More forward-looking OEM automakers have realized the value of ambient lighting in their vehicles to enhance product differentiation in an increasingly competitive market. As a result, electronic devices have become the fastest growing segment of automotive components, surpassing mechanical, pneumatic and hydraulic devices. With the increasing number of ambient lighting applications in the interior of the car, OEM car manufacturers have differentiated and applied LED lighting solutions of different colors. Now it is mainly applied to the intermediate bridge model. The only real problem in the future is how to expand on a large scale. How to achieve the required lighting system at an acceptable cost.
In order to meet the above application requirements and reduce costs and shorten implementation time, it is important to choose the right solution. This article will explain the interior lighting solutions in the following aspects:
(1) Selection of methods for implementing ambient lighting in the vehicle;
(2) Using the control algorithm to achieve the mixing of the RGB LED color, temperature compensation and differential compensation between the LEDs;
(3) Discussion on the implementation of the interior lighting network.
First, the development of the interior lighting system
The basic ambient lighting mechanism began to be applied to cars about five years ago. At this stage, the functions they can support are very limited, and the implementation is also considered to be quite expensive. If the interior lighting is to be used as a market selling point, it is necessary to increase the flexibility of the terminal customer. This has led to the adoption of traditional early passive solutions to more intelligent ambient lighting systems. With the development of fully adjustable ambient atmosphere lights, vehicle drivers will adjust the interior ambiance lights to any color they want based on personal mood or taste in the near future. In future models, the interior ambience lights are very likely to be part of the advanced driver information system. For example, cars in the city
In the middle, when driving on a country road or on a highway, the ambient light color will be different. Red can improve the driver's warning, while yellow helps the driver relax on the highway. In many cases, the interior ambient light can be linked to the value of OEM car manufacturers and personalized positioning to make their products more unique.
Second, the realization of environmental lighting
In order to make the ambient lighting have a practical impact in the vehicle environment and truly let the passengers and drivers experience the comfort, it is necessary to arrange a large number of solid-state light sources in the car - can be placed in the foot space, along the dashboard, wear Pass the top of the top of the car, or the door panel, etc., and use it to illuminate specific parts of the car (door handles, cup holders, etc.). In the field of optoelectronics, there has been progress in color mixing and diffusing the three primary colors of a single package of light-emitting devices throughout the vehicle. However, the cost requirements for interconnecting all RGB LEDs have a very large constraint on the implementation of ambient ambient light. Considering this factor, it may only be used in luxury cars. This is unfortunately unfortunate because it can be widely used in mid-range or entry-level cars if the price is right.
Figure 1 The actual effect of applying interior lighting
1.LIN control
As with any solution, the cost requirement is the first to bear the brunt. Implementing Local Interconnect Network (LIN) communications in in-vehicle ambient lighting solutions is the best practice to help reduce development costs and shorten development cycles, so with existing bus facilities, lightweight, single-line LIN communication is Complementing high performance, costly CAN (Controller Area Network) communications. Each RGB LED module can be connected to a LIN network without the need for additional wiring. As a result, overall system cost can be kept to a minimum, plus the total weight of the car does not increase – thus avoiding increased fuel consumption, and designers can take advantage of the LED package to further enhance the in-car experience.
2.? Constant current source drive
The LED must be driven with a constant current source. Most LEDs have a specified current level at which the LED can achieve maximum brightness and contribute to LED lifetime. In addition, the light energy output of the LED is proportional to the current flowing through it.
There are usually two ways to control the brightness of the LED: one is to change the LED current; the other is to use pulse width modulation (PWM) to change the duty cycle of the constant current source current driven by the LED.
The main disadvantage of the first method is that as the current decreases, the LED efficiency also decreases. For high-power LEDs, the current reduction also causes color deviation. However, the PWM dimming mode drives the LED at full current to avoid problems such as reduced efficiency and color deviation.
Melexis Electronics' latest MLX81106 includes the necessary internal drive technology. The chip can directly drive 4 LEDs (red, green, blue and white) with built-in constant current source, and use PWM gate voltage amplitude adjustment to achieve PWM Air ratio modulation.
Figure 2 MLX81106 driver RGB LED circuit diagram
3.RGB LED color control algorithm and compensation
Use the integrated MCU microcontroller to detect and control RGB LED colors, produce specific colors, and blend colors. For example, the components of RGB can be mixed to produce a particular white light.
By using the red, green, and blue color values ​​and brightness values ​​to produce the desired color, correspondingly using three PWM channels or four PWM channels, this article discusses the use of three PWM channels to achieve color and brightness. Controls are combined in a way that requires higher PWM resolution and a powerful MCU.
The MLX81106 integrates an MCU with 4 outputs capable of driving external RGB LEDs up to 30mA (using internal circuit enhancements up to 48mA). (If you need to output more current to drive white LEDs, you can use the high side of the chip itself. IOs drive external devices to achieve) mixed light produces 2.7 x 1014 different colors - giving users a wider choice.
In addition, the temperature and variability of the LED are part of the LED color factor, so the impact of the two needs to be compensated.
For temperature compensation, a low-cost and efficient way is to use the integrated temperature sensor and comparator inside the chip; or to detect the forward voltage drop of the LED, the MLX81106 can provide the above two ways of implementation hardware, its integrated LED threshold detection capability Not only does the LED temperature compensate, but it also compensates for LED aging without any external components.
Since LEDs are usually caused by differences in light intensity caused by the production process, this causes a difference in the color of the output mixed light. To compensate for this effect, the integrated MLX81106 128-byte EEPROM can be used to perform terminal calibration of LEDs on the module production line to achieve LED differential compensation.
4. Network implementation
Since a large number of solid state light sources are required inside the vehicle, for logistics reasons, it is important to ensure that all modules are identical (hardware and software aspects). In order to simplify the production process of the car, separate control of the individual modules is required, and a system is needed to detect the specific position of each module in the car. In addition, the requirements for networking and diagnostics of automotive interior lighting nodes require the use of suitable low-cost communication protocols. For example, the most cost-effective way to use the newly developed interior lighting for OEM automotive manufacturers is the LIN/J2602 communication protocol. To the need to detect and diagnose each module, some OEM car manufacturers are now considering LIN communication with automatic addressing. This method is implemented based on bus parallel (used in air conditioning systems). Although the cost of silicon is increased compared to standard LIN communication, the system provides the ability to detect all modules in the EOL process of the vehicle, reducing the logistics costs of OEM car manufacturers and Tier 1 suppliers.
Melexis Electronics offers a simple reference circuit design for this solution, including RGB LEDs, driver circuitry and auto-addressing, all on a single 9mm x 9mm PCB.
Figure 3 Schematic diagram of automatic addressing of bus parallel implementation in LIN network
Third, MLX81106 solution introduction
In order to meet the actual needs of automotive OEMs, and to achieve cost-effective environmental lighting solutions, Melexis Electronics introduced the MLX81106 automotive grade chip. The chip is a highly integrated, freely programmable LIN RGB LED slave device with a 0.18μm high voltage CMOS process. The chip itself uses SOIC8 small-size package to meet the customer's PCB design size and space requirements. Second, it integrates multiple functions, including physical layer LIN transceiver, 19.2kbit/s LIN controller (supporting LIN 2.x and SAE J2602), prescaler 10-bit analog-to-digital converter (ADC), voltage regulator And 16-bit microcontroller (with 24/32K byte size Flash, 512 byte RAM, 128 byte EEPROM storage).
In addition, in order to meet the requirements of automotive OEMs for the number of lighting nodes in the car, the R&D personnel made a special design for the internal circuit of the chip, so that up to 25 slave nodes can be connected on the LIN bus, which is much larger than the 16 slave nodes specified by the LIN bus. .
Figure 4 MLX81106 reference PCB design
in conclusion
This article focuses on a highly integrated single-chip implementation of an ambient lighting system solution. This means that the RGB Ambient Light Module can be interconnected via the in-car LIN network, and the solution does not require a long development process or a large amount of human resources. Such technology is bound to become the main selling point of OEM car manufacturers. The ambient ambience function through the LIN network will make the car manufacturers' models more personalized, which will make them stand out from the competition. So in the future, this feature will no longer be limited to high-end cars, but will become more suitable for the high-volume models of OEM car manufacturers.
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