KR20090014903A - Method and apparatus for driving back light of liquid crystal display - Google Patents

Abstract

A backlight driving method of an LCD device and a device thereof are provided to calculate duty values of RGB LEDs that satisfy a color signal value of a target color space, thereby maintaining regular brightness and colors all the time regardless of peripheral environmental changes. To perceive an emission state of an LED backlight unit, an output value of an RGB optical sensor installed around the backlight unit is specialized to estimate the current CIEXYZ values(S22). Based on duty values and the current CIEXYZ values, An XYZ to duty conversion matrix is updated so that an LED emission state of the current LED backlight unit is reflected(S23). Duty values of LEDs are calculated to output a target CIEXYZ value by using the updated results(S24). Driving pulses corresponding to the calculated duty values are generated and outputted to the LED backlight unit(S25).

Description

 TECHNICAL FIELD & APPARATUS FOR DRIVING BACK LIGHT OF LIQUID CRYSTAL DISPLAY}

The present invention relates to a technology for preventing a change in the characteristics of the backlight in the liquid crystal display device, in particular a liquid crystal that can maintain a constant brightness and chromaticity at all times regardless of environmental changes in the backlight using a three-color LED (RGB LED) A backlight driving method and apparatus for a display device are provided.

In general, the liquid crystal display does not emit light by itself and includes a backlight unit for supplying light. The backlight unit mainly uses a Cold Cathode Fluorescent Lamp (CCFL) as a light source, and transmits the light generated by the Cold Cathode Fluorescent Lamp through the light guide plate and is projected onto the LCD screen directly under the liquid crystal display.

However, the cold cathode fluorescent lamp has a problem of using a relatively large amount of mercury, which is a target of environmental regulation, and there is a risk of current leakage because it is connected to the inverter through a power line. In addition, the cold-cathode fluorescent lamp has a disadvantage that it is not suitable for TV use because its lifetime is only about 10,000 to 50,000 hours. In addition, it is weak in vibration and shock in terms of reliability, and in terms of color reproduction, there is a disadvantage in that visibility is much lower than that of existing CRT.

For this reason, in recent years, white LEDs have attracted attention as high reliability light emitting devices capable of overcoming the disadvantages of cold cathode fluorescent lamps because of their excellent color reproducibility, environmental friendliness, and lifetime. Because it is long.

However, in the drive device of the white LED backlight, a ripple occurs in the output voltage of the DC / DC converter, and the forward voltage periodically fluctuates, and the boost-up voltage of the DC / DC converter is generated. There was a problem that can not be easily changed.

Accordingly, recently, a backlight unit that provides white light using color LEDs, that is, three colors (RGB: Red, Green, Blue) LEDs (Light Emitting Diodes) instead of the white LEDs has been in the spotlight.

FIG. 1 is an example of the prior art, which shows a lighting measurement system and method US6344641 of a researcher.

Here, the microdisplay apparatus 14 provided with the at least one photodetector 11a and the intensity detection and control circuit 50 is provided, and the photodetector 11a can be irradiated using the irradiation source 12a. Make sure

Then, the intensity of the irradiation source 12a is measured using the photodetector 11a and transmitted to the intensity detection and control circuit 50. In contrast, the intensity detection and control circuit 50 compares the measured output voltage with the target output voltage and adjusts the irradiation source 12a to a predetermined level according to the comparison result.

However, in such a system, since the photodetector, which is an optical sensor, is a 1-channel sensor, it is not possible to perform a sensing operation for adjusting each LED driving current in a light emitting state in which RGB LEDs are simultaneously turned on, and one LED that is a sensing target among RGB LEDs. It was supposed to light up only.

Therefore, when used in the backlight driving technology of the conventional liquid crystal display device, there is a problem that the process of detecting the backlight light is complicated and takes a lot of detection time.

In addition, FIG. 2 shows another method of the related art (US6448550) for measuring and controlling the spectral content of an LED light source.

This is a semiconductor regulator using closed loop spectral control, which is a red LED (110a, 110b, 110c), green LED (120a, 120b, 120c) and blue LED (130a, 130b, 130c) is installed close to the photosensors 150a, 150b, 150c, and 150d.

The spectral contents of the light emitting diodes 110a-110c, 120a-120c, and 130a-130c are measured by the photosensors 150a-150d, and the light emitting diodes 110a-are measured based on the measured values. The current of 110c, 120a-120c, and 130a-130c is adjusted, and a desired spectral characteristic is achieved.

However, such a system can sense each time at the same time without having to turn on the LEDs in order to sense each of the RGB LEDs, but it requires a lot of installation time and cost accordingly because photosensors must be installed between the LEDs. There was this.

Another example of the prior art is US Patent US6507159B2. This patent uses the LED current temperature and current current as parameters when modeling the LED control current and the transfer function of the LED output light (G_LED), but there are errors in modeling because all LEDs cannot be the same. There was a problem that occurred.

Another example of the prior art is US patent US6630801B2. The overall technology of this patent is the same as the US patent US6507159B2, except that an optical sensor without a filter is added. Therefore, there is a problem in that the cost of the additional installation of the optical sensor without the filter is added.

Another example of the prior art is US Pat. No. 6,923,803 B2. This patent proposes a method of finding the chromaticity value of each RGB LED in a backlight unit in which all of the RGB LEDs are lit. To do this, we need to find the chromaticity values of the RGB three LEDs, so we need three or more white values. In addition, the chromaticity values of the RGB LEDs should be found through inverse matrix operations on three white samples, and the chromaticity values of the RGB LEDs should be found through pseudo inverse matrix operations on three or more white samples. Accordingly, the process of finding the chromaticity value of the RGB LED is complicated and time-consuming.

Accordingly, an object of the present invention is to calculate a duty value of an RGB LED satisfying a color signal value of a target color space in implementing a backlight control system using a tricolor LED (RGB LED) and using It is to maintain constant brightness and chromaticity regardless of environmental changes.

The present invention for achieving the above object, the process of calculating the target CIEXYZ value from the target luminance, chromaticity value; Characterizing an output value of an RGB optical sensor for identifying an emission state of the LED backlight unit to estimate a current CIEXYZ value; Based on the current CIEXYZ and the duty value, the XYZ to Duty conversion matrix is updated to reflect the LED emission state on the LED backlight unit, and the duty value of the LED for outputting the target CIEXYZ value is calculated using the update result. Process of doing; And generating a driving pulse corresponding to the calculated duty value of the LED and outputting the driving pulse to the LED backlight unit.

Another object of the present invention to achieve the above object, the RGB optical sensor for detecting the intensity of the light irradiated from the LED backlight unit and outputs the output signal according to; Characterize the output value of the RGB optical sensor to estimate the current CIEXYZ value, update the XYZ to Duty conversion matrix based on the CIEXYZ and duty value to reflect the LED emission state on the LED backlight unit, and then use the result. A controller for calculating a duty value of the LED for outputting a target CIEXYZ value; And a backlight driving unit generating a driving pulse corresponding to the duty value of the LED calculated by the controller and outputting the driving pulse to the LED backlight unit.

The present invention estimates the CIEXYZ value of the LED backlight unit using an RGB optical sensor, and generates an XYZ to Duty conversion matrix considering the current RGB LED light emission state using the estimated value, and then computes the result of the matrix. By calculating the duty control value of the RGB LED that can maintain a constant CIEXYZ value, it is possible to implement an RGB LED backlight unit that exhibits a constant brightness and chromaticity regardless of environmental changes.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 3 is a signal flow diagram of a backlight driving method of a liquid crystal display device. As shown therein, after setting a target luminance and color temperature value, a chromaticity value is calculated from the color temperature, and the target value is calculated from the luminance and chromaticity value. First steps (S11-S13) for calculating a CIEXYZ value; A second process (S21, S22) of acquiring an output value of an RGB optical sensor installed around the LED backlight unit to determine a current light emitting state and then characterizing the LED backlight unit to estimate a current CIEXYZ value; Based on the current CIEXYZ and the duty value, the XYZ to Duty conversion matrix is updated to reflect the LED light emitting state on the LED backlight unit, and the duty value of the LED for outputting the target CIEXYZ value is obtained using the updated result. A third process of calculating (S23, S24); A fourth process (S25) of outputting a driving pulse corresponding to the calculated duty value of the LED to the LED on the LED backlight unit.

4 is a block diagram of a backlight driving apparatus of a liquid crystal display according to the present invention, and as shown therein, an LED backlight unit 41 having a plurality of LEDs arranged in an array and outputting backlight light; An RGB optical sensor 42 installed around the LED backlight unit 41 to detect an intensity of light emitted from the LED backlight unit 41 and output an output signal according thereto; Characterize the output value of the RGB optical sensor 42 to estimate the current CIEXYZ value, and update the XYZ to Duty conversion matrix based on the CIEXYZ and duty value to reflect the LED emission state on the LED backlight unit. A controller 43 for calculating the duty value of the LED for outputting the target CIEXYZ value using the result; It is composed of a backlight driver 44 for generating a driving pulse corresponding to the duty value of the LED calculated by the controller 43 and outputs it to each LED on the LED backlight unit 41, the operation of the present invention configured as described above When described in detail with reference to the accompanying FIG. 5 as follows.

First, after setting a target color temperature value, the radiation spectrum of a black body is calculated from the target color temperature using Equation 1 below (S11).

Here, 'S' means the radiation spectrum of the black body, 'λ' means the wavelength of the spectrum, 'T' means the absolute temperature. 'H' is Planck's constant, 'c' is the speed of light, and 'k' is Boltzmann's constant. That is, the radiation spectrum of the black body has different spectra depending on the color temperature.

The radiation spectrum of the black body thus obtained is multiplied by the xyz color matching function for each wavelength as shown in Equation 2 below, and then integrated to calculate the color signal (hereinafter referred to as "CIEXYZ") of the target RGB color space. (S12)

For reference, color models include RGB (red, green, blue) model, HSB / HLS model, Munsell color system and CIE color model. The CIE color model is determined by the International Lighting Commission, which sets the standard for lighting devices, and includes CIEXYZ, CIELUV, and CIELAB. The CIEXYZ color model represents the RGB three element values as XYZ, which is a set of other three element values all having a positive sign. Here, X, Y, and Z are commonly referred to as tri-stimulus values.

The CIEXYZ value calculated through the above process is a value that does not yet consider the target luminance. In order to obtain the CIEXYZ value considering the target luminance, the following equation (3) is performed, in which the chromaticity of the CIEXYZ value obtained above is maintained as it is and only the luminance is converted to the target luminance.

Here, 'Y' means target luminance. The 'X', 'Y', and 'Z' values thus obtained are the color signal values of the color space satisfying the target luminance and color temperature at the same time.

The controller 43 acquires the output value of the RGB optical sensor 42 installed on the surface of the LED backlight unit 41 to grasp the light emitting state of the current LED backlight unit 41 and uses the current. Characterization of the RGB optical sensor 42 for estimating the CIEXYZ value of the light emitted by the LED backlight unit 41 is performed as follows (S21, S22).

In order to enable the controller 43 to estimate the CIEXYZ value of the light emitted by the LED backlight unit 41 on the basis of any output value of the RGB optical sensor 42, the system designer is responsible for the LED backlight unit ( The 64 sample lights are set by combining the duty control values of the RGB LEDs arranged in an array form on 41).

Thereafter, the 64 sample lights are sequentially emitted through the RGB LED, and each time the controller 43 obtains an output value proportional to the light intensity from the RGB optical sensor 42. Correspondingly, the 64 sample lights are sequentially measured by a photometer (not shown) that is a spectrophotometer (eg, Minolta CS-1000).

Of course, when the controller 43 secures the XYZ values for the 64 sample lights in advance through a memory or the like, the process of sequentially colorizing the 64 sample lights through the photometer as described above is omitted.

The controller 43 may obtain a 3 × 4 relation matrix between the output value of the RGB optical sensor 42 and the corresponding CIEXYZ value using the colorimetric result. An RGB to XYZ relation matrix for converting an arbitrary output value of the RGB optical sensor 42 into a corresponding CIEXYZ value is used as shown in Equation 4 below.

The RGB to XYZ relation matrix is obtained through a pseudo inverse operation as shown in Equation 5 below.

Here, 'n' means the number of samples 64, and '(R _K , G _K , B _K )' means the k-th output value among the output values of the RGB optical sensor 42. '(X _K , Y _K , Z _K )' means the measured CIEXYZ value of the k th sample. As a result, the RGB to XYZ transformation matrix is 3x4 in size.

Since the output value of the RGB optical sensor 42 or the CIEXYZ value has a linear relationship with the control duty value of the LED, the degree of improvement even if the magnitude of the CIEXYZ value is estimated using a conversion matrix of 3 × 4 or more. Is not large. This is because a combination of the square or cube of the output value of the RGB optical sensor 42 and the CIEXYZ value is used to obtain a conversion matrix of 3 × 4 size or more, and these values are linearly related to each other. This is because a matrix that can sufficiently define a linear relationship can be obtained even if it is used as it is.

5 is a table showing the relationship between the size of the RGB to XYZ conversion matrix and the estimation error of the CIEXYZ value. As shown in FIG. have. It can also be seen that the characterization error using the 3x4 matrix falls within the tolerance range ΔEab <3. After all, this means that the sensor characterization performance using the 3x4 transformation matrix shows satisfactory results.

In order to control the luminance and chromaticity values of the LED backlight unit 41 to output the target luminance and chromaticity values, the controller 43 basically fixes the current driving the RGB LED, and the duty of the current. Use the method of changing the control value appropriately.

In order to obtain a control duty value for the LED backlight unit 41 to produce a target CIEXYZ value, the controller 43 uses a 3x3 transformation matrix that converts the CIEXYZ value into a corresponding duty input value. .

The reason for this is that the duty control value and the measured CIEXYZ value have a linear relationship with each other, so even when using a 3 × 3 matrix, the linear relationship can be sufficiently defined. This is because it is easy.

The XYZ to Duty conversion matrix for calculating the duty value of the LED capable of producing the target CIEXYZ value is used as shown in Equation 6 below (S24).

Here, '[X _T Y _T Z _T ]' is a target CIEXYZ value, and '[D _R D _G D _B ]' is an input duty value of an RGB LED for generating a target CIEXYZ value. The input duty value is normalized to use a value between 0 and 1. Typically, the XYZ to Duty transformation matrix is obtained as shown in Equation 7 below.

Here, '(XR _max YR _max ZR _max )' is the CIEXYZ value measured when the duty value of the red LED is maximized, and '(XG _max YG _max ZG _max )' is the maximum duty value of the green LED. It is measured CIEXYZ value, and '(XB _max YB _max ZB _max )' is the CIEXYZ value measured when the duty value of the blue LED is maximized.

However, it should be noted that even though a constant duty value is applied to the LED backlight unit 41, the luminance and chromaticity of the LED backlight unit 41 do not have a constant value, that is, they do not have a constant CIEXYZ value and change with time. Will. There may be various reasons for this, and a representative reason may be a temperature change of the LED backlight unit 41.

Therefore, in order to obtain a duty value for producing a target CIEXYZ value, the controller 43 does not use a fixed XYZ to Duty conversion matrix, but XYZ continuously changing to match the light emission state of the LED backlight unit 41. to Duty transformation matrix should be used. That is, the controller 43 periodically checks the light emitting state of the current LED backlight unit 41 and continuously updates the XYZ to Duty conversion matrix according to the check result.

In this case, the controller 43 periodically checks the light emitting state of the current LED backlight unit 41 by acquiring an output value of the RGB optical sensor 42 and then performing the sensor characterization process to determine the current LED backlight unit 41. ) Estimates the CIEXYZ value. In this process, the controller 43 checks the light emitting state of the current LED backlight unit 41 and calculates an XYZ to Duty conversion matrix as shown in Equation 8 below (S23).

Here, '[n]' represents the current inspection point, '[n-1]' represents the inspection point one step ahead of the current point in time, and '[n-2]' is the inspection point two steps ahead of the current point in time. Indicates. In addition, since '(D _R D _G D _B )' is an input duty value of the RGB LED output from the controller 43 to the backlight driver 44, the controller 43 already recognizes the value of '(X _R D _G D _B )' ₀ Y ₀ Z ₀ ) 'represents the output CIEXYZ value of the LED backlight unit 41 accordingly.

Therefore, '(D _R [n] D _G [n] D _B [n])' means an input duty value of the RGB LED output from the controller 43 to the backlight driver 44 at the current inspection point, and ( X ₀ [n] Y ₀ [n] Z ₀ [n]) 'is obtained through the sensor characterization process after acquiring the output value of the RGB optical sensor 42 at the current inspection point of the LED backlight unit 41 The output CIEXYZ value. That is, the XYZ to Duty conversion matrix is made based on the duty value and the CIEXYZ value at that time every time the light emission state of the LED backlight unit 41 is checked. Each time a new duty value and CIEXYZ value are input, the duty value and CIEXYZ value of the oldest check point are not used.

In more detail, the 3 × 3 XYZ to Duty transformation matrix generated as described above satisfies the following. Multiplying both sides of a matrix of three CIEXYZ values in the previous equation is expressed by the following equation (9),

This can be thought of as the following equation (10).

As a result, the XYZ to Duty conversion matrix can be referred to as a matrix defining the relationship between the CIEXYZ value and the duty input value that the LED backlight unit 41 can produce in the present light emission state, and at the same time, check one step ahead. It can be referred to as a matrix defining the relationship between the CIEXYZ value according to the light emission state of the LED backlight unit 41 and the duty input value capable of generating the value at the time point and the inspection point two steps earlier.

Since the difference between the light emitting state of the current LED backlight unit 41 becomes larger as the past inspection point with a large time difference between the current inspection point and time, the 3 × 3 matrix is configured by considering only the current inspection point and the inspection point two steps earlier. As time passes, the next check point becomes the current check point [n], and the previous present point in time becomes the check point [n-1] one step ahead.

That is, when the CIEXYZ value and the duty value of the current point of time occupy the first column of the matrix, the value of the previous point of time is eliminated by pushing the value of the previous point into the next column when checking the light emission state of the LED backlight unit 41. It is repeated every time. However, if the same duty input value is calculated, the matrix is not updated.

Equation 11 below shows an example of the XYZ to Duty transformation matrix from the start point to the fourth check point.

The controller 43 outputs the duty value of the LED obtained through the above process to the backlight driver 44. Accordingly, the backlight driver 44 generates a driving signal having a duty value supplied from the controller 43 and outputs the driving signal to the LED backlight unit 41 (S25).

1 is a block diagram of an illumination measuring system of an irradiation source according to the prior art;

2 is a layout view of a common substrate in the apparatus for measuring the spectral content of an LED light source according to the prior art;

3 is a control flowchart of a backlight driving method of a liquid crystal display according to the present invention;

4 is a block diagram of a backlight driving apparatus of the liquid crystal display according to the present invention.

5 is a table showing the relationship between the magnitude of the RGB to XYZ conversion matrix and the estimation error of the CIEXYZ value in the present invention.

*** Description of the symbols for the main parts of the drawings ***

41: LED backlight unit 42: RGB optical sensor

43: controller 44: backlight driver

Claims (10)

A first process of characterizing an output value of an RGB optical sensor installed around the LED backlight unit to determine a light emitting state of the LED backlight unit to estimate a current CIEXYZ value; Based on the current CIEXYZ and the duty value, the XYZ to Duty conversion matrix is updated to reflect the LED emission state on the LED backlight unit, and the duty value of the LED for outputting the target CIEXYZ value is calculated using the update result. A second process of doing; And generating a driving pulse corresponding to the calculated duty value of the LED and outputting the driving pulse to the LED backlight unit. The method of claim 1, wherein the first step further comprises: setting a target luminance and color temperature value, calculating a chromaticity value from the color temperature, and calculating a target CIEXYZ value from the luminance and chromaticity values. A backlight driving method of a liquid crystal display device, characterized in that. The liquid crystal display of claim 2, wherein the calculating of the CIEXYZ value comprises calculating the following Equation to maintain the chromaticity of the CIEXYZ value and change only the luminance to the target luminance. Backlight driving method.

Here, Y: target luminance, X, Y, Z: color signal value of the color space satisfying the target luminance and color temperature at the same time. The backlight driving method of claim 1, wherein the first process uses an RGB to CIEXYZ conversion matrix of about 3 × 4 to estimate the CIEXYZ value. .

Where n is the number of 64 samples, R _K , G _K , B _{K is the} k-th output value, and X _K , Y _K , Z _K is the measured CIEXYZ value of the k-th sample. The backlight driving method of claim 1, wherein the second process uses the following Equation when there is no change in the LED emission state of the backlight unit.

XR _max YR _max ZR _max : CIEXYZ value measured when the duty value of the red LED is maximized, XG _max YG _max ZG _max : CIEXYZ value measured when the duty value of the green LED is maximized, XB _max YB _max ZB _max : CIEXYZ value measured when the duty value of the blue LED is maximized. The method of claim 1, wherein the second process uses the following Equation to update the XYZ to Duty conversion matrix to reflect the LED light emitting state on the LED backlight unit.

Where [n] is the current inspection point, [n-1] is the inspection point one step ahead of the present point, [n-2] is the inspection point two steps ahead of the present point, and D _R D _G D _B : RGB Input duty value of LED, X ₀ Y ₀ Z ₀ : Output CIEXYZ value of backlight unit. The XYZ to Duty transformation matrix of claim 6, wherein the CIEXYZ value and the duty value of the current time point occupy the first column of the matrix and push the previous time value to the next column as shown in the following Equation. The method for repeatedly eliminating the value of is repeated every time the light emitting state of the LED backlight unit is checked, and when the same duty input value is calculated, the matrix is maintained without being updated.

An RGB optical sensor for detecting the intensity of light emitted from the LED backlight unit and outputting an output signal according thereto; Characterize the output value of the RGB optical sensor to estimate the current CIEXYZ value, update the XYZ to Duty conversion matrix based on the CIEXY and duty values to reflect the LED emission state on the LED backlight unit, and then use the result. A controller for calculating a duty value of the LED for outputting a target CIEXYZ value; And a backlight driver which generates a driving pulse corresponding to the duty value of the LED calculated by the controller and outputs the driving pulse to the LED backlight unit. The backlight driving apparatus of claim 8, wherein the controller comprises a photometer that sequentially colorizes a predetermined number of sample lights irradiated from the LED backlight unit and outputs a CIEXYZ value accordingly. The backlight driving apparatus of claim 8, wherein the controller comprises a memory in which CIEXYZ values are stored for a predetermined number of sample lights emitted from the LED backlight unit.

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