JP2007052338A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device with high picture quality that exhibits no decrease in the luminance and NTSC (National Television System Committee) ratio and is free from color mixing at a low temperature, and to provide a liquid crystal display device with high picture quality that can reduce power consumption at a high temperature. <P>SOLUTION: Data of application voltages and pulse widths of an LED are preliminarily measured at a plurality of temperature points so as to obtain a NTSC ratio and luminance at a low temperature almost equal to those at normal temperature, and the data are memorized in a non-volatile memory device. Prior to displaying an image, the environmental temperature is measured, and the LED is activated to emit light at appropriate application voltage and pulse width of the LED based on the data according to the environmental temperature. Data of application voltages and pulse widths of the LED are also preliminarily measured at a plurality of temperature points so as to obtain a NTSC ratio and luminance at a high temperature almost equal to those at normal temperature, and the data are memorized in a non-volatile memory device. Upon practical driving, the LED is activated to emit light at appropriate application voltage and pulse width on the LED based on the data according to the environmental temperature. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a display device used for a portable game machine, a car navigation system, a portable television, a cellular phone, a portable information terminal (PDA), etc., and more particularly to a color liquid crystal display device suitable for high-quality display. is there.

  In a conventional color liquid crystal display device, a backlight using a white LED as a light source is provided on the back of a liquid crystal panel having color filters of three primary colors of red, blue, and green. White light that has passed through the color filter appears to have three primary colors of red, blue, and green, which is the principle of the color liquid crystal panel. However, in the method using a conventional color filter, white light emitted from the LED passes through the color filter, and thus the luminance is reduced to about 1/3. For this reason, the brightness of the display image is reduced. Further, since three sub-pixels for each RGB are required for each pixel, it has been an adverse effect of a high-density screen. On the other hand, the field sequential method employs a system in which LEDs of three primary colors are used as a backlight to emit light sequentially in red, blue, and green, and the liquid crystal is turned on / off in accordance with the light emission of the LED. Since no color filter is used, the luminance is significantly increased over the conventional method. Furthermore, since no subpixel is required, it is possible to display all three primary colors of red, blue, and green with one pixel, so that the pixel density is three times that of the conventional one. Therefore, a field sequential display screen can provide an image with high pixel density and luminance.

In the conventional field sequential type liquid crystal display device, there are methods such as increasing the power efficiency by shortening the red light emission pulse width, increasing the peak luminance, and increasing the green and blue light emission pulse width accordingly. there were. Moreover, there existed a system which changes LED light emission start time with temperature.
Japanese Patent Laid-Open No. 2003-44016 Japanese Patent Laid-Open No. 2005-184020

  In the conventional liquid crystal display device, the reaction speed of the liquid crystal is worse than that at room temperature at a low temperature, both the fall time and rise time of the liquid crystal transmittance are slow, and the maximum transmittance is lower than that at room temperature. Therefore, white balance adjustment is insufficient when only the LED drive voltage is adjusted at low temperatures. As a result, when white is displayed at a low temperature, it becomes bluish, and the NTSC ratio is also lower than that at room temperature. In addition, since the response of the liquid crystal becomes slow when displaying a single color such as red at low temperatures, after the LED light emission in the single color field is finished, the LED in the next field emits light without the liquid crystal transmittance being lowered. As a result, color mixing was caused.

  At a high temperature, the rise time and fall time of the liquid crystal transmittance are shortened and become steep. For this reason, the time during which the LED light is transmitted increases, and the luminance becomes higher than that at room temperature. For this reason, the current flowing through the LED has not been used efficiently.

  Accordingly, an object of the present invention is to provide a high-quality liquid crystal display device in which luminance and NTSC ratio do not decrease at low temperatures and color mixing does not occur, and a high-quality liquid crystal display device that can reduce power consumption at high temperatures.

  In order to solve the above problems, a liquid crystal display device that displays the shutter function of the liquid crystal panel and the light emission of the three primary color LED light sources is connected to a temperature detector that detects the environmental temperature, and the three primary color LED light sources are connected. By controlling the driving conditions, the change in transmittance due to the temperature of the liquid crystal panel was corrected.

  Here, when the temperature detected by the temperature detector is lower than room temperature, the brightness of the red LED of the three primary color LED light sources is increased to correct the blueness of the liquid crystal panel.

  Alternatively, when the temperature detected by the temperature detector is lower than room temperature, the brightness of the blue LED of the three primary color LED light sources is lowered to correct the blueness of the liquid crystal panel.

  Alternatively, when the temperature detected by the temperature detector is lower than the room temperature, the light emission pulse width of the three primary color LED light sources is shortened to correct the change in transmittance due to the temperature of the liquid crystal panel.

  Alternatively, when the temperature detected by the temperature detector is higher than room temperature, the emission pulse width of the three primary color LED light sources is lengthened to correct the change in transmittance due to the temperature of the liquid crystal panel.

  Alternatively, it has a non-volatile memory element in which LED emission pulse width data of three primary colors for maintaining the same brightness and NTSC ratio as normal temperature at a plurality of points in the operating temperature range, and according to the temperature detected by the temperature detector. Thus, the LED light emission pulse width of the three primary colors is changed based on the LED light emission pulse width data written in the nonvolatile memory element, and the change in transmittance due to the temperature of the liquid crystal panel is corrected.

  The conventional liquid crystal display device has a problem that the transmittance of the liquid crystal is lowered at a low temperature, the NTSC ratio of the displayed image is lowered, and the luminance is further lowered. Therefore, in the present invention, the emission pulse width of each field of the backlight and the current flowing to the LED are changed according to the temperature at a low temperature. As a result, it is possible to maintain a display image having a high NTSC ratio and a high luminance without color mixing as at room temperature.

  On the other hand, when the temperature is high, the rising and falling waveforms of the transmittance of the liquid crystal become steeper than at normal temperature, and the opening time of the liquid crystal becomes longer. As a result, the luminance tends to be higher than that at normal temperature. Therefore, it is possible to reduce the LED current consumption by extending the LED light emission pulse width and lowering the voltage applied to the LED.

  A basic configuration of the liquid crystal display device 1 of the present invention is shown in FIG. The liquid crystal display device 1 of the present invention drives a backlight 3 having LEDs of three primary colors that illuminate the liquid crystal panel 2 from the back, a source driver 6 that drives the liquid crystal panel 2, a gate driver 7, and an LED of the backlight 3. LED driver 4 to be operated, temperature sensor 10 for measuring the environmental temperature connected to the LED driver 4, a non-volatile memory 8 for storing liquid crystal driving conditions and the like, and a power supply circuit for maintaining the power supply voltage 8. It is comprised from I / O11 which connects the liquid crystal display device 1 with the exterior. Further, the liquid crystal display device 1 is connected to an external MPU 5 and is controlled and driven by a signal from the MPU 5. Note that there is no problem even if the source driver 6 and the gate driver 7 are integrated in one driver. In addition, the liquid crystal panel may be a TN type, or another liquid crystal element in which liquid crystal molecules have high responsiveness.

  The driving method of the liquid crystal display device 1 at normal temperature will be described in detail with reference to FIG. FIG. 3A shows the voltage level applied to the source electrode, FIG. 3B shows the liquid crystal transmittance, and FIG. 3C shows the voltage level applied to the LED of the backlight 3. In this embodiment, it is normally black. Although the voltage level applied to the source electrode is described with reverse polarity for the sake of explanation, the transmittance of the liquid crystal is actually increased when no voltage is applied. At room temperature, when a voltage is applied to the source electrode, the voltage increases to a level close to 100% over a certain time t1, and when the source electrode is turned off (black reset), the transmittance decreases. The original value returns to the steady state over a certain time t2. On the other hand, for the LED, the voltage is applied to the LED after applying the voltage to the source electrode. Since the LED is lit during the period when the liquid crystal is open and the transmittance is high, the light emitted from the LED is transmitted from the liquid crystal panel 2. A pulse is applied to the source electrode for each field. Since the LED drive voltage Vf depends on conditions such as the type of liquid crystal and the transmittance, it must be adjusted so that high brightness and a high NTSC ratio can be maintained. An arbitrary image can be displayed on the liquid crystal panel 2 by sequentially lighting the LEDs of the three primary colors of green, blue, and red using the driving method described above.

  Next, the transmittance characteristics of the liquid crystal at a low temperature will be described with reference to FIG. 4 (1) shows the voltage applied to the source electrode, FIG. 4 (2) shows the liquid crystal transmittance, and FIG. 4 (3) shows the voltage applied to the LED before the change according to this embodiment. Represents the change with time of the voltage applied to the LED after the change. In this embodiment, it is normally black. Although the voltage level applied to the source electrode is described with reverse polarity for the sake of explanation, the transmittance of the liquid crystal is actually increased when no voltage is applied. In FIG. 4, t1 represents the time from when the source voltage is applied until the liquid crystal transmittance becomes the highest. Also, TR in FIG. 5 is the maximum value of the transmittance of the liquid crystal when a voltage is applied to the source electrode. It can be seen from FIG. 4 that the response of the liquid crystal transmittance is slow compared to the normal temperature, the rise time and the fall time are slow, and the transmittance is also low as a whole. In addition, when it is desired to light a monochromatic field, the fall time of the liquid crystal becomes long in the monochromatic field. For this reason, when the liquid crystal transmittance is not sufficiently lowered, the LED in the next field emits light, which causes color mixing. As a result, the NTSC ratio and the brightness are lowered at low temperatures, which causes a reduction in the image quality of the display image on the liquid crystal panel 2. In addition, the liquid crystal may become bluish on the low temperature side. FIG. 2 is a graph showing the spectral transmittance. In the vicinity of 25 ° C. at room temperature, there is a maximum transmittance in the visible light range, about 470 nm. However, since the maximum value of visible light is shifted to 400 nm or less at around −20 ° C. at low temperatures, the entire liquid crystal panel 2 may be bluish due to all white display or the like.

  At low temperatures, the transmittance of the liquid crystal decreases, the rise and fall times of the transmittance increase, and color mixing may occur. Further, at low temperatures, the liquid crystal panel 2 has a characteristic of being bluish. Therefore, in the case of all red display or the like, in order to transmit the light of the LED effectively in the time when the transmittance of the liquid crystal is high as shown in FIG. When the liquid crystal transmittance is high, the LED is turned on, and when the liquid crystal transmittance is not sufficiently lowered, the LED light emission pulse width is shortened so that the LED in the next field does not emit light and applied to the LED. The voltage to be increased. Further, as shown in FIG. 5 (3) showing the time-dependent change of the LED applied voltage in this embodiment at the time of all white display, in order to prevent the liquid crystal panel 2 from becoming bluish, the light emission time of the blue LED is set from other fields. It was decided to shorten it. Further, the same effect can be obtained by lowering the voltage applied to the blue LED, making the light emission time of the red LED longer than other fields, or setting the voltage applied to the red LED higher. By doing so, high luminance can be maintained without color mixing in one field at low temperatures and without being bluish. By such a driving method, it is possible to prevent deterioration in image quality at low temperatures.

In this example, data was previously collected so that an NTSC ratio of 90% and a luminance of 200 cd / m ² could be realized within the operating temperature range. Table 1 shows a temperature table created by obtaining the LED light emission pulse width and the LED drive voltage Vf at intervals of 5 ° C. within the operating temperature range.

さらに、温度テーブルをＥＥＰＲＯＭ９のような不揮発性の記憶素子に記録した。実際ＥＥＰＲＯＭに書き込むデータは１６進数表示になっている事があるが、説明の為に１０進数で記述した。液晶表示装置１は、画像を液晶パネル２に表示する前に温度センサー１０で環境温度を測定して、ＥＥＰＲＯＭに書き込まれた温度テーブルから環境温度に適切な駆動条件を入手して、高画質が得られる駆動方法で画像を表示することが可能となる。

Further, the temperature table was recorded in a non-volatile storage element such as EEPROM 9. Actually, the data written to the EEPROM may be displayed in hexadecimal, but it is described in decimal for explanation. The liquid crystal display device 1 measures the environmental temperature with the temperature sensor 10 before displaying an image on the liquid crystal panel 2, obtains a driving condition appropriate for the environmental temperature from the temperature table written in the EEPROM, and achieves high image quality. Images can be displayed by the obtained driving method.

  In this embodiment, under the conditions of a frame frequency of 80 Hz and field inversion, the drive voltage Vf of the red LED at room temperature is set to 2 V, the drive voltage Vf of the blue and green LEDs is set to 3.5 V, and the LED light emission pulse width t4 is set to 2 ms. It is. On the low temperature side, the LED drive voltage Vf can be set to 5 V at the maximum. Further, the light emission pulse width t4 of the LED can be set to 0.7 ms as a minimum. The LED driving voltage Vf may differ depending on the temperature characteristics of the liquid crystal and the LED. When the liquid crystal transmittance and the LED characteristics change drastically depending on the temperature and it is desired to finely adjust the temperature, it is necessary to create a temperature table with finer temperature increments. When the liquid crystal transmittance and the LED characteristic change due to temperature are small, there is no problem even if the temperature step is taken and the temperature table is created. Further, when the individual difference between the liquid crystal panel 2 and the LED is large, the temperature characteristics of each liquid crystal display device 1 are measured, and a temperature table for each liquid crystal display device 1 is required. In this embodiment, the EEPROM 9 is incorporated in the liquid crystal display device 1, but there is no problem even if it is recorded in a nonvolatile memory element outside the liquid crystal display device 1.

  With the driving method as described above, the liquid crystal display device 1 has a high NTSC ratio and high brightness that are the same as those at room temperature even at low temperatures by an appropriate driving method according to each temperature based on a temperature table measured in advance. Can be maintained.

  The present embodiment will be described in detail with reference to FIG. FIG. 6 shows changes over time in the voltage applied to the source electrode at high temperature, the transmittance of the pixels of the liquid crystal, and the voltage applied to the LED. In order from the top, FIG. 6 (1) shows the voltage level applied to the source electrode, FIG. 6 (2) shows the liquid crystal transmittance, and FIG. 6 (3) shows the voltage applied to the LED before the change according to this embodiment. FIG. 6 (4) shows the voltage level applied to the LED after the change according to this embodiment. In this embodiment, it is normally black. Although the voltage level applied to the source electrode is described with reverse polarity for the sake of explanation, the transmittance of the liquid crystal is actually increased when no voltage is applied. The response of the liquid crystal transmittance is faster than that at normal temperature, and the rise and fall of the liquid crystal transmittance are both steep. For this reason, the time during which the LED light is transmitted through the liquid crystal is increased, the luminance is increased, and the current consumption of the LED is not efficiently used. Therefore, in order to efficiently use the current consumption of the LED at a high temperature, the driving is performed as follows. That is, when the voltage is applied to the source electrode in FIG. 6A, the voltage is applied to the LED as shown in FIG. 6D, and when the applied voltage falls, the applied voltage of the LED is lowered. At this time, the voltage applied to the LED was set low. In a temperature environment higher than normal temperature, the liquid crystal has high transmittance, so that the same luminance as that at normal temperature can be obtained. Further, since the voltage applied to the LED is low, the power consumption can be suppressed from the normal temperature.

In the present embodiment, as in the first embodiment, the LED light emission pulse width and the amount of current flowing through the LED are set in 5 ° C. increments within the operating temperature range so that an NTSC ratio of 90% and a luminance of 200 cd / m ² can be realized. Table 2 shows the temperature table created by obtaining the data.

さらに温度テーブルをＥＥＰＲＯＭ９に書き込んでいる。液晶表示装置１は、画像を表示する前に温度センサー１０で環境温度を測定して、ＥＥＰＲＯＭ９に書き込まれた温度テーブルから環境温度に適切な駆動方法を入手して、高画質が得られる駆動方法で画像を表示することが可能となる。ＬＥＤ発光パルス幅は、ソース電極に印加するパルス幅ｔ５レベル、約２．９ｍｓまで長くすることができる。ＬＥＤの駆動電圧Ｖｆは、赤で１．５Ｖまで、青、緑は３．０Ｖまで駆動電圧を低くした。ＬＥＤの駆動電圧Ｖｆは、ＬＥＤ、液晶の温度特性によって異なる。温度による液晶透過率、ＬＥＤの特性の変化が激しく、細かい温度調整を行いたい時は、温度の刻みを更に細かくして、温度テーブルを作成する必要がある。逆に温度による液晶透過率、ＬＥＤ特性の変化が小さい時は、温度の刻みを大きくとり、温度テーブルを作成しても問題はない。また個体差が大きいときは、液晶表示装置１それぞれで温度特性を測定して、液晶表示装置１各々の温度テーブルが必要になる。また本実施例では、ＥＥＰＲＯＭ９を液晶表示装置１の中に組み込んだが、液晶表示装置１の外部の不揮発性記憶素子に記録しても問題はない。

Further, a temperature table is written in the EEPROM 9. The liquid crystal display device 1 measures the environmental temperature with the temperature sensor 10 before displaying an image, obtains a driving method suitable for the environmental temperature from the temperature table written in the EEPROM 9, and obtains a high image quality. It becomes possible to display an image. The LED light emission pulse width can be increased to a pulse width t5 level applied to the source electrode up to about 2.9 ms. The drive voltage Vf of the LED was lowered to 1.5V for red and to 3.0V for blue and green. The LED drive voltage Vf varies depending on the temperature characteristics of the LED and the liquid crystal. When the liquid crystal transmittance and the LED characteristics change drastically depending on the temperature and it is desired to finely adjust the temperature, it is necessary to create a temperature table with finer temperature increments. Conversely, when the change in liquid crystal transmittance and LED characteristics due to temperature is small, there is no problem even if a temperature table is created by increasing the temperature increment. When the individual difference is large, the temperature characteristics of each liquid crystal display device 1 are measured, and a temperature table for each liquid crystal display device 1 is required. In this embodiment, the EEPROM 9 is incorporated in the liquid crystal display device 1, but there is no problem even if it is recorded in a nonvolatile memory element outside the liquid crystal display device 1.

  By using the driving method as described above, the liquid crystal display device 1 has a high NTSC ratio, which is the same as that at normal temperature, and a high driving method according to each temperature at high temperatures, based on a temperature table measured in advance. It is possible to reduce power consumption while maintaining the luminance.

  By controlling the applied voltage and light emission pulse width of the LED at a low temperature, the same image quality as that at a normal temperature can be maintained. In addition, at high temperatures, power consumption can be reduced by controlling the LED applied voltage and LED light emission pulse width with temperature. That is, high image quality can be maintained within the operating range.

It is a block diagram which shows the basic composition of the liquid crystal display device of this invention. It is a chart showing spectral transmittance. It is explanatory drawing explaining the drive at the normal temperature. It is explanatory drawing showing the specific drive of all the red display at the time of low temperature. It is explanatory drawing showing the specific drive of the all white display at the time of low temperature. It is explanatory drawing showing the specific drive of all the red display at the time of high temperature.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid crystal display device 2 Liquid crystal panel 3 Backlight 4 LED driver 5 MPU
6 Source driver 7 Gate driver 8 Power supply circuit 9 EEPROM
10 Temperature sensor 11 I / O
12 Data bus

Claims (6)

  A liquid crystal panel, a backlight having LEDs of three primary colors that irradiate the liquid crystal panel, a liquid crystal driver that drives the liquid crystal panel, an LED driver that drives the LEDs of the three primary colors, and an LED driver that is connected to the environmental temperature. And a temperature detector for detecting the liquid crystal display device, wherein a change in transmittance due to a temperature of the liquid crystal panel is corrected by controlling a driving condition of the LEDs of the three primary colors.   2. The blue LED of the liquid crystal panel is corrected by increasing the brightness of the red LED when the three primary color LEDs include a red LED and the temperature detected by the temperature detector is lower than room temperature. A liquid crystal display device according to 1.   2. The blue LED of the liquid crystal panel is corrected by lowering the luminance of the blue LED when the three primary color LEDs include a blue LED and the temperature detected by the temperature detector is lower than room temperature. A liquid crystal display device according to 1.   2. The change in transmittance due to the temperature of the liquid crystal panel is corrected by shortening the light emission pulse width of the three primary color LEDs when the temperature detected by the temperature detector is lower than room temperature. The liquid crystal display device described.   2. The change in transmittance due to the temperature of the liquid crystal panel is corrected by increasing the light emission pulse width of the three primary color LEDs when the temperature detected by the temperature detector is higher than room temperature. The liquid crystal display device described.   A non-volatile memory element in which the LED emission pulse width data of the three primary colors for maintaining the same brightness and NTSC ratio as normal temperature at a plurality of points in the operating temperature range is provided, and according to the temperature detected by the temperature detector Then, the LED emission pulse width of the three primary colors is changed based on the LED emission pulse width data written in the nonvolatile memory element, and the change in transmittance due to the temperature of the liquid crystal panel is corrected. Item 2. A liquid crystal display device according to item 1.

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