Multiple color sources combined with red, green, and blue (RGB) light-emitting diodes ( LEDs ) can produce a variety of color outputs, while the LED itself has considerable stability and high efficiency, but the use of RGB LEDs to produce multiple color sources And to maintain high quality, there are still some challenges that must be overcome. This article will introduce techniques that can address these challenges.
With RGB LED
The simplest multi-color LED source consists of three sets of LEDs, red, green and blue, each driven by a separate drive module. Therefore, the color of the resulting light source is affected by the relative luminous intensity between the red, green and blue LEDs. The luminous intensity of the LED can be controlled by the drive current, or by using a Pulse Width Modulation (PWM) change to drive the LED signal and the effective period rate. Among them, PWM is more common because the relationship between the period coefficient and the luminous intensity is more linear than the relationship between current and luminous intensity.
(Figure 1) The CIE1931 color space has a potential open-loop architecture for LED light sources. Since the optical characteristics of LEDs are affected by operating conditions, the brightness and chromaticity of the combined RGB source output will vary. At the same time, each LED component is not the same, thus causing more changes in the output of the RGB light source. (Figure 2) and (Figure 3) describe several examples of LED variations.
(Figure 2) Example of RGB LED output spectrum affected by temperature
(Fig. 3) An example of the relative intensity of LEDs being affected by temperature. A solution of 25oC is to use optical feedback to generate a closed loop system. The basic setup includes a light sensor that records the brightness of the LED source, and The light sensor measurement results to adjust the control method of the light source output, which will allow the brightness of the LED light source to remain stable as each LED changes, that is, although the various parts vary, the total remains unchanged.
In (Fig. 4), the integrating circuit labeled 22 can output a voltage controlled by the amount of light on the photodiode (11a). This voltage is compared with VSET. The output of the comparator can control the increase or decrease of the counter value. It is used to drive a digital-to-analog converter (37), which in turn controls the drive current of the LED.
Another more advanced optical feedback method is the use of a three-color light sensor, usually consisting of three separate light sensors and an upper three-color filter, allowing these light sensors to record color information, not just brightness, which can be further controlled. The luminous intensity ratio of red, green and blue LEDs is critical because it allows the brightness and chromaticity of the RGB source to be controlled, while ASSP plays an important role in the tri-color optical feedback design.
(Fig. 4) Circuit for realizing optical feedback
Three-color optical feedback system
Basically, a three-color light sensor produces a three-dimensional color specification system, hence the RGB sensor color space. This system allows a specific color to be specified by the sensor's output voltage. For example, D65 white light with a specific brightness can be recorded as: (Vred , Vgreen, Vblue) = (2.0, 2.2, 1.9) volts.
As shown in (Figure 5), assuming that the D65 used in the above example is the target color, the feedback system will continuously measure the red, green and blue sensors periodically, collectively referred to as a three-color light sensor, and compare the measured color values ​​with the target color. . The purpose of the feedback system is to adjust the error between the measured color and the target color to zero.
(Figure 5) Three-color optical feedback system
(Figure 6) This concept is described in different ways. All possible target color set points are specified by the coordinate values ​​in the color space of the RGB sensor formed by the red, green and blue light sensors. When the characteristics of the LED change, The measured color will deviate from the target, ASSP will detect this change and adjust the LED signal output of the LED at any time.
Another point is quite important. At the same time, it must be noted that the longer the LED is used, the lower the light output intensity. Therefore, after a period of time, the maximum output brightness of the RGB LED system will decrease, although in most applications, Both can accept gradual and stable brightness attenuation, but sometimes it is unacceptable for the chromaticity of the RGB illumination system. ASSP has the function of stably controlling the luminosity attenuation of the RGB illumination system, such as maintaining the stability of the chromaticity within a certain tolerance. Even when the highest output brightness drops.
While the brightness of the system must remain constant throughout the life of the application, the user must ensure that the highest selectable brightness is below the maximum achievable brightness over the overall required life, as shown in Figure 7.
Although the RGB lighting system is quite attractive, it also faces the challenge of the widespread use of this technology, thus causing the need to hide the complexities of three-color optical feedback behind a simple use interface. The following will introduce ASSP. How to achieve this requirement.
(Figure 6) Three-line coordinate system for RGB sensor color space
(Figure 7) ASSP will maintain the chromaticity of the RGB source (here, 1931x, y coordinates) within a certain tolerance range, even when the maximum achievable brightness decreases with time.
No external processing required
ASSP integrates a series of algorithms that can analyze the color information of the three-color light sensor and calculate the set point of the target color and the size of the PWM drive signal. The ASSP samples the light sensor at a rate of about one hundred times per second to ensure that the periodic adjustment of the PWM signal is not noticeable by the human eye. As mentioned earlier, the ASSP also includes an RGB light source output that prevents LED aging. The algorithm for chromaticity change.
Therefore, no other calculations are required to achieve and maintain the target color.
Standardization of color space
This is related to the device correlation of the target color set point. The RGB sensor color space will vary according to the light sensor output, light sensor position, LED, LED driver circuit, and other factors. (Figure 9) describes this problem, each Each system has a slight gap in the RGB sensor color space, so the D65 specification set in System A may be different from System B.
E.g:
System A (Vred, Vgreen, Vblue) = (2.0, 2.2, 1.9) volts;
System B (Vred, Vgreen, Vblue) = (2.1, 2.4, 2.3) volts.
The three-color light sensor in System A will generate the above voltage level when the D65 light output is achieved, but the light sensor of System B, although achieving the same D65 light output as the A system, will produce different voltage level combinations. . In other words, the color specification system defined by the RGB sensor color space is different in each system.
ASSP integrates the calibration process so that each system can use the standard color specification system. CIE1931 xyY and CIE RGB are two built-in systems of ASSP. Users can use the same color space input through the standard color space. It is sent to different systems and can be assured that each system produces a color output within the same tolerance tolerance.
(Figure 8) ASSP samples directly from the tri-color light sensor and converts the result to the desired LED PWM signal
(Fig. 9) The variation of the color space of the RGB sensor can be solved by using ASSP to adjust the color space of the RGB sensor to the standard color system.
For example, the 1931 CIE xyY allows each system to select a target color using a standard color system.
Easy design import
Under normal circumstances, ASSP only needs to support passive components and an external PROM to store the calibration data. In most cases, the memory space can be shared with the system and the periphery, because the calibration data only needs 31 bytes.
The ASSP has a standard two-wire 100 kHz I2C interface, while all major functions correspond to 8-bit addressing space. For example, to perform the tuning operation, just write 0x01 to the CTRL2 buffer. For details of other designs, please refer to the data specification of the component.
During the production phase, the system can be calibrated using a standard CIE camera. The calibration data must be stored in an external, short-lived memory, and the system does not need to be calibrated after importing into the application. In the application, the user first configures the device, and then writes the previously stored calibration data to the calibration buffer. This is a simple program that reads and writes. After the completion, the system can accept the target color. input of.
The choice of color is quite simple. Taking the above example as an example, the target color D65 is specified by the sensor voltage. In practical applications, the target color can be specified by the coordinates of the CIE 1931xyY system. Of course, other methods such as CIE uvY and CIE RGB can also be used. Color system. For example, to select the illumination E as the target color, simply send the value of (x, y, Y) = (330, 330, 200) to the appropriate buffer in the ASSP.
Illuminance E CIE x, y coordinates are 0.33, 0.33;
Multiply them by 1000 to get 330, 330;
â— Select the relative brightness size Y = 250;
- Write 250 to buffer addresses 237 and 236 to set the brightness (Y value);
- Write 330 to buffer addresses 235 and 234 to set x-axis chromaticity coordinates;
- Write 330 to buffer addresses 233 and 232 to set y-axis chromaticity coordinates;
• Write 0x12 to buffer address 1 (CTRL1) to update to the new target color.
The ASSP will change the RGB light output as soon as the relative bit in the update buffer is set.
(Figure 10) Typical configuration of the three-color management system (Note: Since the internal reference circuit and oscillator selection are enabled, this component can be supported only with passive components. If the system already provides memory space, then No EEPROM required.)
(Figure 11) ASSP part of the buffer space example, each bit corresponds to a function
(Figure 12) Typical design flow
Experimental result
(Figure 13) shows the difference in performance between the open-loop and closed-loop RGB light source systems. The experiment uses a 9000K white target color and uses duv as the evaluation index.
(Formula 1)
Where (U25, v25) = 1976 CIE u, v chromaticity coordinates at 25oC;
(uT, vT) = 1976 CIE u, v Chromaticity coordinates at temperature T.
A basic method for judging performance is to use duv = 0.005 as the smallest change in the chromaticity before the change can be perceived by the human eye.
(Fig. 13) Measurement of chromaticity due to temperature change
(Fig. 14) describes the significant influence on the LED spectrum when the temperature rises. This data is obtained by a closed loop system of 9000K white light target color. Although the spectral curve appears to be greatly shifted, the duv remains below 0.005.
(Fig. 14) The shift of the spectrum due to temperature, the shift of the chromaticity duv < 0.005
Conclusion
RGB LED light source can be said to be a very attractive lighting solution, but due to changes in LED characteristics, the RGB light source output shifts the target color. Although the three-color optical feedback is a good solution, it is operational. However, it is somewhat complicated, and it is necessary to simplify the implementation of such systems through a good feedback controller design.
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