JP5259496B2 - Illumination device and liquid crystal display device - Google Patents

Description

  The present invention relates to an illumination device having a semiconductor light emitting element (LED) and a liquid crystal display device using the illumination device having a semiconductor light emitting element as a backlight.

  Light emitting diodes (LEDs), which are semiconductor light emitting devices, are small and light, can be selected from a variety of light emission colors, and can be driven at relatively low voltages. It is getting used. As a field of application of light emitting diode elements, in addition to lights such as indoor lighting, it is also used as a backlight for liquid crystal display devices. In general, there are units such as brightness and luminous flux as indicators for the light characteristics of such lighting devices, and there are units such as chromaticity, color temperature, and color rendering for color types. It is properly used according to the situation.

  Here, the liquid crystal display device is a device that is configured by combining a liquid crystal panel that controls transmittance for each pixel and a backlight, and displays an image based on an input video signal. The characteristics of the light emitting element used as the backlight of the liquid crystal display device will affect the image quality of the display screen. When the LED is used as a light emitting element of a backlight, the characteristics of the light emitting element that affect the image quality of the display screen include brightness, chromaticity, efficiency, response speed, variation, temperature characteristics, and the like.

  According to Non-Patent Document 1, in television, the chromaticity of the three primary colors is determined by standards such as the NTSC system and the high vision system. For example, in the illumination device used for the backlight of a liquid crystal display device, the color of the light emitting element It is desirable that the degree is stable. For this reason, Patent Document 1 discloses that since the emission chromaticity changes depending on the operating condition of the LED, the fluctuation of the emission chromaticity is suppressed by controlling the drive current. Specifically, in Patent Document 1, on the assumption that the characteristics of the LED fluctuate, it is possible to generate a drive signal for each LED in order to realize the stability of the emission color formed by the combination of RGB three-color LEDs. Disclosed. This patent document 1 aims to stabilize the chromaticity (especially white) of backlight emission.

  Moreover, in the illuminating device used for the backlight of a liquid crystal display device, it is known to reduce power consumption by controlling the light emission luminance of the illuminating device according to the content of the display screen. With respect to the control of the light emission luminance, for example, in Patent Document 2, a current control signal for driving the LED provided in the backlight is generated for the control of the light emission luminance, and this LED is used to control the LED to a predetermined brightness. It is disclosed that the light is turned on. Therefore, in the liquid crystal display device disclosed in Patent Document 2, when the LED is controlled by the current value, the light emission luminance of the LED backlight is dynamically controlled based on the video signal input to the liquid crystal panel. The

  As a method for controlling the brightness of the LED, the brightness of the LED is controlled by controlling the cycle and width of the lighting pulse by utilizing the fast response speed of the LED (the time from the input of the drive signal to the light emission). A PWM method for changing the frequency is generally known. In the PWM method, the brightness is controlled by variably setting the pulse repetition period (pulse period) or pulse frequency and the duty ratio (ratio between the pulse period and the pulse width). A duty ratio ranging from 0% to 100% is used.

  In LED backlight lighting circuits, the above-described constant current PWM method is widely adopted as a method for realizing both chromaticity stability and light emission amount variability. Here, the set value of the constant current may be equal to or less than the maximum rating of the LED driving current at a duty ratio of 100% (DC lighting).

  Further, Non-Patent Document 2 discusses the visual psychological brightness at the time of short-time pulse emission from a viewpoint different from the above. Here, FIGS. 11A, 11B, and 11C are diagrams corresponding to FIGS. 11A, 11B, and 11C are diagrams illustrating experimental results showing the relationship between input power and luminance at each duty ratio. In Non-Patent Document 2, the brightness of the LED that is pulse-driven is interpreted based on visual psychology. However, no detailed examination has been made on this figure.

JP 2008-135220 A JP 2006-30588 A

The Color Society of Japan "New Color Science Handbook (2nd edition)" The University of Tokyo Press, 1998 first edition Morita, et al. “Improvement of effective brightness of pulse-driven LED by visual psychological approach”, Illumination Society Study Group, Light Generation / Related Systems Research Special Committee, LS-08-07

  According to FIGS. 11A, 11B, and 11C (FIGS. 1 to 3 of Non-Patent Document 2), the duty ratio decreases from 100% (DC lighting) to 70%, 40%, and 10%. Accordingly, there is a tendency that the luminance decreases, and the luminance is substantially the same when the duty ratio is 10% and 5%. Therefore, if the short-time pulse driving in which the duty ratio is 10% or less is compared with the pulse driving in which the duty ratio is larger than 10%, the latter emission efficiency is deteriorated. For this reason, if a PWM system that controls brightness using a duty ratio of 0% to 100% is used, the light emission efficiency cannot be sufficiently improved.

  In the constant current PWM method, the brightness is controlled by the duty ratio of the drive signal, focusing on the fact that the chromaticity of LED emission is stabilized by keeping the current value constant. However, factors that change the chromaticity of LED emission include not only the current value, but also temperature, life, etc., and these factors need to be considered in order to stabilize the chromaticity, only the constant current PWM method However, brightness control with stable chromaticity cannot be realized sufficiently.

  In view of the above problems, an object of the present invention is to provide an illumination device that improves luminous efficiency and stabilizes chromaticity, and a liquid crystal display device that uses the illumination device as a backlight.

  In order to solve the above problems, an illumination device according to the present invention is a lighting device including a light-emitting diode and a drive circuit that drives the light-emitting diode by supplying a light-emission pulse. Pulse generation means for periodically generating a light emission pulse at a predetermined duty ratio; and current value control means for controlling a current value of the light emission pulse generated at the predetermined duty ratio, the predetermined duty The ratio is a duty ratio that suppresses the temperature rise of the light emitting diode, and the light emission amount of the light emitting diode is controlled by the current value control means.

  The lighting device according to the aspect of the invention further includes a luminance input receiving unit that receives an input of luminance to emit light, and the pulse generation unit suppresses a temperature rise of the light emitting diode according to the input. The predetermined duty ratio is determined within a range of duty ratios to generate the light emission pulse, and the current value control means inputs the current value of the light emission pulse generated at the predetermined duty ratio as the input Control may be performed according to the above.

  In one aspect of the lighting device according to the present invention, a luminance input receiving unit that receives an input of luminance to emit light, and a duty ratio and a current value of a light emitting pulse generated by the light emitting pulse generating unit with respect to the luminance input. A table holding means for holding a table associated with the pulse generator, the pulse generating means determines the predetermined duty ratio according to the luminance input and the table, and generates the light emission pulse, The current value control means may control the current value of the light emission pulse in accordance with the luminance input and the table.

  Moreover, in one aspect of the lighting device according to the present invention, the predetermined duty ratio may be a duty ratio of 10% or less.

  Further, in one aspect of the lighting device according to the present invention, the current value control means is controlled so that an average value of a current value of the light emission pulse for a predetermined period does not exceed a rated current of the light emitting diode. It may be.

  In the lighting device according to the aspect of the present invention, the light emitting diode may be a light emitting diode that emits green or blue light.

  Further, in one aspect of the lighting device according to the present invention, the lighting device includes a plurality of light emitting diodes, and the driving circuit includes the light emitting pulses in each of the light emitting diodes so that the phases of the light emitting pulses are different. You may make it supply.

  In one aspect of the lighting device according to the present invention, the predetermined duty ratio is a duty ratio of 10% or less, and the light emission pulse is supplied to the light emitting diode at a period of 60 Hz or more. Also good.

  In order to solve the above problem, a liquid crystal display device according to the present invention includes a backlight having a light emitting diode and a driving circuit for driving the light emitting diode by supplying a light emitting pulse, and between two substrates. A liquid crystal display device comprising: a liquid crystal panel that drives the sandwiched liquid crystal layer according to a video signal and displays an image by selectively transmitting light supplied from the backlight; A pulse generation means for periodically generating the light emission pulse at a predetermined duty ratio; and a current value control means for controlling a current value of the light emission pulse generated at the predetermined duty ratio. The duty ratio of the light emitting diode is a duty ratio that suppresses a temperature rise of the light emitting diode, and the light emission amount of the light emitting diode is controlled by the current value control. Is controlled by stage, characterized in that.

  In the liquid crystal display device according to one aspect of the present invention, the liquid crystal panel includes a light emission condition acquisition unit that acquires a condition for causing the light emitting diode to emit light from the drive circuit, and is acquired by the light emission condition acquisition unit. Further, the chromaticity given to the image by the backlight may be compensated by compensating the video signal in accordance with the conditions.

  In one aspect of the liquid crystal display device according to the present invention, the light emission pulse supplied to the light emitting diode is compensated so that the predetermined duty ratio or the current value brings the chromaticity of the backlight closer to white. You may make it.

  In one aspect of the liquid crystal display device according to the present invention, the backlight includes chromaticity information acquisition means for acquiring chromaticity information of an image to be displayed from the video signal, and the drive circuit includes the color According to the degree information, the chromaticity of the backlight is compensated to be close to any of RGB, the light emission pulse is supplied to the light emitting diode, and the liquid crystal panel is compensated so that the chromaticity is close to any of RGB The image may be displayed by driving the liquid crystal layer according to the light of the backlight.

  In the liquid crystal display device according to the aspect of the invention, the liquid crystal panel may drive the liquid crystal layer according to a predetermined frame period, and the drive circuit may periodically emit a plurality of the light emission pulses according to the predetermined duty ratio. The light-emitting pulse is supplied to the light-emitting diode so as to have a lighting period for supplying the light-emitting diode to the light-emitting diode and a light-off period for stopping the supply of the light-emitting pulse to the light-emitting diode. The lighting period in one of the above and the lighting period in a frame period different from the one may be started by shifting the timing with respect to each frame period.

    In the liquid crystal display device according to the aspect of the invention, the driving circuit may include a luminance target value receiving unit that sequentially receives a target value of luminance emitted from the light emitting diode, and a plurality of the light emission pulses to the light emitting diode. Switching means for supplying the light emitting pulse to the light emitting diode so as to have a lighting period that is periodically supplied at a predetermined duty ratio, and a light extinction period in which the supply of the light emitting pulse to the light emitting diode is stopped; Cumulative amount storage means for accumulating the amount of light emitted by the light emission pulse supplied to the light emitting diode and storing the cumulative amount; and determining whether the cumulative amount during the lighting period is equal to or greater than the target value. Comparing means, wherein the switching means receives the target value when the luminance target value receiving means receives the target value. When the comparison unit determines that the accumulated amount is equal to or greater than the target value, the lighting period is ended, and the accumulated amount storage unit is configured so that the comparison unit determines that the accumulated amount is equal to or greater than the target value. When it is determined that the difference has occurred, the difference between the accumulated amount and the target value may be left, and the difference may be carried over to the next lighting period.

  The lighting device according to the present invention can improve luminous efficiency and stabilize chromaticity. In addition, according to the liquid crystal display device according to the present invention, the light emission efficiency of the backlight can be improved and the chromaticity can be stabilized.

1 is a diagram illustrating a schematic configuration of a liquid crystal display device according to Embodiment 1. FIG. It is a graph which shows the relationship between the duty ratio of a light emitting diode, and permissible forward current. In the liquid crystal display device according to Embodiment 1, it is a diagram illustrating a state in which a video signal is supplied to a liquid crystal panel and a backlight. It is a figure which shows an example of the brightness | luminance of the light emitting diode to which the light emission pulse was supplied based on the video signal to input, and the moving average in predetermined appropriate time width. It is a figure which shows the circuit structural example of the pulse generation means concerning Embodiment 1, and an electric current value setting means. It is a figure which shows the circuit structural example of the pulse generation means concerning Embodiment 1, and an electric current value setting means. In the backlight or illumination device provided with N light emitting diodes, the phase of the pulse period of each light emitting diode is shifted, and the figure shows how light is emitted by sequentially turning on light emitting pulses with a low duty ratio. is there. The state of the video signal that switches the transmittance of the liquid crystal panel between 0% and 100% for each frame period, the response of the liquid crystal molecules corresponding to this video signal, and the state that the light emission pulse is output in synchronization with the frame period. FIG. 6 is a diagram illustrating an example of a functional configuration of a backlight drive circuit according to a second embodiment; FIG. It is a schematic diagram which shows a mode that the error produced with respect to the luminance target value in the lighting period which consists of a several light emission pulse is carried over to the next lighting period, and is canceled. FIG. 6 is a schematic diagram showing a state in which errors generated with respect to a luminance target value are accumulated and canceled in the case of emitting light by combining RGB three primary colors. It is a figure which shows the relationship between the electric power in each duty ratio of blue LED, and a light beam characteristic. It is a figure which shows the relationship between the electric power and luminous flux characteristic in each duty ratio of green LED. It is a figure which shows the relationship between the electric power in each duty ratio of red LED, and a light beam characteristic.

[Embodiment 1]
FIG. 1 is a diagram illustrating a schematic configuration of a liquid crystal display device according to the present embodiment, and the liquid crystal display device includes a backlight 100 and a liquid crystal panel 200. In particular, the backlight 100 is a lighting device that emits light in a planar shape, and includes one or a plurality of light emitting diodes 101 and a flexible printed circuit board 109 (FPC board) connected to the light emitting diodes 101. A drive circuit for driving the light emitting diode 101 is included in the backlight 100 by being mounted on the flexible printed circuit board 109, for example. On the other hand, the liquid crystal panel 200 includes an upper polarizing plate 201, a lower polarizing plate 202, and a liquid crystal cell 203 interposed therebetween. The liquid crystal cell 203 has two glass substrates and a liquid crystal layer sandwiched between them, and one of the two glass substrates is a pixel in which a plurality of pixels are partitioned in a matrix. A circuit is formed, and a color filter layer is formed on the other glass substrate. Further, the liquid crystal cell 203 has a peripheral circuit that supplies a video signal and a scanning signal to the pixel circuit, and signals are supplied from the peripheral circuit to the pixel electrode and the counter electrode of each pixel partitioned in a matrix. Thus, the transmittance of each pixel is controlled. The light emitted from the light emitting diode 101 of the backlight 100 is supplied to the liquid crystal panel 200 through an optical member such as a light guide plate or a diffusing member, and each pixel whose transmittance of the liquid crystal panel 200 is controlled is supplied to the backlight 100. The light is transmitted, so that an image is displayed.

  FIG. 2 is a graph showing the relationship between the duty ratio of the light emitting diode 101 and the allowable forward current. In the graph shown in FIG. 2, the horizontal axis represents the duty ratio (%), the vertical axis represents the current value (mA), and the solid line 10 represents a typical maximum rating that can be set in the light emitting diode 101. A conventional constant current PWM operation line is indicated by a broken line 11 in the figure. In the case of the broken line 11, the current value is fixedly set so as to be within the maximum rating when the duty ratio is 100% (DC lighting), and the duty ratio is variable based on the target value of the luminance input to the light emitting diode 101. When set, an LED drive signal is output.

  In the present embodiment, the pulse width of the light emitting diode 101 is set so as to have a duty ratio of 10% or less of a pulse period of a predetermined period (time from transmission of one pulse to transmission of the next pulse). . FIG. 2 does not show an isoluminance curve (a plot of the current value and the duty ratio at which the light emission luminance of the LED becomes equal), but the combination of the current value and the duty ratio that realizes the input target value (luminance) is unique. Instead, there are multiple combinations. For this reason, the duty ratio is fixedly set to any value of 10% or less, the maximum rated current at the duty ratio of 10% is set as the upper limit of the drive current, and the current value below the upper limit is appropriately set. The input target value is realized by controlling the brightness. As the target value to be input, the light emission luminance for displaying the display image (based on the pixel requiring the highest luminance among the display images) is set as the target value, and the target value is set at the fixed duty ratio. The drive circuit outputs a light emission pulse as an LED drive signal by calculating a current value that realizes the target value.

  Here, in the graph of FIG. 2, the operation line of the light emitting diode 101 of the present embodiment is indicated by a solid line 12. In the driving method of the light emitting diode 101 in the present embodiment, since the pulse is a short time that completes the light emitting mechanism before the temperature rise due to local heat generation during driving, light is emitted with high light emission efficiency. To do. When the duty ratio is fixedly set to 10% or less in each pulse period, a period during which the light emission pulse is not output is secured longer than the period during which the light emission pulse is output. The short period during which the light emission pulse is output reduces the heat generation of the light emitting diode 101, and the long period during which the light emission pulse is not output increases the heat dissipation period mainly due to heat transfer. It will equilibrate at a relatively low temperature. In this way, the light emission efficiency can be improved in light that is always lit visually by light emission by repeating short-time pulses.

  Further, in the present embodiment, when setting the current value and the duty ratio for realizing the input target value (luminance), the setting is fixed so as to be selected from a plurality of duty ratios in a range where the duty ratio is 10% or less. . The means for selecting the current value and the duty ratio for the target value is selected by preparing a table in which the target value to be input is associated with the set value of the current value and the duty ratio, for example. Alternatively, an arithmetic expression for calculating the duty ratio and the current value with respect to the target value may be prepared. Thus, by combining a duty ratio of 10% or less and a current value that can be set at a duty ratio of 10% or less, it is possible to emit light while suppressing a temperature rise of the light emitting diode 101, thereby improving luminous efficiency. Become. In the present embodiment, the duty ratio is set to 10% or less. However, since this value varies depending on the characteristics of the light emitting diode 101, the present invention is not limited to this value, and a low temperature even at the maximum rated current value. It is sufficient to use a duty ratio that balances at.

  FIG. 3 is a diagram illustrating a state in which the video signal 110 is supplied to the liquid crystal panel 200 and the backlight 100 in the liquid crystal display device according to the present embodiment, and the hardware configuration of the liquid crystal display device according to the present embodiment. Functional configuration is shown.

  As shown in FIG. 3, first, the video signal separation unit 301 sets a target value of the luminance of the backlight 100 according to the video signal 110 input from the outside, and outputs it to the backlight 100, and also the liquid crystal panel 200. The transmittance of each pixel is also set and output to the liquid crystal panel 200. The backlight 100 includes a light emitting diode 101, a drive circuit 102 that emits light from the light emitting diode 101, a luminance input acquisition unit 103, and a table holding unit 104. The luminance input acquisition unit 103 and the table holding unit 104 are configured to be included in a flexible printed circuit board 109 connected to the light emitting diode 101 together with the driving circuit 102. The target luminance value output from the video signal separation unit 301 is acquired by the luminance input acquisition unit 103 as the luminance input 110 </ b> A of the backlight 100. Then, the luminance input acquisition unit 103 further acquires the luminance input of each light emitting diode 101 in the backlight 100 from the luminance input 110 </ b> A, and supplies the LED luminance input 112 to the drive circuit 102.

  The table holding unit 104 holds a table in which the LED luminance input 112 that is a target value of the luminance of the light emitting diode 101 is associated with the duty ratio and current value of the light emitting diode 101. The drive circuit 102 inquires of the table holding means 104 about the duty ratio and the current value corresponding to the LED luminance input 112 and acquires them. Then, the pulse generation unit 102 </ b> A generates a light emission pulse having a duty ratio obtained from the table holding unit 104 and supplies it to the light emitting diode 101. The light emission pulse supplied to the light emitting diode 101 is controlled by the current value setting (control) means 102B to the current value obtained in the same manner from the table holding means 104. The light-emitting pulse is supplied as an LED drive signal 115 for driving the light-emitting diode 101, and the light-emitting diode 101 emits light with a duty ratio of 10% or less by the LED drive signal 115, and the temperature rise is suppressed. .

  The liquid crystal panel 200 includes a display signal processing unit 210 and a pixel circuit 220. The display signal processing unit 210 is mounted, for example, in a peripheral circuit in the liquid crystal cell 203, and receives the transmittance target value 110B of each pixel output from the video signal separation unit 301. Then, the display signal separation unit 210 generates a pixel drive signal 116 according to the transmittance target value 110B in each pixel and supplies it to the pixel circuit 220.

  Note that the backlight 100 may include an external environment detection unit that separately acquires the brightness of the external environment with a sensor. In this case, the external environment detection is performed with respect to the luminance input received by the luminance input acquisition unit 103. Reflecting the brightness detected by the means, the brightness of the backlight 100 (the brightness of the light emitting diode 101) is set.

  The video signal 110 is a signal including a synchronization signal based on a standard such as NTSC. The luminance input acquisition unit 103 receives the backlight luminance input 110 </ b> A from the video signal separation unit 301. The video signal separating unit 301 detects the maximum value of the video signal in one frame displayed on the liquid crystal panel 200 and outputs the maximum value to the backlight 100 as the luminance value of the backlight. If the amplitude range of the video signal is 0 to 255 and the detected maximum value in one frame is 100, the luminance value of the backlight in the frame period is set to 100/255. For the sake of simplicity, the description will be made ignoring the gamma characteristic of the signal. Here, if the transmittance set in each pixel of the liquid crystal panel 101 is 0% to 100%, the transmittance corresponding to the maximum value 100 of the video signal is 100% in the frame period. Set to. That is, the video signal separating unit 301 multiplies the video signal by (100 / (maximum amplitude of luminance of the video signal)), calculates the transmittance of each pixel, and sets the display signal processing unit 210 to the transmittance target value 110B. Output. As a result, the transmittance of the pixel taking the maximum value of the video signal becomes 100%.

  The drive circuit 102 derives a generation condition of the light emission pulse (LED drive signal 115) from the LED luminance input 112 acquired from the luminance input acquisition unit 103. In this embodiment, the drive circuit 102 inquires the table holding unit 104 for the duty ratio and current value corresponding to the luminance input value, and derives the relationship, for example, the relationship between the luminance input value, the duty ratio, the current value, and the like. You may make it determine using a type | formula. In this case, for example, two relational expressions of a relational expression in which the duty ratio is determined according to the luminance input value and a relational expression in which the current value is determined in accordance with the luminance input value and the duty ratio. Is held by the recording means. The pulse generation means 102A in the drive circuit 102 generates a light emission pulse having a duty ratio obtained from the table holding means 104, and the current value of the light emission pulse is controlled by the current value setting means 102B. In this way, a light emitting pulse is provided to the light emitting diode 101 to emit light, and the liquid crystal panel 200 selectively transmits light emitted from the backlight 100 to form a display image. In this embodiment, the video signal separation unit 301 is provided in the liquid crystal display device. However, the luminance input acquisition unit 103 and the display signal processing unit 210 receive the video signal directly without providing the video signal separation unit 301. It may be. In this case, the luminance input acquisition unit 103 may calculate and acquire the luminance target value of each light emitting diode 101 by identifying the maximum value of the video signal for each frame.

  Here, the chromaticity of light emission of the light emitting diode 101 changes depending on the driving conditions that combine the driving current value, temperature, elapsed time, and the like. For example, when an illumination unit using the light emitting diode 101 is used as the backlight 100 of the liquid crystal display device, a change in chromaticity causes image quality degradation. Examples of the backlight 100 include an illuminating unit combining RGB (red, blue and green) light emitting diodes 101, an illuminating unit using a W (white) light emitting diode 101, and the like. Examples of the image quality deterioration of the liquid crystal display device caused by the characteristic variation of the light emitting diode 101 include a chromaticity change of RGB three primary colors, a chromaticity change of white, and a luminance change.

  In this embodiment, it is expected that the change in chromaticity is reduced by the effect of reducing the temperature rise of the element by using short-time pulse light emission with a duty ratio of 10% or less, but it eliminates all the above factors. However, on the other hand, the chromaticity may change due to a larger range in which the drive current value fluctuates than in the PWM method or the like. Therefore, the display signal processing unit 210 is further provided with a chromaticity compensation unit 210A that compensates for a change in chromaticity based on a set value of the drive current of the light emitting diode 101 or the like. The display signal processing unit 210 acquires light emission information 113 such as a current set value from the drive circuit 102 in the backlight 100 as information on conditions for causing the light emitting diode 101 to emit light. In other words, the display signal processing unit 210 includes a light emission condition acquisition unit that acquires information on conditions under which the light emitting diode 101 emits light. Based on the acquired light emission information 113, the chromaticity compensation unit 210A compensates for the chromaticity change in the backlight 100. Further, the display signal processing unit 210 generates a pixel drive signal 116 in which the change in chromaticity is compensated, and supplies it to the pixel circuit 220. The change in chromaticity generated in the backlight 100 is caused by the transmittance of each pixel in the liquid crystal panel 200. Therefore, compensation is made for each color. Note that the light emission information 113 in the present embodiment is information on the current value in the LED drive signal 115, but the information on the LED luminance input 112, the information on the temperature of the light emitting diode 101, or the current value information and the duty ratio. The information may be received by the display signal processing unit 210 as the light emission information 113 to compensate for the chromaticity change. When the chromaticity compensation unit 210A compensates the chromaticity change, the chromaticity compensation table (transparency compensation amount for the transmittance target value 110B and the current value information 113) separately held in the liquid crystal panel 200 is used. May refer to a table associated with each color).

  Signals to be compensated in the liquid crystal display device include an LED drive signal 115 for driving the light emitting diode 101 constituting the backlight 100 and a pixel drive signal 116 for controlling the transmittance of each pixel. Compensation operation based on the characteristic change of the light emitting diode 101 includes a method of compensating the chromaticity change only by the pixel drive signal 116, a method of compensating the chromaticity change by both the LED drive signal 115 and the pixel drive signal 116, There is. In the present embodiment, the pixel drive signal 116 is compensated by the current value information 113 as described above, and the chromaticity change is also compensated for in the LED drive signal 115. The compensation in the LED drive signal 115 is to maintain the white chromaticity (white point) by the combination of the RGB three-color light emitting diodes 101, for example, and is held by the table holding unit 104, for example. By maintaining the compensated value in the table, the LED drive signal 115 is compensated. The LED drive signal 115 may be compensated according to the chromaticity within one frame displayed by the liquid crystal display device. In this case, first, the backlight 100 separately acquires chromaticity information indicating which chromaticity of RGB of the image to be displayed is higher from the video signal 110. (The backlight 100 may separately acquire the transmittance target values 110B of the light-emitting diodes 101 for each of RGB colors, and determine the chromaticity of the image to be displayed from these transmittance target values 110B.) According to the chromaticity information, the backlight 100 emits light with the chromaticity approaching one of the three RGB colors, and the chromaticity compensation unit 210A further compensates the transmittance of each pixel according to the chromaticity emitted by the backlight 100. It may be.

  In general, the specification of the light emitting diode defines the maximum rating of the LED drive current that does not cause destruction of the element at the set duty ratio. Therefore, when using a light emitting diode, it is required to use it so as to keep these maximum ratings. By the way, when the light emitting diode 101 is used as the light emitting means of the backlight 100 of the liquid crystal display device, the brightness of the display screen may be set so that the maximum value of the input video signal 110 can be displayed. However, in practice, the video signal 110 generally has amplitudes that take various values. This amplitude also changes on the two-dimensional coordinates of the screen and every frame, and the average value of the amplitude depends on the content of the video signal 110. The format of the video signal itself is determined by standards such as NTSC, but there is no standard that determines the content of the video signal and the brightness when it is displayed on the display screen.

  Therefore, in the present embodiment, the current value setting unit 102B sets the drive current of the light emitting diode 101 in the backlight 100 so that the average is not more than the maximum rating. FIG. 4 is a diagram illustrating an example of a current value (solid line) of a light emission pulse supplied to the light emitting diode 101 based on an input video signal and a moving average (broken line) in a predetermined time width. Even if the light emission pulse supplied to the light emitting diode 101 exceeds the maximum rating due to the instantaneous peak of the input video signal, the light emission pulse supplied to the light emitting diode 101 is lower than the maximum rating on average. It becomes. In the present embodiment, as described above, the LED drive signal 115 is set so that the light emission pulse for a predetermined period is less than or equal to the maximum rating. Here, the method for calculating the average value is not particularly limited. For example, the time width may be appropriately set to 1 second, 1 minute, or the like. In this embodiment, the maximum rating of the LED driving current is set to be equal to or less than the maximum rating on average, but the temperature of the element of the light emitting diode 101 is set to a certain value or less regardless of the current value of the maximum rating. The current value may be maintained on average.

  In addition, the temperature of the light emitting diode 101 can measure the temperature of an LED element, for example using a temperature sensor. Alternatively, the element temperature can be derived by measuring the LED light emission amount using a luminance sensor and using the relational expression between the light emission amount variation and the element temperature. Alternatively, by measuring the voltage applied to the LED and the flowing current, the element temperature can be derived by using the relational expression of the voltage, current and element temperature of the LED element.

  5A and 5B are diagrams showing circuit configuration examples of the pulse generation means 102A and the current value setting means 102B. The current value of the light emission pulse is controlled by a digital signal. The pulse generator 102A in this embodiment generates a pulse signal when the connection with the voltage source 601 is turned on / off by the pulse drive switch 602 at a predetermined frequency and a duty ratio of 10% or less. As the pulse drive switch 602, a semiconductor element such as a transistor or an FET can be used. Further, the current value setting means 102B in the present embodiment includes a current limiting element 603 and a changeover switch 604, and the current flowing through the light emitting diode 101 is determined by these. A plurality of resistors or constant current diodes are used for the current limiting element 603, and the former is used in FIG. 5A and the latter in FIG. 5B. The changeover switch 604 is a semiconductor element such as a plurality of transistors or FETs. The changeover switch 604 switches the combination of the current limiting elements 603 by being turned on / off by a digital signal that sets an input current value. When the current value setting signal input here is an N-bit digital signal, 2 to the Nth power is the upper limit of the number of combinations of current value settings by the current limiting element 603. Can be set. As a simple example, the input digital signal Din is a 5-bit current switching signal. When five constant current diodes are used as the current limiting element 603 shown in the figure, if the current ID of each constant current diode is used, the changeover switch 604 is turned on / off using this digital signal Din. Thus, the current flowing through the light emitting diode 101 can be switched by variably setting the number of combinations of constant current diodes by the changeover switch 604 in the range of 0 to 5. Here, if the current limit value of the current limiting element 603 corresponding to each bit is set with a power of 2 power, a current value in binary representation can be set. Thus, by switching the current value of the LED drive signal 115 by setting Din, it can be used as a method of switching the brightness of the illumination, for example. In order to increase the accuracy of current value control by the current value setting means 102B, the number of current limiting elements 603 may be increased and the number of bits of the signal Din for switching may be increased.

  In the present embodiment, an illuminating device in which a light emission amount is controlled by periodically generating a light emission pulse at a duty ratio of 10% or less and controlling a current value of the light emission pulse is a liquid crystal display device. It is used as a backlight 100 configured to include. However, it goes without saying that the illumination device according to the present embodiment may be used for illumination devices other than the backlight 100 of the liquid crystal display device. The present invention is not limited by the description of the first embodiment described above and the description of the second and later embodiments described below, and various changes and modifications can be made by those skilled in the art within the scope of the technical idea. Moreover, the form which combined the form currently disclosed in each of these embodiment shall also be contained in this invention within the range of the technical idea.

  Further, in a backlight or lighting device having light emitting diodes of RGB colors, the light emitting diodes of any of the light emitting colors are driven using the LED driving signal 115 as described above while suppressing the temperature rise, and the remaining light emitting diodes In this case, it may be driven by a PWM method or the like. In particular, as shown in FIG. 11A, FIG. 11B, and FIG. 11C, the red light emitting diode does not improve the light emission efficiency even when the duty ratio is low. The green light emitting diode may be driven by the method using the LED driving signal 115 as described above.

  Further, FIG. 6 shows a case in which a backlight 100 or an illuminating device including N light emitting diodes 101 as light emitting means shifts the phase of each pulse period and sequentially turns on with light emission pulses having a low duty ratio. FIG. In the figure, the phase of the pulse period of each light emitting means is provided to be shifted by 2π / N period.

  In the configuration as shown in FIG. 6, the individual light emitting diodes 101 are set to have a high light emission efficiency by suppressing the light emission duty ratio as described above, and the whole light emitting means is combined. Since this corresponds to shortening the lighting cycle, the duty ratio becomes relatively high. Thereby, a reduction in luminance can be prevented, and flickering due to the lighting cycle can be reduced.

  The pulse period or phase relationship of each of the plurality of light emitting means may be set at random, but if it is random, the phase may be aligned in a situation where the lighting device emits light for a long period of time. It becomes flickering of lighting. Therefore, it is desirable to configure the drive circuit 102 in advance so that the phase of the pulse period of each light emitting means does not match. In order to realize this, a digital pulse generation circuit such as a pulse counter and a shift register may be combined, or an analog pulse generation circuit may be used.

  The number of light emitting means to be combined is not limited. For example, if two are combined, the luminance is doubled, and if N, the luminance is N times, so that a great effect can be obtained in luminance improvement. A plurality of light emitting means may be mounted in a single case, or may be divided into one or a plurality of independent cases that are dispersed in position. Here, the meaning of the above case is not limited, but this case can be, for example, one closed system sharing a pedestal. And, for example, when a plurality of light emitting means are arranged on the ceiling as room lighting, if a plurality of cases for storing the light emitting means are individually arranged so as to cover the entire surface of the ceiling, the combination of the light emitting means for the entire ceiling It can be set as room lighting which illuminates the whole room. By driving each light emitting means with light emission pulses having different phases as described above, it is possible to increase the light emission efficiency while reducing flickering, that is, to obtain the effect of reducing power consumption. Here, the pulse generation circuit is shared for a plurality of cases, and driving is performed while maintaining the phase relationship. Specifically, the light emission timings of the plurality of light emitting means can be controlled by preparing signal lines from the pulse generating circuit to the plurality of light emitting means and multiplexing the control signals instructing the plurality of light emission timings. . Alternatively, the control signal can be multiplexed on the power supply line without using a signal line for new control. Alternatively, an optical transmission system that multiplexes a control signal with illumination light can be configured by combining a light emitting means and an optical sensor.

  In the description of the first embodiment, the ratio of the light emission pulse in the pulse period of the light emitting diode 101 is determined as the duty ratio, and the pulse generation means 102A emits the light emission pulse at a duty ratio of 10% or less at a predetermined frequency. Is generated periodically. The duty ratio itself is not a numerical value that determines the pulse period (the reciprocal of the frequency). When the duty ratio is the same, the longer the period, the easier it is to visually observe flicker. In general, in a television receiver, screen rewriting is performed at 60 Hz or more (30 Hz in the NTSC standard for interlaced scanning) in order to suppress visual flicker. Therefore, the pulse period of the light emitting diode 101 as the light emitting means in this embodiment is desirably 60 Hz or more. A circuit that generates such a light emission pulse that defines the duty ratio and period (or frequency) can be appropriately manufactured by a digital circuit or an analog circuit.

[Embodiment 2]
In the first embodiment, the relationship between the timing for outputting the light emission pulse 115 and the frame period for switching the display image on the liquid crystal panel 200 is not particularly defined. On the other hand, in the second embodiment, a lighting period in which a plurality of light emission pulses (pulse groups) having a duty ratio of 10% or less is output is provided. One or a plurality of the lighting periods are present in the frame period of the video signal 110, and a light-out period during which no light emission pulse is output is provided between the lighting period and the lighting period. In particular, the light emission pulse group is generated so that the timing is shifted between the lighting period in any one of the frame periods and the lighting period in a frame period different from the frame period. Specifically, as shown in FIG. 7, the timing of the lighting period is shifted in two adjacent frame periods. Except for this point, the second embodiment is substantially the same as the first embodiment.

  FIG. 7 shows a state of a 60 Hz video signal in which one frame cycle for switching the transmittance of the liquid crystal panel between 0% and 100% for each frame cycle is 1/60 seconds, and the response of liquid crystal molecules corresponding to this video signal. It is a figure which shows the mode of an example of the LED drive signal 115 output synchronizing with a frame period. The video signal 110 transmits data of one screen repeatedly at a frame period. For the sake of convenience, FIG. 7 shows an example in which the transmittances are repeated 100% and 0% for each frame period (FIG. 7A). Therefore, the liquid crystal panel 200 repeatedly displays white and black based on the video signal 110, but the response speed of the liquid crystal element for each pixel is finite and does not rise or fall immediately. In FIG. 7B, the liquid crystal response is approximately shown using an EXP function. However, if the response is sufficiently fast, the step function approaches the step function.

  In the lighting period for turning on the backlight 100, three lighting periods are arranged in the frame period # 1, and two lighting periods are arranged by inverting the lighting period and the extinguishing period in the frame period # 2. ing. By providing the light-off period in this way, the moving image display characteristics of the liquid crystal display device are improved. Then, the timing of the lighting period is shifted in two different frame periods, so that the deviation of the driving state of the liquid crystal element at the timing when the backlight 100 blinks is eliminated, and the moving image characteristics are further improved.

  FIG. 7C is a diagram showing a state where two lighting periods in the range of the arrow portion in FIG. 7B are enlarged. As shown in FIG. 7C, the lighting period is configured by arranging a plurality of light emission pulses as a light emission pulse group. The light emission pulse refers to a pulse signal of the minimum unit for lighting the backlight within the backlight lighting period. During the lighting period, the light emitting diode 101 emits light with a duty ratio of 10% or less. In the second embodiment, the waveform of the lighting period is maintained, and the light emission luminance of the backlight 100 is adjusted by adjusting the current value and pulse width (duty ratio) of the light emission pulse 115 arranged in the lighting period. The In the second embodiment, as in the first embodiment, the light emitting diode 101 emits light with a short-time pulse with a duty ratio of 10% or less, and a light emission pulse group is configured by a plurality of light emission pulses.

  In the second embodiment, the timing of the lighting period is shifted in two adjacent frame periods, and the lighting period and the extinguishing period are reversed. Instead of synchronizing the period with the frame period, the period may be set asynchronously with the frame period. In the latter case, it is desirable to start the lighting period and the extinguishing period to be completely random with respect to the frame period. By doing so, the driving state of the liquid crystal element at the timing when the backlight 100 blinks Will be eliminated.

  FIG. 8 is a diagram illustrating an example of a functional configuration of the drive circuit 102 according to the second embodiment. In the second embodiment, light emission pulses are arranged in the lighting period so as to satisfy the set brightness condition while maintaining the lighting period. In order to satisfy the brightness condition, the duty ratio of the light emission pulse is made variable in the range of 0% to 10%, and the light emission pulse current value is made variable in the range of 0 mA to the rated current. The brightness within the lighting period is controlled by a value obtained by multiplying the light emission pulse width (the pulse width is derived from the frequency of the light emission pulse and the duty ratio) and the light emission pulse current value. First, the pulse generation means 102A receives the light emission pulse frequency information 119 and the LED luminance input 112 inputted from the outside. The pulse generation unit 102A determines the number of light emission pulses and the duty ratio according to the received information, and outputs the number of light emission pulses and the frequency information of the light emission pulses to the timing control unit 102D. Then, the pulse generation unit 102A sequentially generates and outputs the light emission pulse to the current value setting unit 102B, and the current value setting unit 102B controls the current value of the light emission pulse and outputs it to the switch unit 102C. Further, the lighting period is repeatedly output with the light extinction period interposed therebetween, and for these timings, the timing control means 102D manages the output of the light emission period and the sandwiching of the light extinction period, and switches the switch means 102C. Used to switch on / off the output of the light emission pulse group. By switching the lighting period and the extinguishing period by the switch means 102C, the period during which the LED drive signal 115 is supplied to the light emitting diode 101 and the period during which it is not supplied are controlled. The pixel clock generation unit 102E receives the horizontal synchronization signal 117 and the clock signal 118 of the light emission pulse, and provides the clock signal to the timing control unit 102D. In the second embodiment, the timing control unit 102D synchronizes the frame period of the liquid crystal display device with the timing at which the lighting period and the extinguishing period are provided. When the target value of the LED luminance input 112 is large, the duty ratio and current amplitude of the light emission pulse in the lighting period should be increased and the number of light emission pulses should be increased (the lamp period should be longer in the frame period). Just fine). When the target value of the LED luminance input 112 is small, the number of light emission pulses is reduced, or the duty ratio and current amplitude of the light emission pulses within the lighting period are set low. In the second embodiment, the frequency in the LED drive signal 115 is a frequency in a range suitable for driving the light emitting diode 101, and it is desirable to use a frequency of 60 Hz or higher as in the first embodiment.

[Embodiment 3]
In Embodiments 1 and 2 above, no particular consideration is given to errors in the brightness target values of the backlight 100 and the light emitting diode 101 emitted by the light emission pulse, but in Embodiment 3, these brightness target values and the light emitting diode 101 Consider an error (difference) from the light emission amount. The third embodiment is substantially the same as the second embodiment except for this point.

  Here, since the lighting period consisting of a plurality of light emission pulses is periodically repeated at high speed, even if there is a slight error in a single lighting period, if the error is canceled as a cumulative error, image quality deterioration due to the error will occur. The impact is suppressed.

  FIG. 9 is a schematic diagram showing how an error generated with respect to the luminance target value is carried over (propagated) and canceled in the next lighting period in the lighting period composed of a plurality of light emission pulses. The horizontal axis schematically shows the time axis, lighting periods 1 to 5 are provided in order from the left side in the figure, and there are further extinguishing periods between the lighting periods. The backlight 100 blinks periodically by combining the lighting period with an appropriate extinguishing period. The reciprocal of this cycle is the lighting frequency. The vertical axis represents the light emission amount of the light emitting diode 101. In FIG. 9, the cumulative value of the light emission amount by the light emission pulse in each lighting period is shown, and the target value in the LED luminance input 112 is a plurality of values in the lighting period. This is achieved by the cumulative value of the amount of light emitted by the light emission pulses.

  In the third embodiment, for convenience of explanation, the pulse width is fixed and the current value is also fixed at a constant value by making the frequency and duty ratio of the light emission pulse constant during the lighting period. And the target value of the LED luminance input 112 in each lighting period 1-5 is also constant. The frequency of the light emission pulse, the duty ratio, the current value, and the target value based on the luminance input 112 are constant for convenience of explanation in the third embodiment, and are set so as to be different for each light emission pulse group. Alternatively, the light emission amount may be controlled by intentionally changing the duty ratio in the light emission pulse during the lighting period.

  Note that the light emission amount (luminance value) by the single light emission pulse of the light emitting diode 101 is substantially proportional to the current value or pulse width. That is, the target value of light emission shown on the vertical axis can be converted into a product value of the drive current and the pulse width. The minimum unit for determining the light emission amount of the light emitting diode 101 can be converted by the current value which is the minimum unit determined by the current setting circuit connected to the DA converter when the pulse width is constant. At this time, the set number of light emission amounts of the light-emitting diode 101 is the same as the number of bits of the DA converter (that is, the set number of stages), and a high-precision DA converter is required to control the brightness with high accuracy. It becomes.

  Here, the product of the current value and the pulse width, which are constant in the third embodiment, is set as the light emission amount W, and this light emission amount W is accumulated as a minimum unit, thereby corresponding to the target value input by the LED luminance input 112. The light emitting diode 101 emits light for the amount of light emitted.

ここでＩＮＴは整数化演算であり、切り上げとする。実際の発光量はＰｗとＷを掛け算した値になり、目標値Ｗｔと誤差が生じて、その大きさは、下記の式（２）で表される。

実施形態３では、ある点灯期間で発生する誤差Ｅｗが、その次の点灯期間に伝播される。 In FIG. 9, the number of accumulated light emission amounts W for each lighting cycle is numbered as (1), (2), (3),. Here, the target value of the light amount of the light emitting diode 101 obtained from the LED luminance input 112 in each lighting period is set to Wt. Then, when the light emission amount W is stacked in the vertical axis direction so as to achieve this target value, the number Pw of stacked light emission amounts W for realizing this is an integer value obtained by dividing the target value Wt by the light emission amount W. (Equation (1)).

Here, INT is an integer operation and is rounded up. The actual light emission amount is a value obtained by multiplying Pw and W, and an error occurs with the target value Wt. The magnitude is expressed by the following equation (2).

In the third embodiment, an error Ew that occurs in a certain lighting period is propagated to the next lighting period.

  In the third embodiment, the drive circuit 102 first accumulates the amount of light emitted by each light emission pulse, and the accumulated amount and target value each time the light emission pulse is emitted and the amount of light emission is accumulated. Comparing means for comparing whether or not the cumulative amount is equal to or greater than the target value is provided. Then, when the cumulative amount becomes equal to or greater than the target value in the comparison means, the cumulative amount storage means carries over the difference between the stored cumulative amount and the target value as the cumulative amount in the next lighting period. When the accumulated amount is equal to or greater than the target value, the switch means 102C terminates the supply of the light emission pulse to the light emitting diode 101 by the pulse generation means 102A, and the light extinction period is interposed. When the timing control means 102D causes the switch means 102C to start the next lighting period, the target value of the light emission amount to be emitted to the light emitting diode 101 in the next lighting period is obtained by subtracting the carried light emission quantity. . In this case, the backlight 100 may include a light emission amount detection unit that detects the light emission amount and accumulates it in the accumulation amount storage unit.

  In FIG. 9, the state in which the error Ew is propagated in the next lighting period (the difference between the light emission amount in the lighting period and the target value of the light emission amount is carried over to the next lighting period) is indicated by arrows. The error component is already emitted in the lighting period that is the propagation source, and does not emit light in the lighting period that is the propagation destination, but the magnitude is regarded as being emitted in the lighting period that is the propagation destination and is accumulated. . In the lighting period of the propagation destination, the amount of light emission W by each light emission pulse in the lighting period is accumulated with the propagated error Ew as an initial value. If the accumulated value exceeds the target value Wt, the error propagation procedure is performed again. By repeating this procedure, the target value can be achieved on average. In the third embodiment, the target value Wt and the light emission amount W have been described as being constant, but it goes without saying that these can be changed for each lighting period.

  The error propagation type pulse generation procedure as described above can also be used for lighting control of the backlight 100 combining multicolor light sources such as RGB (red blue green) or RGBW (red blue green white). Here, a generation procedure of a backlight-driven light emission pulse for controlling the brightness of white light emission using the light emitting diodes 101 of RGB three colors will be described.

  FIG. 10 is a schematic diagram showing a state in which errors generated with respect to the luminance target value are accumulated and canceled in the case of emitting light by combining the three primary colors of RGB. It is known that the emission wavelength distribution of the light emitting diode 101 can be replaced with tristimulus values X, Y, and Z in consideration of human visual sensitivity. A numerical value obtained by converting the emission wavelength distribution of the light emitting diode 101 of RGB three colors into the tristimulus values XYZ is set as a calculation target in the pulse generation procedure. For each of XYZ, the cumulative value and the target value are compared, and the operation procedure for outputting a light emission pulse based on the comparison result is the same as in the case of the white LED, and each of the XYZ characteristic values is a light emitting diode of RGB three colors. It is different in that it is accumulated according to the amount of light emission 101. Here, the target values of the tristimulus values are assumed to be Xt, Yt, and Zt (for convenience of explanation, Xt, Yt, and Zt are displayed as common values in FIG. 10, but different values may be used. Needless to say. Xt, Yt, and Zt are characteristic values that realize white (white point) by turning on LEDs of three colors RGB. The light emitting diodes 101 of RGB colors represent the light emission amount when the duty ratio is 100% (DC lighting) in units of XYZ, the characteristic values of the red LEDs are Xr, Yr, Zr, and the green LED are characteristic values of Xg. , Yg, Zg, and blue LED characteristic values are Xb, Yb, Zb.

In order to obtain the LED drive signal 115 with high accuracy, the frequency of the pulse may be simply increased. On the other hand, in the third embodiment, in order to achieve high accuracy by combining a plurality of pulses, calculation of error propagation as in the above formulas (1) and (2) is performed on the XYZ converted signal. A pulse drive signal (light emission pulse) P having a product of current value and pulse width of the minimum 0 and maximum 1 is prepared, the red LED pulse drive signal Pr, the green LED pulse drive signal Pg, blue If the LED pulse drive signal is Pb, the output of each of the RGB three-color light emitting diodes 101 can be expressed by the product of the drive signal of the red, green and blue light emitting means and the XYZ characteristic matrix.

The drive signals (Pr, Pg, Pb) of the light emitting diodes 101 for each color RGB for realizing the output target value can be calculated by multiplying the target value by the inverse of the XYZ characteristic matrix.

  However, the calculation of the inverse of the matrix is complicated, and it is not suitable for a simple apparatus configuration considering that the characteristic value fluctuates under various light emission conditions such as temperature. Therefore, in the third embodiment, in order to obtain the drive signal by directly using the components of the XYZ characteristic matrix, the above-described cumulative amount storage means or means for comparing the cumulative quantity with the target value is used. In the third embodiment, in order to calculate a specific drive signal, the drive signals pr, pg, and pb, which are the minimum units, are used to accumulate the actual drive signals. The characteristic values of the emission characteristics XYZ of the red LED by the drive signals pr, pg, pb as the minimum unit are Xpr, Ypr, Zpr, and the characteristic values of the emission characteristics XYZ of the green LED are Xpg, Ypg, Zpg, The characteristic values of the light emission characteristics XYZ of the blue LED are Xpb, Ypb, and Zpb.

The drive signals pr, pg, and pb can be converted into the product of the pulse width and the current value, the number of times of accumulation corresponds to the digital data of the DA converter, and the maximum number of times of accumulation is the bit width. Equivalent to. In the third embodiment, a high accuracy is realized as a combination of a plurality of DA conversion signals while a relatively small bit width, that is, a relatively coarse DA conversion signal. For the light emitted by the drive signals pr, pg, pb, the cumulative value of X is Xa, the cumulative value of Y is Ya, the cumulative value of Z is Za, the target values Xt, Yt, Zt and the cumulative values Xa, Ya , Za are compared, and if the accumulated value does not reach the target value, the pulse drive signals pr, pg, and pb are individually output as in the case of the white LED described above.
If you rewrite
IF (Xt> Xa) pr = 1
ELSE pr = 0
IF (Yt> Ya) pg = 1
ELSE pg = 0
IF (Zt> Za) pb = 1
ELSE pb = 0

If the pulse drive signal pr = 1, Xpr is added to the cumulative value Xa, Ypr is added to the cumulative value Ya, Zpr is added to the cumulative value Za,
If the pulse drive signal pg = 1, Xpg is added to the accumulated value Xa, Ypg is added to the accumulated value Ya, Zpg is added to the accumulated value Za,
If the pulse drive signal pb = 1, Xpb is added to the accumulated value Xa, Ypb is added to the accumulated value Ya, and Zpb is added to the accumulated value Za.
If you rewrite
IF (pr == 1) {Xa + = Xpr, Ya + = Ypr, Za + = Zpr}
IF (pg == 1) {Xa + = Xpg, Ya + = Ypg, Za + = Zpg}
IF (pb == 1) {Xa + = Xpb, Ya + = Ypb, Za + = Zpb}

As shown by the lighting period 1 in FIG. 10, the above processing is repeated within a single lighting period, and the pulse drive signal is generated when the accumulated values Xa, Ya, Za exceed the target values Xt, Yt, Zt, respectively. Complete the generation procedure. The number of the accumulated pulse drive signals pr, pg, and pb is used as a digital value of a DA converter that sets a current in the lighting period. Then, the accumulated value is compared with the target value, and the error is propagated to the next lighting period. Error calculation is performed for each of the three colors (formula (5)).

  Using these error values Xe, Ye, and Ze as initial values of the accumulated values Xa, Ya, and Za of the next lighting period, generation of a pulse drive signal that takes into account the wavelength distribution of each of the light emitting diodes 101 of RGB three colors And output can be realized. Since the target value can be set in the XYZ format, there is a feature that it is possible to set a light emission color to be an arbitrary stimulus value. When the characteristics of the light source change due to some factor, it is only necessary to change the XYZ values such as the characteristic values Xpr, Ypr, and Zpr at an arbitrary timing. For example, if the XYZ value of LED emission due to temperature varies, the XYZ value may be corrected in accordance with the variation. Similarly, if there is a change in the XYZ value of LED light emission due to the lifetime or the like, the XYZ value may be corrected in accordance with the change. If the inverse calculation of Expression (4) is used, it can be realized very easily compared to the case where a huge calculation load is applied to reflect the change in the characteristic value.

As a variation of the above, the present invention can be applied to light emission of four colors obtained by adding white (W) to three colors of red, green, and blue (RGB). W has characteristic values Xw, Yw, and Zw of XYZ. Then, the XYZ values in the minimum unit of the white LED pulse drive signal are defined as Xpw, Ypw, and Zpw. The target value of XYZ is compared with the cumulative value. If the cumulative value does not reach the target value for all of XYZ, W is output as a pulse.
If you rewrite
IF ((Xt> Xa) &(Yt> Ya) &(Zt> Za)) pw = 1
ELSE pw = 0
Then, Xpw, Ypw, and Zpw are added to the accumulated values Xa, Ya, and Za of XYZ.
If you rewrite
IF (pw == 1) {Xa + = Xpw, Ya + = Ypw, Za + = Zpw}
In this way, the pulse drive signal for realizing the target value is calculated using the RGBW4 colors.

  In the third embodiment, the accuracy of the output signal is improved by error propagation as described above with a simple apparatus configuration without using the inverse calculation means of the characteristic matrix and without operating the circuit at a high frequency by shortening the pulse period. There is an effect that can be realized.

  In the third embodiment, the frequency of the light emission pulse, the duty ratio, the current value, and the target value based on the luminance input 112 are constant, and the light emission amount W by each light emission pulse is the same. However, in actuality, these are set to fluctuate for each lighting period, and the light emission amount W may be different for each lighting period. Even in the same lighting period, the frequency, duty ratio, and current value of each light emission pulse In some cases, it is used with intentionally fluctuating. In Embodiment 3 described above, a target value that causes a difference with respect to the accumulated amount of the light emission amount W, which is the minimum unit of the light emission amount, without considering the case where an error occurs in the light emission amount due to each light emission pulse. Is set. However, even when the light emission amount varies from one light emission pulse to another or when an error occurs in the light emission amount of the light emission pulse, as in the third embodiment, the accumulated amount of the light emission amount is equal to or greater than the target value. The error is carried over to the next lighting period. As a result, an error is allowed in the current value and pulse width of a single pulse, the error is accumulated for each of a plurality of light emission pulses, and the light emission amount exceeding the target value is carried forward as the initial value of the next target value. . Therefore, a circuit configuration that realizes high accuracy as an average output of the light emission amount of the light emitting diode 101 is realized with a simple circuit configuration. In the third embodiment, the light-out period is interposed between the light-on periods as needed. However, by providing the LED luminance input 112 to the pulse generation unit 102A in sequence, the light emission amount that becomes the target value is effective without providing the light-out period. The lighting period to be activated may be provided continuously.

  DESCRIPTION OF SYMBOLS 10 Maximum rating in light emitting diode, 11 Operation line of constant current PWM system, 12 Operation line of light emitting diode according to embodiment 1, 100 backlight, 101 light emitting diode, 102 drive circuit, 102A pulse generation means, 102B current value setting means , 102C switch means, 102D timing control means, 102E pixel clock generation means, 103 luminance input acquisition means, 104 table holding means, 109 flexible printed circuit board, 110 video signal, 110A luminance input (backlight luminance input), 110B transmittance target Value, 112 LED luminance input, 113 light emission information (current value information), 115 LED drive signal (light emission pulse), 116 pixel drive signal, 117 horizontal synchronization signal, 118 light emission pulse clock signal, 119 frequency information, 00 liquid crystal panel, 201 upper polarizing plate, 202 lower polarizing plate, 203 liquid crystal cell, 210 display signal processing means, 210A chromaticity compensation means, 220 pixel circuit, 301 video signal separation means, 601 voltage source, 602 pulse drive switch, 603 Current limiting element, 604 selector switch.

Claims (6)

A backlight having a light emitting diode and a drive circuit for driving the light emitting diode by supplying a light emission pulse ;
A liquid crystal display device including a liquid crystal panel that displays an image by driving a liquid crystal layer sandwiched between two substrates according to a video signal and selectively transmitting light supplied from the backlight. There,
The drive circuit is
Pulse generating means for periodically generating the light emission pulse at a predetermined duty ratio;
Current value control means for controlling a current value of the light emission pulse generated at the predetermined duty ratio;
Luminance input receiving means for receiving an input of luminance to emit light ,
The predetermined duty ratio is a duty ratio of 10% or less ,
The pulse generation means determines the predetermined duty ratio by setting to any value of 10% or less according to the input, and generates the light emission pulse,
The light emission amount of the light emitting diode is controlled by the current value control means, and the current value control means controls the current value of the light emission pulse generated at the predetermined duty ratio according to the input,
A lighting period for periodically supplying a plurality of the light emission pulses to the light emitting diode at the predetermined duty ratio is configured to be synchronized with a frame period for driving the liquid crystal layer in the liquid crystal panel.
A liquid crystal display device characterized by the above. The liquid crystal display device according to claim 1 .
The liquid crystal panel is
From the drive circuit, having a light emission condition acquisition means for acquiring a condition for causing the light emitting diode to emit light,
Compensating for the chromaticity that the backlight gives to the image by compensating the video signal according to the condition acquired by the light emission condition acquisition means,
A liquid crystal display device characterized by the above. The liquid crystal display device according to claim 1 .
The light emission pulse supplied to the light emitting diode is compensated so that the predetermined duty ratio or the current value brings the chromaticity of the backlight closer to white.
A liquid crystal display device characterized by the above. The liquid crystal display device according to claim 1 .
The backlight is
Chromaticity information acquisition means for acquiring chromaticity information of an image to be displayed from the video signal;
The drive circuit compensates the chromaticity of the backlight to be close to any of RGB according to the chromaticity information, and supplies the light emission pulse to the light emitting diode,
The liquid crystal panel is
Displaying the image by driving the liquid crystal layer according to the light of the backlight that is compensated so that the chromaticity approaches one of RGB.
A liquid crystal display device characterized by the above. The liquid crystal display device according to claim 1 .
Before Symbol drive circuit,
Said lighting period, and extinction period for stopping the supply to the light emitting diodes of the light emitting pulses, to have supplies the light emitting pulses to the light emitting diode,
Starting the lighting period in one of the frame periods and the lighting period in a frame period different from the one by shifting the timing with respect to each frame period;
A liquid crystal display device characterized by the above. The liquid crystal display device according to claim 1 .
The drive circuit is
Luminance target value receiving means for sequentially receiving the target value of the luminance emitted by the light emitting diode;
Said lighting period, and extinction period for stopping the supply to the light emitting diodes of the light emitting pulses, so as to have a switching means for supplying the light emitting pulses to the light emitting diode,
A cumulative amount storage means for accumulating the light emission amount by the light emission pulse supplied to the light emitting diode and storing the cumulative amount;
Comparing means for determining whether or not the accumulated amount in the lighting period is equal to or greater than the target value;
The switching means starts the lighting period when the brightness target value receiving means accepts the target value, and the lighting means is turned on when the comparing means determines that the accumulated amount is equal to or greater than the target value. End the period,
The accumulated amount storage means, when the comparing means determines that the accumulated amount is equal to or greater than the target value, leaves the difference between the accumulated amount and the target value, and the difference is stored in the next lighting period. carry forward,
A liquid crystal display device characterized by the above.

Images