JP2008139753A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To lower power consumption of a liquid crystal display device which uses external light whose luminance is varied as a light source. <P>SOLUTION: An external light detecting circuit 1S detects variation in the luminance of the external light and a detection signal D indicating the luminance is input to a driver IC. At this time, a frame frequency having 0 flicker rate is selected by a first table memory 28 according to the luminance of the external light that the detection signal D indicates. A driving clock having a frequency corresponding to the frame frequency is generated by an oscillator 27, and a display panel 1 is driven on the basis of the driving clock. Namely, since the display panel 1 is driven by a frame frequency which is suitable for each luminance of the external light, the flicker rate becomes 0 in each luminance and the power consumption can be suppressed as much as possible. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that uses external light whose luminance changes as a light source.

  A liquid crystal display device requires a light source such as a backlight and external light. In a liquid crystal display device using a backlight as a light source, the backlight is switched between two stages, on and off, as necessary, in order to reduce unnecessary power consumption of the backlight.

  In general, when the frame frequency of the liquid crystal display device (the number of frames per second) is low, flicker is easy to see, but when the backlight brightness is low, flicker is difficult to see even when the frame frequency is low. .

  Therefore, in the liquid crystal display device in which the backlight is switched on and off, the power consumption is reduced by switching the frame frequency in two steps of high and low according to the switching of the backlight on and off.

That is, in this system, the frame frequency is increased when the backlight is on, and the frame frequency is decreased when the backlight is off, thereby reducing power consumption associated with driving the liquid crystal display device. This type of liquid crystal display device is described in Patent Document 1.
JP-A-6-83501

  However, for a liquid crystal display device that uses external light whose luminance changes as a light source, such as a reflective or transflective liquid crystal display device, low power consumption is still insufficient.

  Therefore, the present invention aims to reduce power consumption in a liquid crystal display device that uses external light whose luminance changes as a light source.

  The liquid crystal display device of the present invention includes a liquid crystal display panel including a plurality of pixels that use external light as a light source, an external light detection circuit that detects the brightness of external light, and a change in the brightness of external light detected by the external light detection circuit. And a control circuit that changes the frame frequency of the liquid crystal display panel so that the flicker rate of the liquid crystal display panel becomes a certain value or less.

  According to such a configuration, the power consumption of the liquid crystal display device can be reduced by switching the frame frequency to a plurality of stages according to the brightness of external light.

  In addition, the liquid crystal display device of the present invention includes a liquid crystal display panel including a plurality of pixels that use external light as a light source, an external light detection circuit that detects the brightness of external light, and a change in the brightness of external light by the external light detection circuit. And a control circuit for changing the number of frames to be written, which is the number of frames in which display data is written to the liquid crystal display panel, so that the flicker rate of the liquid crystal display panel becomes a certain value or less. To do.

  According to such a configuration, the power consumption of the liquid crystal display device can be reduced by switching the source driver on and off at a frequency according to the brightness of external light.

  According to the liquid crystal display device of the present invention, in a liquid crystal display device that uses external light whose luminance changes as a light source, it is possible to reduce power consumption while preventing flickering.

  Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a liquid crystal display device according to a first embodiment of the present invention. In FIG. 1, a liquid crystal display panel (hereinafter abbreviated as “display panel”) 1 is shown in a cross-sectional view, and other components are shown in a block configuration diagram. 2 is a block diagram showing the liquid crystal display device of FIG. 1, and shows an external light detection circuit 1S and a driver IC 20 provided in the display panel 1. As shown in FIG. The display panel 1 is a reflection type that performs display by controlling the amount of light transmitted through the liquid crystal LC using external light as a light source.

  First, a schematic configuration of the liquid crystal display device will be described. As shown in FIG. 1, the display panel 1 is composed of a liquid crystal LC sealed by a first insulating substrate 11 and a second insulating substrate 14. On the first insulating substrate 11, a pixel transistor 12 to which a display signal Vsig is supplied is formed for each pixel 1P. Each pixel transistor 12 is provided with a holding capacitor (not shown) that holds the display signal Vsig for a certain period. Each pixel transistor 12 is connected to a pixel electrode 13 to which a display signal Vsig is applied through the pixel transistor 12. The pixel electrode 13 is composed of a reflective metal layer that reflects external light, or a combination of a transparent metal layer and a reflective material that reflects external light. A common electrode 15 made of a transparent metal and applied with a common potential is formed on the second insulating substrate 14.

  Although not shown, the first insulating substrate 11 is vertically supplied with a horizontal driver that supplies the display signal Vsig to the pixel 1P to perform horizontal scanning and a scanning signal that sequentially selects the pixel 1P for each row. A vertical driver for scanning is formed.

  Further, as a feature of the present embodiment, the display panel 1 is provided with an external light detection circuit 1S that detects the luminance of external light incident on the display panel 1. As shown in FIG. 2, the external light detection circuit 1S includes a thin film transistor (hereinafter abbreviated as “TFT”) 2 having a gate and a source grounded and a drain connected to a bias voltage 2B.

  When external light is incident on the TFT 2, a drain current Id flows as a light leakage current in accordance with the luminance. The drain current Id is a leakage current generated when an external light is incident on a depletion layer existing in an active layer (not shown) of the TFT 2 to generate an electron-hole pair.

The result of the measurement of the relationship between the drain current Id of the TFT 2 and the brightness of external light by the inventors of the present invention is shown in the characteristic graph of FIG. In FIG. 3, the vertical axis corresponds to the drain current Id (light leakage current) [pA], and the horizontal axis corresponds to the luminance [cd / m ² ] of external light. As is apparent from FIG. 3, since a substantially correlation is recognized between the drain current Id and the brightness of external light, the drain current Id can be handled as a detection signal D of the brightness of external light.

  The detection signal D is input to the current detection circuit 31 of the driver IC 20 connected to the display panel 1 as shown in FIGS. The current detection circuit 31 will be described later. In response to the detection signal D, the driver IC 20 supplies drive signals such as a display signal Vsig, a vertical start pulse STV, a vertical clock CKV, a horizontal start pulse STH, and a horizontal clock CKH to the display panel 1 and controls the frame frequency. .

  The external light detection circuit 1S may have a configuration other than the above as long as it outputs a detection signal D indicating the luminance of external light. For example, other types of transistors, light receiving elements, or electronic devices may be provided as long as they output a current corresponding to the brightness of external light instead of the TFT 2.

As a result of optical measurement by the inventors of the present application on the display panel 1 described above, the relationship between the flicker rate and the frame frequency of the display panel 1 as shown in FIG. 4 became clear. FIG. 4 is a characteristic graph showing the relationship between the flicker rate and the frame frequency of the display panel 1. The vertical axis corresponds to the flicker rate and the horizontal axis corresponds to the frame frequency [Hz]. FIG. 4 shows a measurement result when each luminance of external light assumed to be incident on the display panel 1 is 2700, 2100, 1500, and 800 [cd / m ² ].

  Here, the flicker rate is a value obtained by dividing the flicker effective value when the display light of the display panel 1 (that is, external light reflected by the pixel electrode 13) is measured by an optical measuring instrument by the luminance. The frame frequency is the number of frames (number of screens) per second.

As can be seen from FIG. 4, at each luminance, the flicker rate becomes 0 when the frame frequency exceeds a certain value. For example, at 2700 [cd / m ² ], the flicker rate becomes 0 at 50 [Hz], and the flicker is not visually recognized on the display panel 1. Similarly, when 2100 [cd / m ² ] is 40 [Hz], 1500 [cd / m ² ] is 30 [Hz], 800 [cd / m ² ] is 20 [Hz] The flicker rate is zero.

Based on the characteristic graph of FIG. 4, when the relationship between each luminance and the frame frequency when the flicker rate becomes 0, the characteristic graph as shown in FIG. 5 is obtained. In FIG. 5, the vertical axis corresponds to the frame frequency [Hz], and the horizontal axis corresponds to the luminance [cd / m ² ] of outside light. As is apparent from FIG. 5, since almost linearity is recognized for each luminance and frame frequency, it is also possible to obtain a frame frequency at which the flicker rate is 0 at luminances other than those described above.

  In general, when the frame frequency is increased, the flicker rate is reduced, but the power consumption is increased. On the other hand, in the present embodiment, every time a change in the brightness of the external light is detected by the external light detection circuit 1S, in order to achieve both a reduction in the flicker rate and suppression of power consumption as much as possible. Each frame frequency at which the rate becomes 0 is determined according to the characteristic graph of FIG. 5, and the display panel 1 is driven by the frame frequency.

  Next, details of such a liquid crystal display device will be described with reference to the drawings. As shown in FIG. 2, the driver IC 20 connected to the display panel 1 has a memory controller 21 controlled in accordance with a command CMD input through a CPU interface (hereinafter abbreviated as “CPU I / F”) 21C. Is provided. In addition, a video memory 22 in which video data as digital data is read and written through the memory controller 21, an output controller 23 that controls output of the video data written in the video memory 22, and video data output from the output controller 23 A gradation selector 24 for selecting a gray scale and a first source driver 25 for amplifying the video data and outputting it to the display panel 1 are provided.

  The driver IC 20 is provided with a timing controller 26 that controls the timing of the output controller 23 and an oscillator 27 that generates a drive clock having a frequency corresponding to a predetermined frame frequency and inputs the drive clock to the timing controller 26. Yes.

  Further, the driver IC 20 is provided with a first table memory 28 in which a table of frame frequencies corresponding to the brightness of each external light determined according to the characteristic graph of FIG. 5 is stored.

Here, in the table stored in the first table memory 28, the brightness of the external light that is assumed to be incident on the display panel 1 is, for example, 2700, 2100, 1500, and 800 [cd / m ² ]. Then, according to the characteristic graph of FIG. 5, frame frequencies of 50, 40, 30, and 20 [Hz] correspond to each luminance. A table in this case is shown in FIG.

  In order to cope with various luminances of external light, the luminance interval in the table of the first table memory 28 may be smaller than the above. In this case, the first table memory 28 stores a frame frequency table for setting the flicker rate to 0 corresponding to the brightness other than the above in accordance with the characteristic graph of FIG.

  The driver IC 20 includes a current detection circuit 31 that amplifies a minute drain current Id (light leakage current) input from the TFT 2 of the external light detection circuit 1S, and an A / D that converts the drain current Id from analog to digital. A converter 32 is provided.

  Next, the operation of this liquid crystal display device will be described. Video data (digital data) input to the driver IC 20 is written into the video memory 22 by the memory controller 21 in accordance with a command CMD input through the CPU I / F 21C. This video data is output to the gradation selector 24 by the output controller 23 under the control of the timing controller 26. After the gray scale is adjusted by the gradation selector 24, the video data is amplified as an analog signal by the first source driver 25 and supplied to the display panel 1 as the display signal Vsig.

  The display panel 1 supplies the display signal Vsig to the pixel 1P by horizontal scanning and vertical scanning. At the time of display, the display signal Vsig is applied to the pixel electrode 13, and the transmitted light amount of the external light reflected by the pixel electrode 13 is changed by changing the alignment direction of the liquid crystal LC according to the electric field between the common electrode 15. Is controlled.

  The horizontal scanning and the vertical scanning are performed at the frame frequency switched in accordance with each luminance and frame frequency table of the first table memory 28.

  That is, when the luminance of the external light incident on the display panel 1 changes, the drain current Id that is the light leakage current of the TFT 2 of the external light detection circuit 1S changes. The external light detection circuit 1S inputs the drain current Id flowing according to the luminance of the external light as a detection signal D to the driver IC. Since this detection signal D is a minute current, it is amplified by the current detection circuit 31 and converted into digital data by the A / D converter 32.

  Then, the first table memory 28 refers to the detection signal D that has become digital data, and inputs data of a frame frequency corresponding to the luminance (or luminance close to it) of the detection signal D to the oscillator 27.

  The oscillator 27 generates a drive clock having a frequency corresponding to the data of the frame frequency, and inputs this to the timing controller 26. The timing controller 26 generates a vertical start pulse STV, a vertical clock based on a driving clock of the oscillator 27 and various driving signals inputted separately from the driving clock, for example, a horizontal synchronizing signal HSYNC, a vertical synchronizing signal VSYNC, a reference clock PCLK, and the like. Drive signals such as CKV, horizontal start pulse STH, and horizontal clock CKH are generated.

  The generated drive signal is input to the display panel 1. In the display panel 1, a scanning signal is sequentially applied to the pixel transistor 12 of the pixel 1P by a vertical driver (not shown) according to the vertical start pulse STV and the vertical clock CKV, and vertical scanning is performed at a frame frequency corresponding to the luminance of external light. Is done. Further, a horizontal driver (not shown) sequentially supplies the display signal Vsig to the pixel 1P in accordance with the horizontal start pulse STH and the horizontal clock CKH to perform horizontal scanning.

  This operation is repeated every time a change in the brightness of the external light is detected by the external light detection circuit 1S, and a frame frequency corresponding to the brightness of each external light is selected by the first table memory 28.

  Thus, since the display panel 1 is driven by switching to the optimum frame frequency according to the brightness of each external light, the flicker rate becomes 0 and the power consumption can be suppressed as much as possible.

  Below, 2nd Embodiment concerning this embodiment is described with reference to drawings. FIG. 7 is a block diagram showing the liquid crystal display device according to this embodiment. The schematic configuration of this liquid crystal display device is the same as that of the first embodiment shown in FIG. In FIG. 7, the same components as those shown in FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted.

  In the first embodiment, according to the characteristic graph of FIG. 5, the frame frequency is switched according to the brightness of the external light detected by the external light detection circuit 1S, and the drive clock frequency of the oscillator 27 is changed according to the frame frequency. Was. In contrast, in the present embodiment, the frame frequency is constant, that is, the frequency of the drive clock generated by the oscillator 27 is constant. In addition, the number of frames to which the display signal Vsig is written is thinned out according to the luminance of each external light detected by the external light detection circuit 1S, thereby changing the number of write frames and suppressing power consumption while preventing flickering. It is characterized by that.

  As shown in FIG. 7, the driver IC 40 connected to the display panel 1 has a second table memory 48 connected to the second source driver 45 and the timing controller 26 instead of the first table memory 28. Is provided. The second table memory 48 stores a table of each luminance of external light that is assumed to be incident on the display panel 1 and the number of write frames per second corresponding to the luminance.

  The number of write frames indicates how many frames per second that the display signal Vsig is written to, that is, the frames where the display signal Vsig is supplied to the pixel 1P when the frequency of the drive clock is constant. . In a frame in which the display signal Vsig is not written, display is performed while the display signal Vsig of the immediately preceding frame is held in a holding capacitor (not shown) of the pixel 1P. Hereinafter, this frame is referred to as a holding frame.

The number of writing frames corresponds to the frame frequency at which the flicker rate is 0 at each luminance in FIG. Here, assuming that the brightness of external light is 2700, 2100, 1500, and 800 [cd / m ² ], for example, at a brightness of 2700 [cd / m ² ], the frame frequency at which the flicker rate is 0 is 50 Since it is [Hz], the number of write frames is 50 [frames / second]. In the luminance of 2100 [cd / m ² ], the frame frequency at which the flicker rate is 0 is 40 [Hz], and thus the number of write frames is 40 [frames / second]. Similarly, in the luminance of 1500 [cd / m ² ], the number of writing frames is 30 [frame / second], and in the luminance of 800 [cd / m ² ], the number of writing frames is 20 [frame / second]. Become. A table in this case is shown in FIG.

  It should be noted that the brightness interval in the table of the second table memory 48 may be smaller than the above in order to cope with various brightnesses of external light. In this case, the second table memory 48 stores a table of the number of write frames for setting the flicker rate to 0 corresponding to the brightness other than the above, according to the characteristic graph of FIG.

The second table memory 48 may store the number of retained frames corresponding to the number of write frames instead of the number of write frames. For example, if each luminance of the external light is 2700, 2100, 1500, 800 [cd / m ² ] and the frequency of the drive clock is 50 [Hz], 0, 10, 20, 30 for each luminance. Corresponds to the number of frames held in [frame / second]. The number of holding frames is obtained by the difference between the frequency value of the drive clock and each value of the frame frequency corresponding to each luminance in FIG.

  Next, the operation of this liquid crystal display device will be described. The operation until the video data input to the driver IC 40 reaches the second source driver 45 is the same as that in the first embodiment. Similar to the first source driver 25 of the first embodiment, the second source driver 45 amplifies the video data as an analog signal and supplies it to the display panel 1 as a display signal Vsig. However, unlike the first embodiment, the second source driver 45 refers to the table of the second table memory 48 to thereby determine the brightness of the external light detected by the external light detection circuit 1S, that is, the detection signal. The supply of the display signal Vsig to the display panel 1 and the stop of the supply are controlled in accordance with the number of writing frames corresponding to the luminance of D (or luminance approximate thereto).

  In the frame in which the display signal Vsig is written, the display signal Vsig is supplied from a horizontal driver (not shown) to the pixel 1P according to the horizontal start pulse STH and the horizontal clock CKH from the timing controller 26, and horizontal scanning is performed. This horizontal scanning is sequentially repeated in accordance with the vertical start pulse STV and the vertical clock CKV to perform vertical scanning for one frame.

  On the other hand, the operation of the second source driver 45 is stopped in a frame in which the display signal Vsig is not written, that is, a holding frame. The timing controller 26 stops supplying the vertical start pulse STV, the vertical clock CKV, the horizontal start pulse STH, and the horizontal clock CKH. Thereby, the pixel transistor 12 is turned off in the frame, and writing of the display signal Vsig to the pixel 1P is stopped. The storage signal (not shown) of the pixel 1P holds the display signal Vsig written immediately before, and display is performed by the display signal Vsig.

  In this way, by changing the number of write frames corresponding to each luminance according to the table of FIG. 8, the frame frequency corresponding to each luminance is equivalently changed, and the occurrence of the flicker rate is prevented.

  Further, since the operation of the second source driver 45 is stopped in the holding frame, the power consumption on the driver IC 40 side can be suppressed as much as possible. In addition, since the pixel transistor 12 is turned off in the holding frame and the writing of the display signal Vsig to the pixel 1P is stopped, the number of times of charging / discharging a holding capacitor (not shown) of the pixel 1P is reduced. Power consumption can be reduced.

  Note that in the first and second embodiments, the frame frequency corresponding to each luminance in the characteristic graph of FIG. 5 is a predetermined value other than 0 if the flicker rate is such that a desired display quality is obtained. For example, it may be determined to be substantially zero. In this case, the minimum frame frequency capable of obtaining a predetermined flicker rate at each luminance is extracted based on FIG. 4, and a characteristic graph showing the relationship between the luminance and the frame frequency as in FIG. 5 may be obtained. Each table of the first table memory 28 or the second table memory 48 is determined based on this characteristic graph.

  In the first and second embodiments, the relationship between the luminance and the frame frequency in FIG. 5 has a substantially linearity. Therefore, the linear curve is approximated by a function, and external light detection is performed based on the function. You may obtain | require the frame frequency with respect to each brightness | luminance of the external light which the circuit 1S detected, or the number of writing frames. In this case, instead of the first table memory 28 or the second table memory 48, an arithmetic circuit for obtaining the frame frequency or the number of writing frames for each luminance based on the above function is provided.

  In the first and second embodiments, the display panel 1 of the liquid crystal display device is a reflection type as shown in FIG. 1, but a transmissive region having a transparent pixel electrode with a backlight as a light source. In addition, a transflective display panel including a reflective region that uses external light as a light source may be used similarly to the pixel 1P of FIG. In the case of a transflective display panel, data of the frame frequency and the number of write frames when the backlight is on is stored in the first and second table memories 28 and 48 in advance, and the backlight is turned on / off. Based on the signal, when the backlight is on, the display may be controlled based on the data of the frame frequency and the number of writing frames stored in advance. That is, in the first embodiment, when the backlight is on, the data of the frame frequency when the backlight is on is input from the first table memory 28 to the oscillator 27, while on the other hand, in the second embodiment When the backlight is on, the display control is performed by inputting the data of the number of write frames when the backlight is on from the second table memory 48 to the second source driver 45. The same applies to the case where the number of retained frames corresponding to the number of write frames is stored in the second table memory 48 instead of the number of write frames.

1 is a schematic configuration diagram of a liquid crystal display device according to a first embodiment of the present invention. 1 is a block configuration diagram showing a liquid crystal display device according to a first embodiment of the present invention. It is a characteristic graph which shows the relationship between the drain current (light leakage current) of a thin-film transistor, and the brightness | luminance of external light. It is a characteristic graph which shows the relationship between a flicker rate and a frame frequency. 4 is a characteristic graph showing the relationship between the frame frequency and the luminance of external light of the liquid crystal display device according to the first embodiment of the present invention. It is a table | surface of the brightness | luminance and frame frequency which are stored in the 1st table memory. It is a block block diagram which shows the liquid crystal display device concerning the 2nd Embodiment of this invention. It is a table | surface of the brightness | luminance stored in the 2nd table memory, and the number of writing frames.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Display panel 1P Pixel 1S External light detection circuit 2 Thin film transistor (TFT)
Reference Signs List 11 first insulating substrate 12 pixel transistor 13 pixel electrode 14 second insulating substrate 15 common electrode 20, 40 driver IC 21S CPU interface 21 memory controller 22 video memory 23 output controller 24 gradation selector 25 first source driver 26 Timing controller 27 Oscillator 28 First table memory 31 Current detection circuit 32 A / D converter 45 Second source driver 48 Second table memory LC Liquid crystal

Claims (8)

A liquid crystal display panel including a plurality of pixels using external light as a light source;
An external light detection circuit for detecting the brightness of the external light;
A control circuit that changes a frame frequency of the liquid crystal display panel so that a flicker rate of the liquid crystal display panel becomes a predetermined value or less when a change in luminance of the external light is detected by the external light detection circuit; A liquid crystal display device comprising: A memory storing data indicating a relationship between a luminance of the external light and a frame frequency at which a flicker rate of the liquid crystal display panel is equal to or less than a predetermined value; and the control circuit, based on the data stored in the memory, The liquid crystal display device according to claim 1, wherein the frame frequency of the liquid crystal display panel is changed. 3. The liquid crystal display device according to claim 1, wherein the data stored in the memory indicates a relationship between a luminance of the external light and a frame frequency at which a flicker rate of the liquid crystal display panel is zero. A liquid crystal display panel including a plurality of pixels using external light as a light source;
An external light detection circuit for detecting the brightness of the external light;
This is the number of frames in which display data is written in the liquid crystal display panel so that the flicker rate of the liquid crystal display panel becomes a certain value or less when a change in luminance of the external light is detected by the external light detection circuit. A liquid crystal display device comprising a control circuit for changing the number of writing frames. The memory includes data indicating the relationship between the brightness of the external light and the number of write frames in which the flicker rate of the liquid crystal display panel is a predetermined value or less, and the control circuit is based on the data stored in the memory. The liquid crystal display device according to claim 4, wherein the number of writing frames of the liquid crystal display panel is changed. 6. The liquid crystal display device according to claim 4, wherein the data stored in the memory indicates a relationship between the luminance of the external light and the number of write frames in which the flicker rate of the liquid crystal display panel is zero. 7. A source driver for writing display data to the liquid crystal display panel, wherein the control circuit stops the operation of the source driver in a frame in which display data is not written. The liquid crystal display device according to any one of the above. 8. The external light detection circuit includes a transistor that generates a leakage current according to the luminance of external light, and detects a change in luminance of the external light by increasing or decreasing the leakage current. The liquid crystal display device according to any one of the above.

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