signal to each signal electrode in a dummy scanning period in which no scanning electrode is scanned.
[0015]
Next, the operation will be described. In a simple matrix type liquid crystal display device, line-sequential driving is generally performed. For example, in the scan electrode group 4 shown in FIG. 2, each scan line (scan electrode) is scanned sequentially from the top to the bottom by the row driver 21. Then, image data corresponding to the selected scanning line is applied from the column driver 22 to each signal electrode. Normally, when scanning reaches the bottom scanning line, the top scanning line is scanned again. However, as described above, the duty number may be larger than the actual scanning line.
[0016]
For example, when the number of scanning electrodes is 122 and the duty number is 128, 6 selection periods after scanning of 122 scanning lines are dummy scanning periods. In the dummy scanning period, the column driver 22 applies a dummy signal to each signal electrode. The dummy signal is generated in the control unit 23, for example. FIG. 3 is an explanatory diagram in which virtual dummy scanning lines are clearly shown in order to facilitate understanding of the dummy scanning period. FIG. 3 illustrates six dummy scanning lines. The dummy scanning lines are only virtual and are not connected to the row driver 21.
[0017]
In this embodiment, as shown in FIG. 4, when the first, third and fifth dummy scanning lines (the first, third and fifth dummy scanning lines from the top in the dummy scanning period) are scanned, that is, In the odd-numbered period among the six selection periods in the dummy scanning period, the column driver 22 applies a voltage corresponding to OFF to each signal electrode. In addition, when the second, fourth, and sixth dummy scanning lines are scanned, that is, in the even-numbered period among the six selection periods in the dummy scanning period, the column driver 22 responds to each signal electrode being turned on. Apply the selected voltage. In FIG. 4, it is assumed that pixels indicated by diagonal lines are on-pixels. That is, in the dummy scanning period, a signal corresponding to OFF and a signal corresponding to ON are alternately applied to each signal electrode.
[0018]
FIG. 5A is a waveform diagram showing a voltage waveform applied to the liquid crystal display element when the signal application as shown in FIG. 4 is performed. FIG. 5B is a waveform diagram showing voltage waveforms applied to the liquid crystal display element when a signal corresponding to ON is always applied in the dummy scanning period, that is, when a conventional driving method is used. In FIG. 5, VD and VB indicate applied voltage levels. As shown in FIG. 5A, according to the driving method in this embodiment, a high frequency component is added in the dummy scanning period. FIG. 6A is a waveform diagram showing an example of a voltage applied to the scan electrode for generating the voltage waveform shown in FIG. 5A, and FIG. 6B is applied to the signal electrode. It is a wave form diagram which shows the example of a signal voltage waveform. In FIG. 6, V0 to V5 indicate applied voltage levels. Specific examples will be described below.
[0019]
[Example 1] As a liquid crystal cell, an STN element having a twist angle of 240 °, a dielectric anisotropy of +14.8, Δnd of 0.86, and a retardation plate 11 made of polycarbonate having a retardation value of 585 nm was used. A voltage as shown in FIG. 4 was applied to the liquid crystal display element using a monochrome STN panel. Scan electrode group 4 and signal electrode group 5 are each formed of ITO on a glass substrate. The number of dots in the display portion is vertical (scanning electrode side) 122 × horizontal (signal electrode side) 100. In addition, HD66750 manufactured by Hitachi, Ltd. was used as the driving IC. This drive IC has a duty number of 128. Accordingly, dummy scanning periods corresponding to six selection periods are generated.
[0020]
Using such a liquid crystal display element and a driving IC, driving was performed at a duty ratio of 1 / 128-bias ratio of 1/9 and a frame frequency of 70 Hz in an environment of 60 ° C. At this time, alternating current was performed for each frame, and the entire display image was turned on. Then, a dummy signal was applied during the dummy scanning period as shown in FIG.
[0021]
Then, the occurrence of display unevenness was prevented. The reason for this is thought to be that since a high frequency was applied during the dummy scanning period, the movement of ionic impurities in the liquid crystal was suppressed, and uneven conduction was prevented.
[0022]
[Comparative Example] Under the liquid crystal display element, the driving IC and the environment used in Example 1, as in the conventional driving method, that is, as shown in FIG. Applied. Then, severe display unevenness occurred after about 24 hours.
[0023]
[Example 2] A checkered pattern signal as shown in FIG. 8 was applied to each signal electrode in the dummy scanning period under the liquid crystal display element, driving IC and environment used in Example 1. That is, in the odd-numbered (1, 3, 5th) period among the 6 selection periods in the dummy scanning period, a voltage corresponding to OFF is applied to the odd-numbered (1, 3,..., 99th) signal electrode. The voltage corresponding to ON was applied to the even-numbered (2, 4,..., 100th) signal electrodes. In FIG. 8, it is assumed that pixels indicated by diagonal lines are on-pixels. In addition, in the even-numbered (2, 4, 6th) period of the six selection periods in the dummy scanning period, a voltage corresponding to ON is applied to the odd-numbered signal electrode, and OFF is applied to the even-numbered signal electrode. A voltage was applied.
[0024]
Then, display unevenness was prevented. In this case, since the driving voltage is applied to the liquid crystal display element at a higher frequency in the dummy scanning period, the movement of the ionic impurities in the liquid crystal is further suppressed, and the effect of suppressing the unevenness in conduction is further increased.
[0025]
[Example 3] Under the liquid crystal display element, the driving IC and the environment used in Example 1, in the dummy scanning period, as shown in FIG. 9, the AC conversion cycle is 32 rows, and the signal of the pattern shown in FIG. Applied to the electrode. Also in this case, unevenness in energization was suppressed and occurrence of unevenness in display was prevented. In this example, the non-uniformity of current supply can be almost completely prevented by the synergistic effect of the low frequency suppression by alternating current and the high frequency in the dummy scanning period.
[0026]
As described above, in order to prevent display unevenness, it is effective to increase the high frequency component in the frequency of the drive voltage applied to the liquid crystal display element. This is because, as described above, according to the high-frequency driving waveform, it is considered that the mobility of the ionic impurities that cause the unevenness in energization is lowered. In this embodiment, the dummy signal applied to each signal electrode is devised so that the drive waveform in the dummy scanning period has a high frequency. Therefore, the high frequency component of the drive waveform for the liquid crystal display element is increased, and the movement and uneven distribution of ionic impurities in the liquid crystal can be suppressed. As a result, the occurrence of display unevenness due to uneven current supply is prevented.
[0027]
A driving method for driving a liquid crystal display device in which each scanning electrode of a liquid crystal panel having a plurality of scanning electrodes and a plurality of signal electrodes is selected and driven line-sequentially and has a higher duty number than the number of scanning electrodes. In a driving method in which a signal that causes the liquid crystal driving waveform to be high frequency is applied to the signal electrode during a period in which no scan electrode is selected, if the AC timing is every frame, suppression of unevenness in energization The effect is great. When the AC timing is every frame, as shown in FIG. 5B, the drive waveform tends to be the lowest frequency. That is, ionic impurities are likely to move. If the driving method according to the present invention is applied to such a driving system, the effect of suppressing the unevenness in energization increases.
[0028]
In addition, when the driving method according to the present invention is applied to the driving method of the liquid crystal display device having a duty number of 64 or more, the effect of suppressing the unevenness of energization is still great. This is because unevenness in energization tends to occur as the duty becomes higher. In particular, in a driving method of a liquid crystal display device having a duty number of 64 or more, the effect of suppressing the unevenness in energization becomes greater as the alternating period is shorter.
[0029]
Further, when the liquid crystal display device uses an STN liquid crystal element, the STN type liquid crystal display device is widely used in a portable terminal such as a mobile phone because it can be driven at high cost at a low cost. In such a widely used mobile terminal, display unevenness is prevented. That is, since display unevenness is prevented in a liquid crystal display device supplied in large quantities on the market, it is meaningful to apply the present invention to an STN type liquid crystal display device.
[0030]
It is also effective to apply the present invention to a liquid crystal display having a liquid crystal display element having a threshold voltage of 2 V or less. This is because, in general, a low voltage liquid crystal display element having a threshold voltage of 2 V or less contains a large amount of ionic impurities that cause unevenness in conduction, and thus the effect of the present invention is remarkably exhibited.
[0031]
In the above embodiment, linear scanning electrodes and signal electrodes are used. However, the present invention can be applied even if the shape of these electrodes is not linear. In addition, the arrangement is not limited to the case where they are orthogonal to each other, and the present invention can be applied to a modified arrangement. Furthermore, in a liquid crystal driving device in which a portion other than the matrix driving portion, for example, an icon portion that is driven by a stick, is provided on the liquid crystal panel, the present invention can be applied to the matrix driving portion.
[0032]
In the above embodiment, the scanning electrodes are scanned from the top to the bottom of the display panel. However, the scanning direction is not limited to such a direction, and may be, for example, a direction from the bottom to the top or a left-right direction. Regarding the dummy scanning period, it is preferable to insert the dummy scanning period into the actual scanning period from the viewpoint of the effect of suppressing the low-frequency components, not after the scanning of all the scanning electrodes is completed.
[0033]
Furthermore, although the STN type liquid crystal display device is used in the above embodiment, the present invention is applied to a TN type, a STN type that does not use a retardation plate, and a two-layer cell type. be able to.
[0034]
【The invention's effect】
As described above, according to the present invention, there is an effect that unevenness in energization is suppressed by effectively utilizing the dummy scanning period, and as a result, display unevenness is prevented. In the present invention, since the signal waveform in the dummy scanning period is devised, optical characteristics such as contrast and response of the display unit are hardly affected. In particular, it is possible to suppress display unevenness that is likely to occur when the drive duty is increased in order to increase the display density of a high duty drive unit or when a liquid crystal having a low threshold voltage is used to reduce power consumption. Provided is a driving method suitable for a liquid crystal display device used for a purpose of reducing power consumption of a portable terminal such as a cellular phone.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an example of a liquid crystal panel.
FIG. 2 is a block diagram showing a scan electrode and a signal electrode together with a drive circuit.
FIG. 3 is an explanatory diagram for explaining a dummy scanning period.
FIG. 4 is an explanatory diagram for explaining an example of a driving method of a liquid crystal display device according to the present invention.
FIG. 5 is a waveform diagram showing an example of a drive waveform.
FIG. 6 is a waveform diagram showing an example of applied voltages to scan electrodes and signal electrodes.
FIG. 7 is an explanatory diagram for explaining a comparative example of a driving method of a liquid crystal display device.
FIG. 8 is an explanatory diagram for explaining another example of a method for driving a liquid crystal display device according to the present invention.
FIG. 9 is a waveform diagram showing another example of a drive waveform.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Liquid crystal panel 2, 3 Transparent electrode 4 Scan electrode group 5 Signal electrode group 6, 7 Alignment film 8 Liquid crystal layer 21 Row driver 22 Column driver 23 Control part

Claims (3)

Each scan electrode of the liquid crystal panel having a plurality of scan electrodes and a plurality of signal electrodes is selected and driven line-sequentially, and after the first scan electrode is selected, the first scan electrode is then selected. In a driving method for driving a simple matrix type liquid crystal display device in which the number of duties, which is the reciprocal of the ratio of the time for selecting one scan electrode to the time until the time until, is larger than the number of scan electrodes,
In order to suppress the migration and uneven distribution of ionic impurities in the liquid crystal, an on signal and an off signal are always applied alternately to the signal electrode during a period when no scan electrode is selected. A driving method of a liquid crystal driving device. When no scanning electrode is selected, an off signal is applied when an on signal is applied to the adjacent signal electrode, and an on signal when an off signal is applied to the adjacent signal electrode the driving method of the liquid crystal drive apparatus of claim 1, wherein but for applying a signal to the signal electrodes as is applied. The driving method of the liquid crystal drive apparatus of claim 1 or claim 2, wherein performing alternating at a predetermined period.

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