JP2005091385A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate flickers generated, when the surface temperature of a liquid crystal display panel is high, by changing the drive frequency condition, and to reduce the electric power consumption, particularly in a liquid crystal display device driven at a low frequency. <P>SOLUTION: In the liquid crystal display, a temperature sensor 6 is mounted on the liquid crystal display panel 1. The temperature sensor 6 is connected to a frequency modulation circuit 5, and the frequency modulation circuit 5 is provided between a liquid crystal display device drive circuit 4, a gate driver 2 and a source driver 3. The drive signal, outputted from the liquid crystal display device driving circuit 4, is modulated by the frequency modulation circuit 5, according to the temperature detected by the temperature sensor 6. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device in an information communication device such as a mobile phone, which requires a wide range of operating temperature conditions and low power consumption.

  Recently, liquid crystal display devices have been adopted in many flat panels because they consume less power than CRTs and can save space. In particular, an active matrix liquid crystal display device including a switching element such as a thin film transistor for each pixel can be suitably used for a flat panel or the like.

  FIG. 11 shows a configuration example of a general active matrix liquid crystal display device (liquid crystal display device). As shown in FIG. 11, the liquid crystal display device includes a gate driver 101, a source driver 102, a liquid crystal display panel (display unit) 103, and a liquid crystal display device driving circuit 107 on a glass substrate or the like. The display unit 103 includes a large number of picture elements 108, 108,..., Gate bus lines G1, G2,..., Gn and source bus lines S1, S2,. Each picture element 108 includes a thin film transistor (TFT) 104, a picture element capacity 105, and an auxiliary capacity 106.

  The gate bus lines G1, G2,..., Gn are connected to the gate portion of each TFT 104. Source bus lines S 1, S 2,..., Sm are connected to the source portion of each TFT 104. The drain side of each TFT 104 is connected to a pixel capacitor 105 composed of a transparent electrode (TFT drain side) and a counter electrode, and an auxiliary capacitor (Cs) 106 connected in parallel with the pixel capacitor 105.

  As shown in FIG. 12, the source driver 102 includes a sampling pulse generation circuit 201 and an analog switch 202 for sampling the video signal video (R, G, B) input to the source driver 102. .

  A driving signal for the display unit 103 is output from the liquid crystal display device driving circuit 107. From the liquid crystal display device driving circuit 107, the gate driver 101 receives a start pulse spg, a clock signal ckg, and the like, and the source driver 102 receives a start pulse sp, a clock signal ck, and video (R, R) as video signals. G, B) are input.

  The gate driver 101 sequentially outputs selection signals to the gate bus lines G1, G2,..., Gn according to the clock signal ckg.

  The source driver 102 outputs a display data signal to the source bus lines S1, S2,. The sampling pulse generation circuit 201 sequentially outputs video sampling pulses Q1, Q2, Q3... That are video signals according to the clock signal ck, as shown in the timing chart of FIG. That is, the source driver 102 samples the video signal video input at the timing shown in FIG. 13 with the above-described sampling pulses Q1, Q2, Q3,..., And displays an image on each source bus line S1, S2,. Output data.

  Here, the operation method of the conventional liquid crystal display device will be described in more detail.

  First, a start pulse sp and a clock signal ck, which are control signals, are input to the sampling pulse generation circuit 201.

  The gate bus line Gn, which is a gate driver output, is active while image data is written to the source bus lines S1, S2,... Sm by the sampling pulses Q1, Q2, Q3. Image data written to the source bus lines S1, S2,..., Sm via the TFTs connected to Gn are sequentially stored in the picture element capacity and the auxiliary capacity constituting the picture element in the display unit. After the sampling of the image data for one horizontal period is completed and the image data is written in the picture element capacity and the auxiliary capacity, the gate bus line Gn becomes inactive and the picture data is written until the image data in the next frame period is written. The image display of the liquid crystal display device is performed by holding the image data written in the elementary capacity and the auxiliary capacity.

  The auxiliary capacitor (Cs) is provided to hold written image data. The auxiliary capacity (Cs) is in a connection position in parallel with the pixel capacity in each picture element, and charges are stored at the same time as data is written into the pixel capacity. When the gate bus line Gn becomes inactive after the image data is written in the pixel capacitor, it serves to reduce a decrease in the holding voltage of the pixel capacitor due to the charge stored in the auxiliary capacitor (Cs).

  The liquid crystal display device as described above often operates at a driving frequency of about 60 Hz. However, in consideration of low power consumption, driving at a relatively low frequency such as 30 Hz may be performed. When driving at a low frequency, flicker (flickering of the screen) becomes an important problem in display quality. Flicker is a phenomenon that occurs when the screen update cycle (frame cycle) is close to the afterimage time of the eye. The flicker is made so fast that the frame cycle cannot be followed visually, and the flickering of the display screen It can be suppressed by reducing the amount of change in luminance to such an extent that it cannot be discriminated visually. Since flicker is very obtrusive, various removal methods have been developed.

  Here, the relationship between the flicker and the change amount of the drive frequency / luminance will be described with reference to FIG.

  FIG. 14 shows the holding voltage of the pixel capacity of the liquid crystal display device and the change in the luminance of the liquid crystal display panel. Here, a case where the drive frequency in the liquid crystal display device is low and a case where the drive frequency is high will be described as examples.

In the case the drive frequency is low (corresponding to the solid line in FIG. 14), to the pixel capacitance and an auxiliary capacitance, charging is started at time t _0, to complete the charge at time t _1, the charge until the time t ₂ is Retained. Pixel capacitance time t _1, the charge accumulated in the auxiliary capacitor, flows out through the OFF resistance of the TFT portion little by little over time. As a result, the holding voltage A of the pixel capacity at time t ₁ gradually decreases and becomes holding voltage B at time t ₂ .

Further, if the driving frequency is high (corresponding to the dotted line in FIG. 14), to the pixel capacitance and an auxiliary capacitance, charging is started at time t _0, to complete the charge at time t _1, until the time t ₃ Charge is retained. Pixel capacitance time t _1, the charge accumulated in the auxiliary capacitor, flows out through the OFF resistance of the TFT portion little by little over time. As a result, the holding voltage A of the pixel capacity at time t ₁ gradually decreases and becomes holding voltage C at time t ₃ .

  Hereinafter, for the sake of simplicity, the liquid crystal display device will be described on the assumption that a normally white type liquid crystal is used. For normally white (white display when no voltage is applied to the liquid crystal) type liquid crystal, as the charge stored in the pixel capacitance gradually leaks, the black display (low transmittance and low brightness) changes to a whitish display. (Transmittance is high and brightness is high).

The decrease in the holding voltage due to the charge leakage of the pixel capacitor is such that when the driving frequency is high, polarity inversion is performed in the time of (t ₃ -t ₁ ) and writing into the pixel capacitor starts. That's it. However, when the drive frequency is low, it takes a long time (t ₂ −t ₁ ) for the electric charge to flow out through the OFF resistance of the TFT portion as compared with the case where the drive frequency is high, so the drive frequency is high. Compared to the case, the decrease in holding voltage is as large as voltage C. The luminance when the voltage D is decreased is F, and the luminance when the voltage C is decreased is E. In other words, when the decrease in the holding voltage of the pixel capacity increases, the luminance change amount (comparing the magnitudes of E and F) increases accordingly, and flicker becomes easy to see.

  In general, since the drive frequency and the power consumption are in a proportional relationship, the drive frequency may be lowered in order to reduce the power consumption of the liquid crystal display device. However, when the drive frequency is lowered, there is a problem that flicker is easily seen as described with reference to FIG.

  In addition, the liquid crystal display device has no problem when the panel surface temperature is room temperature, but has a problem that flicker occurs when the temperature rises. This is mainly due to the fact that the TFT characteristics change and the off-current increases (charge leakage from the pixel capacity and auxiliary capacity per unit time increases) as the temperature increases. As described above, when driving at low frequency, flicker is more visible than when driving at a normal high frequency. Therefore, countermeasures against flicker that becomes easier to see when temperature rises become important.

For example, in Patent Document 1, in order to suppress a display abnormality caused by a temperature rise, a method of cooling a liquid crystal panel by attaching a temperature sensor to the liquid crystal panel surface and operating a cooling fan when the liquid crystal panel surface temperature rises is cited. Yes.
JP-A-8-29265 (Publication date: February 2, 1996) JP-A-1-167734 (publication date: July 3, 1989) JP-A-3-36519 (Publication date: February 18, 1991)

  However, the liquid crystal display device described in Patent Document 1 has a demerit that it is expensive to install a cooling fan. In addition, the consumption of electric power by moving the fan is a disadvantage in the liquid crystal display device with low frequency driving in mind. Furthermore, there is also a problem that the liquid crystal display device is increased in size by increasing the number of parts for installing the fan. In particular, in mobile devices such as mobile phones that require low power consumption, the liquid crystal panel surface temperature tends to be high, and the problem of flicker becomes significant.

  The present invention has been made in view of the above problems, and provides a liquid crystal display device that can eliminate flicker that occurs when the panel surface temperature becomes high when the liquid crystal display device is driven at a low frequency. It is to provide.

  In order to solve the above problems, a liquid crystal display device of the present invention includes a display unit in which picture elements are arranged in a matrix on a substrate, and the picture elements are output by a drive signal output from a liquid crystal display device driving circuit. In an active matrix type liquid crystal display device for driving the display, a temperature detection means for detecting the temperature of the display section, and a frequency modulation circuit for modulating the drive frequency of the drive signal according to the temperature detected by the temperature detection means, It is characterized by having.

  In the liquid crystal display device, it is preferable that the frequency modulation circuit modulates the drive frequency of the drive signal stepwise with respect to a certain range of temperature detected by the temperature detection means.

  In the liquid crystal display device, the frequency modulation circuit is 30 Hz when the temperature is lower than 40 ° C., 35 Hz or higher when the temperature is 40 ° C. or higher and lower than 50 ° C., 40 Hz or higher, 60 ° C. or higher when the temperature is 50 ° C. or higher and lower than 60 ° C. It is preferable to modulate to 45 Hz or more when the temperature is less than 70 ° C., and to 50 Hz when the temperature is 70 ° C. or more.

  In the liquid crystal display device, it is preferable that the temperature detecting means is provided in a non-transmissive portion of the display portion.

  The liquid crystal display device preferably includes an illuminating unit that performs display on the display unit and an illuminating unit driving circuit that adjusts the luminance of the illuminating unit.

  In the liquid crystal display device, it is preferable that the illumination means driving circuit adjusts the luminance of the illumination means according to the temperature detected by the temperature detection means.

  In the liquid crystal display device, it is preferable that the illumination unit driving circuit adjusts the luminance of the illumination unit to be lower as the temperature detected by the temperature detection unit is higher.

  In the liquid crystal display device, it is preferable that the temperature detection unit, the frequency modulation circuit, and the illumination unit driving circuit are monolithically formed on the substrate.

  As described above, the liquid crystal display device according to the present invention includes a temperature detection unit that detects the temperature of the display unit, and a frequency modulation circuit that modulates the drive frequency of the drive signal in accordance with the temperature detected by the temperature detection unit. Therefore, the drive frequency of the drive signal is modulated to a modulation drive frequency at which flicker is not visible in accordance with the temperature of the display unit. Flicker is a phenomenon that occurs when the drive frequency is slow and the amount of change in luminance of the display portion is large. Therefore, flicker is eliminated by increasing the drive frequency according to the temperature (as the temperature increases). Can do. At this time, in the liquid crystal display device, the frequency modulation circuit preferably modulates the drive frequency of the drive signal stepwise with respect to a certain range of temperature detected by the temperature detection means. 30 Hz when the temperature is less than 50 ° C., 35 Hz or more when the temperature is 40 ° C. or higher and lower than 50 ° C., 40 Hz or higher when the temperature is 50 ° C. or higher and lower than 60 ° C., 45 Hz when the temperature is 60 ° C. or higher and lower than 70 ° C. It is preferable to modulate to. That is, the liquid crystal display device of the present invention is a liquid crystal display device driven at a low frequency with low power consumption in mind, and when the surface temperature of the display unit rises, flickering occurs. The flicker can be eliminated by increasing the driving frequency of the liquid crystal display device in a stepwise manner. Therefore, as compared with the case where a cooling fan is attached, it is only necessary to add or improve the driver, so that the liquid crystal display device can be downsized.

  As described above, in the liquid crystal display device of the present invention, since the temperature detection means is provided in the non-transmissive portion of the display portion, the display of the liquid crystal display device is not hindered.

  As described above, the liquid crystal display device of the present invention further includes the illumination unit that displays the display unit and the illumination unit drive circuit that adjusts the luminance of the illumination unit, so that the luminance of the illumination unit can be adjusted. . Therefore, since flicker is generally easier to see as the luminance of the illumination means becomes brighter, flicker can be eliminated by adjusting so as to reduce the luminance of the illumination means. Further, when the surface temperature of the display unit rises, the frequency is increased, and at the same time the luminance of the illuminating means is reduced, so that the power consumption increased by the increase of the driving frequency can be canceled out. Thereby, low power consumption can be maintained. The illumination means driving circuit preferably adjusts the brightness of the illumination means according to the temperature detected by the temperature detection means. The higher the temperature detected by the temperature detection means, the higher the brightness of the illumination means. It is preferable to adjust it low.

  In the liquid crystal display device, the temperature detection means, the frequency modulation circuit, and the illumination means drive circuit can be formed monolithically with CG-Si or the like together with other drivers (gate driver, source driver). As a result, the entire module can be reduced in size and the manufacturing cost can be reduced.

[Embodiment 1]
An embodiment of the present invention will be described with reference to FIGS. 1 to 9 as follows.

  A configuration example of a liquid crystal display device according to this embodiment is shown in FIG. As shown in FIG. 1, the liquid crystal display device includes a liquid crystal display panel (display unit) 1, a gate driver 2, a source driver 3, a liquid crystal display device driving circuit (liquid crystal display device driving means) 4, a frequency modulation circuit (frequency modulation). Means) 5, a temperature sensor (temperature detection means) 6, a backlight drive circuit (illumination means drive circuit, illumination means drive means) 7, and a backlight (illumination means) 8. In the liquid crystal display panel 1, display is performed by light from the backlight 8.

  In the present embodiment, the liquid crystal display panel 1 includes a start pulse spg, a clock signal ckg, a start pulse sp, a clock signal ck, and a video signal (video (video signal), which are drive signals output from the liquid crystal display device drive circuit 4. R, G, B) are a start pulse spg *, a clock signal ckg *, a start pulse sp *, a clock signal ck *, which are modulation drive signals in the frequency modulation circuit 5 according to the temperature detection signal of the temperature sensor 6, respectively. After being modulated into video * (R, G, B) which is a video signal, it is input via the gate driver 2 and source driver 3.

  The liquid crystal display panel 1 has a matrix substrate, a counter substrate provided in parallel with the substrate, and a liquid crystal filled between the substrates. On the matrix substrate, a plurality of gate bus lines G (1), G (2),..., G (n) and a plurality of source bus lines S (1), S (2),. m) and picture elements 9... are provided.

  The picture element 9 is formed in a region surrounded by two adjacent gate bus lines G (j) and G (j + 1) and two adjacent source bus lines S (i) and S (i + 1). ing. That is, the picture elements 9 are formed in a matrix in the liquid crystal display panel 1. The picture element 9 includes a TFT 10, a picture element capacity 11, and an auxiliary capacity 12. The gate bus lines G (1), G (2),..., G (n) are connected to the gate portion of each TFT 10. Source bus lines S (1), S (2),..., S (m) are connected to the source portion of each TFT 10. The drain side of each TFT 10 is connected to a transparent electrode as a display electrode, and further, a pixel capacitor 11 composed of this transparent electrode and a counter electrode, and an auxiliary capacitor (Cs) connected in parallel to the pixel capacitor 11. ) 12. The counter electrode is provided on the counter substrate so as to be common to all the picture elements 9.

  In the picture element 9, a common voltage is applied to the counter electrode. As a result, a voltage is written from the source bus line S (i) while the TFT 10 is ON, and the transmittance or reflectance of the liquid crystal is modulated by the potential difference between this voltage and the common voltage, and is input to the picture element 9. An image corresponding to the image data is displayed. Further, in each picture element 9, the charge accumulated in the picture element capacitance 11 is held for a certain period, so that even when the TFT 10 is turned off, the image display is maintained accordingly.

  The liquid crystal display device drive circuit 4 outputs a drive signal for driving the liquid crystal display panel 1. This drive signal includes a start pulse spg for the gate bus line, a clock signal ckg, a start pulse sp for the source bus line, a clock signal ck, and video (R, G, B) which is a video signal. A timing chart of this drive signal is shown in FIG. The timing chart of FIG. 2 also shows the voltage of each signal. As shown in FIG. 2, the voltage of each signal is 0V to 3.3V for sp, ck, spg, and ckg, and the center is 6V ± 4.5V for video (R, B, G). When the driving frequency is 30 Hz, the interval between the start pulses (spg) of the gate bus line is 1/30 second.

  The temperature sensor 6 detects the temperature of the liquid crystal display panel 1 and generates a temperature detection signal according to the detected temperature. The temperature sensor 6 outputs the temperature detection signal to the frequency modulation circuit 5 and the backlight drive circuit 7. The temperature sensor 6 may be provided in the picture element 9 of the liquid crystal display panel 1 or in the vicinity thereof.

  The frequency modulation circuit 5 modulates the drive signal output from the liquid crystal display device drive circuit 4 in accordance with the temperature of the liquid crystal display panel 1. That is, the frequency modulation circuit 5 responds to the temperature detection signal output from the temperature sensor 6 with the start pulse spg, the clock signal ckg, the start pulse sp, the clock signal ck, and the video signal, which are the drive signals. (R, G, B) is modulated into a start pulse spg * that is a modulation drive signal, a clock signal ckg *, a start pulse sp *, a clock signal ck *, and a video signal that is a video signal (R, G, B). It is supposed to be. The frequency modulation circuit 5 outputs a modulation drive signal to the gate driver 2 and the source driver 3. The gate driver 2 outputs spg * and ckg *, and the source driver 3 outputs sp *, ck *, and video * (R, G, B).

  A block diagram of the frequency modulation circuit 5 is shown in FIG. As shown in FIG. 3, the frequency modulation circuit 5 includes an A / D converter 21, a frame memory 22, a D / A converter 23, a buffer circuit (Buf circuit) 24, a write clock circuit 25, and a memory read clock control circuit 26. It has.

  The operation in the frequency modulation circuit 5 will be described. The drive signals (spg, ckg, sp, ck, and video (R, G, B)) input from the liquid crystal display device drive circuit 4 were converted from analog signals to digital signals by the A / D converter 21. Thereafter, the data is written into the frame memory 22 by a memory write clock signal (30 Hz) transmitted from the write clock circuit 25. Here, the memory read clock control circuit 26 outputs a memory read clock signal (fHz) corresponding to the temperature detection signal from the temperature sensor 6 to the frame memory 22. The frequency f of the memory read clock signal is changed according to the temperature detected by the temperature sensor 6. The frequency f is, for example, 30 Hz when the surface temperature of the liquid crystal display panel 1 is less than 40 ° C., 35 Hz or more from 40 ° C. to 50 ° C., 40 Hz or more from 50 ° C. to 60 ° C., 45 Hz or more from 60 ° C. to 70 ° C., When the surface temperature of the liquid crystal display panel 1 is increased, it is changed to 50 Hz or higher at 70 ° C. or higher so as to increase stepwise. Digital signals (spg, ckg, sp, ck, video (R, B, G)) recorded in the frame memory 22 are read out at the frequency of the memory read clock signal and converted into analog signals by the D / A converter 23. , A frequency-converted (modulated) signal (spg *, ckg, sp *, ck *, video * (R, B, G)). The signals (spg *, ckg, sp *, ck *, video * (R, B, G)) thus frequency-converted are output to the gate driver 2 and the source driver 3 via the Buf circuit 24. FIG. 4 shows a timing chart of the frequency-converted (modulated) signals (spg *, ckg, sp *, ck *, video * (R, B, G)). Each signal is modulated by the frequency modulation circuit 5, but the voltage is not changed. The interval between the start pulses (spg *) of the modulated gate bus lines is 1 / f second.

  The gate driver 2 receives a start pulse spg *, a clock signal ckg *, and the like, and the source driver 3 receives a start pulse sp *, a clock signal ck *, and a video signal (video * (R, G, B)). Is entered.

  As shown in the timing chart of FIG. 5, the gate driver 2 sends selection signals (output signals) gbus (1), gbus (2) to the gate bus lines G (1), G (2),. , Gbus (3),..., Gbus (n) are sequentially output according to the clock signal ckg *. The voltage of each output signal gbus is 0V to 15.5V.

  The source driver 3 outputs an image data signal to the source bus lines S (1), S (2), ..., S (m). The source driver 3 includes a sampling pulse generation circuit and an analog switch for sampling the video signal video * (R, G, B), as in the conventional case. In the source driver 3, as shown in the timing chart of FIG. 6, the sampling pulse generation circuit performs sampling pulses Q (1), Q () of video * (R, B, G) that are video signals according to the clock signal ck *. 2), Q (3),..., Q (m) are sequentially output. That is, the source driver 3 converts the video signal video * (R, B, G) input at the timing shown in FIG. 6 into the above-described sampling pulses Q (1), Q (2), Q (3),. , Q (m), and image data signals (output signals) sbus1 (R, B, G), sbus2 (R) are applied to the source bus lines S (1), S (2),. , B, G), sbus3 (R, B, G),..., Sbusm (R, B, G). The voltages of the sampling pulse Q and the output signal sbus to each source bus line are 0V to 15.5V and the center 6V ± 4.5V, respectively. Each output signal sbus is output in response to the sampling pulse Q (i) and held until the next sampling pulse Q (i + 1).

  The backlight drive circuit 7 drives the backlight 8 and adjusts the luminance. The backlight drive circuit 7 adjusts the luminance of the backlight 8 based on the temperature detection signal output from the temperature sensor 6.

As described above, in the liquid crystal display device of the present embodiment, the temperature sensor 6 is incorporated in the liquid crystal display panel 1, and the temperature detection signal detected by the temperature sensor 6 is used as the frequency modulation circuit 5 and the backlight drive circuit. 7 is output. The frequency modulation circuit 7 generates an optimum flicker according to the temperature detection signal detected by the temperature sensor 6 from the drive signal (video, sp, ck, spg, ckg) output from the liquid crystal display device drive circuit 4. caused not the frequency of the modulated drive signal ^{(video *, sp *, ck} *, spg *, ckg *) is modulated, and outputs to the source driver 3, a gate driver 2. Thereby, it is possible to realize a liquid crystal display device having high display quality without flicker regardless of the surface temperature of the liquid crystal display panel.

  Table 1 and FIG. 7 show the results of experiments conducted by the inventors to examine the correlation between flicker visibility and frequency and the flicker visibility and the surface temperature of the liquid crystal display panel.

The measuring method is as follows.
1) Place the liquid crystal display device in a thermostatic chamber and change the drive frequency from 30 Hz to 50 Hz in 5 Hz increments.
2) When the surface temperature of the liquid crystal display panel is changed from about 30 ° C to 70 ° C in increments of 10 ° C for each driving frequency, three observers are at various angles and distances to the liquid crystal display panel. To check flicker. The evaluation is performed in four stages: 0 (no flicker is visible), 1 (the flicker is slightly), 2 (the flicker is weak), and 3 (the flicker is clearly visible).

表１に示した３、１、０．７などの数値は３人の評価値(０〜３)の平均値である。また、図７に示したグラフは横軸に駆動周波数、縦軸にフリッカ評価値の３人の平均値をとって表を見やすくしたものである。図５のグラフより、同じ駆動周波数では液晶表示パネルの表面温度が高いほどフリッカがよく見え、同じ温度では駆動周波数が低くなるほどフリッカがよく見えることがわかる。

Numerical values such as 3, 1, 0.7 shown in Table 1 are average values of the evaluation values (0 to 3) of three people. In the graph shown in FIG. 7, the horizontal axis represents the driving frequency, and the vertical axis represents the average value of the flicker evaluation values of the three persons to make the table easier to read. From the graph of FIG. 5, it can be seen that the flicker looks better as the surface temperature of the liquid crystal display panel is higher at the same driving frequency, and the flicker appears better as the driving frequency is lower at the same temperature.

  Next, FIGS. 8 and 9 show the measurement results of the relationship between the visibility of flicker and the luminance during white display (no liquid crystal applied voltage). The measurement method is to apply a voltage assuming various leaks of amplitude and charge to a liquid crystal display panel that does not have a TFT, and change the drive frequency to change the frequency at the boundary between where the flicker is visible and where it cannot be seen ( Is defined as a critical frequency), and the brightness change of the panel at that time is measured. In other words, the critical frequency is a frequency at which flicker can be seen when the driving frequency is made lower than a specific frequency. FIG. 8 shows the relationship between the critical frequency and the luminance change amount at that time, and FIG. 9 shows the relationship between the retention rate and the critical frequency. Note that the holding ratio is a holding voltage when there is no leakage of charges of the pixel capacitor and the auxiliary capacitor is 100%, and a holding voltage when all charges are leaked is 0%. In the case where the driving frequency is low in FIG. 14 shown in the prior art, the value corresponds to (B / A) × 100.

The above measurement conducted at room temperature, the display screen brightness of the liquid crystal display panel when the liquid crystal in the no voltage application is made to ^{80cd / m 2, 100cd / m} 2, 3 conditions 150 cd / ^{m 2.} The display screen brightness of the liquid crystal display panel when no voltage was applied to the liquid crystal was adjusted by adjusting the brightness of the backlight. As shown in FIG. 8, with the same amount of change in luminance, flicker can be seen even at higher frequencies as the luminance during white display increases. Further, as shown in FIG. 9, even with the same retention rate, flicker can be seen at higher frequencies as the luminance during white display increases. In other words, it was concluded that flicker is generally more visible when the backlight brightness is brighter.

  From the result of FIG. 7 described above, in the liquid crystal display device of the present embodiment, when the surface temperature of the liquid crystal display panel 1 is 30 ° C., no flicker is observed even when driven at 30 Hz, so there is no problem with the driving frequency. When the surface temperature of the liquid crystal display panel 1 is less than 40 ° C., it is 30 Hz, from 40 ° C. to 50 ° C., 35 Hz or more, from 50 ° C. to 60 ° C., 40 Hz or more, from 60 ° C. to 70 ° C., 45 Hz or more, and from 70 ° C. or more, 50 Hz. As described above, if the drive frequency is increased stepwise as the surface temperature of the liquid crystal display panel 1 becomes higher, it is possible to always display beautifully without causing flicker. That is, based on the temperature detection signal from the temperature sensor 6, the frequency modulation circuit 5 is 30Hz when the surface temperature of the liquid crystal display panel 1 is less than 40 ° C, 35Hz or more from 40 ° C to 50 ° C, and 50 ° C to 60 ° C. In this case, it is preferable to increase the drive frequency of the drive signal stepwise as the surface temperature of the liquid crystal display panel 1 increases, such as 40 Hz or higher, 45 Hz or higher from 60 ° C. to 70 ° C., 50 Hz or higher at 70 ° C. or higher.

  Further, it can be seen from the results of FIGS. 8 and 9 that flicker is more easily seen as the luminance during white display increases. The backlight drive circuit 7 preferably drives the backlight 8 (decreases the applied voltage) so that the brightness decreases as the surface temperature of the liquid crystal display panel 1 increases by the temperature detection signal received from the temperature sensor 6. . Thereby, the visibility of flicker can be reduced.

  Further, when the frequency of the drive signal is increased stepwise as the surface temperature of the liquid crystal display panel increases, the power consumption increases. However, by reducing the brightness of the backlight 8, the entire liquid crystal display device is consumed. The increase in power can be suppressed as much as possible. Furthermore, reducing the luminance of the backlight 8 as described above also makes the flicker difficult to see, and thus has a synergistic effect on making the flicker difficult to see by modulating the driving frequency of the liquid crystal display panel. is there.

  As described above, in the liquid crystal display device of this embodiment, display with low power consumption and no flicker is possible in a wide temperature range.

[Embodiment 2]
The following will describe another embodiment of the present invention with reference to FIGS. 10 (a) and 10 (b). For convenience of explanation, members having the same functions as those shown in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  The liquid crystal display device of the present embodiment has a configuration in which the temperature sensor is installed on the auxiliary capacitor 12 in the picture element 9 of the liquid crystal display panel 1 in the liquid crystal display device of the first embodiment. As shown in FIG. 10A, the liquid crystal display device of the present embodiment is provided with a plurality of temperature sensors 31. The temperature sensors 31... Detect the surface temperature of the liquid crystal display panel 30 and output a temperature detection signal corresponding to the surface temperature to the frequency modulation circuit 5 and the backlight drive circuit 7.

  Further, as shown in FIG. 10B, the temperature sensor 31 is formed on the auxiliary capacitor (Cs) 12 in the picture element 9 of the liquid crystal display panel 30. The auxiliary capacitor 12 corresponds to a non-transmission portion in the picture element 9 and does not reduce the light transmittance in the picture element 9. Therefore, the display in the liquid crystal display device is not hindered. Further, the temperature sensor 31 may be provided in a black matrix or the like formed around the picture element 9.

  Further, as shown in FIG. 10A, the temperature sensors 31 are not necessarily installed on the auxiliary capacitors 12 of all the picture elements 9, and the liquid crystal display panel depends on the size of the liquid crystal display panel 30. The center of 30 and the four corners are sufficient. This is because the temperature of the adjacent picture element 9 does not change substantially.

  In addition, various circuits (a gate driver, a source driver, a backlight driving circuit, a frequency modulation circuit, a liquid crystal display device driving circuit) and a temperature sensor described in Embodiment 1 are provided on a matrix substrate such as a glass substrate, for example. By using CG-Si, it can be formed similarly to a TFT. That is, the temperature sensor, the frequency modulation circuit, and the backlight driving circuit can be formed monolithically together with other drivers. As a result, the entire module can be reduced in size and the manufacturing cost can be reduced.

  By increasing the panel drive frequency at the same time as the temperature rises, and by lowering the backlight brightness, it is possible to minimize flicker-free display quality over a wide temperature range and to minimize power consumption. It is useful for information communication equipment such as mobile phones. The liquid crystal always-on device can be used for a liquid crystal panel of a mobile device that requires low power consumption and miniaturization, such as a mobile phone.

It is a block diagram which shows the principal part structure of the liquid crystal display device which concerns on one Embodiment of this invention. It is a timing chart of the signal output from the liquid crystal display device drive circuit in the liquid crystal display device. It is a block diagram which shows the principal part structure of the frequency modulation circuit in the said liquid crystal display device. It is a timing chart of the signal output from the said frequency modulator. 4 is a timing chart of input / output signals of a gate driver in the liquid crystal display device. 4 is a timing chart of input / output signals of a source driver in the liquid crystal display device. It is a graph which shows the temperature dependence and frequency dependence of the flicker in the said liquid crystal display device. It is a graph which shows the relationship between the critical frequency in the said liquid crystal display device, and the luminance variation | change_quantity at that time. It is a graph which shows the relationship between the retention in the said liquid crystal display device, and a critical frequency. (A) is a block diagram which shows the principal part structure of the liquid crystal display device concerning other embodiment, (b) is a figure which shows the pixel provided with the temperature sensor in the liquid crystal display device. It is a block diagram which shows the principal part structure of the liquid crystal display device of a prior art. It is a block diagram which shows the principal part structure of the source driver in the liquid crystal display device of a prior art. It is a timing chart of the input-output signal of the source driver in the liquid crystal display device of a prior art. It is a graph which shows the holding voltage of the pixel capacity | capacitance of a general liquid crystal display device, and the change of the brightness | luminance of a panel accompanying it.

Explanation of symbols

1 LCD panel (display unit)
2 Gate driver 3 Source driver 4 Liquid crystal display drive circuit (Liquid crystal display drive means)
5 Frequency modulation circuit 6 Temperature sensor (temperature detection means)
7 Backlight drive circuit (illumination means drive circuit, illumination means drive means)
8 Backlight (lighting means)

Claims (8)

In an active matrix liquid crystal display device comprising a display unit in which picture elements are arranged in a matrix on a substrate and driving the picture elements by a drive signal output from a liquid crystal display device drive circuit,
Temperature detecting means for detecting the temperature of the display unit;
A liquid crystal display device comprising: a frequency modulation circuit that modulates the drive frequency of the drive signal in accordance with the temperature detected by the temperature detection means.   2. The liquid crystal display device according to claim 1, wherein the frequency modulation circuit modulates the driving frequency of the driving signal in a stepwise manner with respect to a temperature in a certain range detected by the temperature detecting means.   The frequency modulation circuit is 30 Hz when the temperature is lower than 40 ° C, 35 Hz or higher when the temperature is 40 ° C or higher and lower than 50 ° C, 40 Hz or higher when the temperature is 50 ° C or higher and lower than 60 ° C, and 60 ° C or higher and lower than 70 ° C. 3. The liquid crystal display device according to claim 2, wherein the liquid crystal display device modulates to 50 Hz when it is 45 Hz or higher and 70 ° C. or higher.   The liquid crystal display device according to claim 1, wherein the temperature detection unit is provided in a non-transmission part of the display part.   The liquid crystal display device according to claim 1, further comprising: an illuminating unit that performs display on the display unit; and an illuminating unit driving circuit that adjusts luminance of the illuminating unit.   6. The liquid crystal display device according to claim 5, wherein the illumination means driving circuit adjusts the luminance of the illumination means in accordance with the temperature detected by the temperature detection means.   The liquid crystal display device according to claim 6, wherein the illumination unit driving circuit lowers the luminance of the illumination unit as the temperature detected by the temperature detection unit increases.   The liquid crystal display device according to claim 5, wherein the temperature detection unit, the frequency modulation circuit, and the illumination unit drive circuit are monolithically formed on the substrate.

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