Countless specks of light in the Rolls-Royce Phantom make you feel like you’re driving beneath a sky of stars, conveying a sense of luxury and exclusivity. Unlike chrome, light can perform many more functions. Inside vehicles, it can influence the occupants’ emotions depending on the brightness and color—they feel at ease, comfortable and less stressed, or cheered up and encouraged. Cognitive abilities can also be improved with light, preventing the driver from tiring as quickly and helping them to better concentrate.
But colors can also have negative effects. This is why an additional HMI system may be useful to ensure that the color of the lighting matches the physical and emotional condition of the driver. These usually employ what are known as “themes,” which involves various parameters being preconfigured. Such a system can contain several microcontrollers and up to 300 RGB modules, comprising LED chips and drivers.
Out of the Comfort Zone
While interior LED lighting was initially only used in luxury models, and purely as a comfort factor, they are now commonly being used in mid-class vehicles. The main objective here is to establish LEDs as a functional feature for enhancing safety. The selective use of light makes it easier for the driver to find their way around the vehicle interior, and control elements or indicators can be highlighted or hidden as needed. Warnings, for example when an object is too close, are more readily noticed when shown in red light. Information on the status of the vehicle or on incoming phone calls can also be elegantly communicated via LEDs to be quickly interpreted by the driver.
But it is essential to avoid overkill—the light must not dazzle or distract the driver or other people. And when too many lighting elements are used, their impact is lost.
Smaller LEDs Move Closer Together Technical evolutions offer up even more artistic and application opportunities. For example, LED strips can be integrated on flexible PCBs designed for challenging conditions along the edges of the roof and in the seating upholstery.
Components are also getting smaller and smaller, with recent LED modules measuring just 8 ×18 mm². This is necessary, because the space available to install electronic components also shrinks with every development cycle. This means that in smaller modules, the diffusor gets closer and closer to the light source, the LED chip.
If the distance between the LED or LED module and the diffusor is reduced, they need to be fitted more tightly together to generate a consistent light surface. Generally, the distance between the LED and diffusor should be equal to the distance between two LEDs, which means that more LEDs need to be fitted.
For small light strips, an optical fiber cable can be illuminated from both ends. However, this only works up to a length of around 15 to 20 cm beyond that, the light is not strong enough in the middle of the cable to ensure a consistent, homogeneous color. This is why, with roof lighting for example, it is necessary to place a series of LEDs or LED modules at regular distances.
LED Drivers for Limited Space
For applications such as the center console, a multi-channel driver such as the E521.38 from Elmos is recommended. This can control four LEDs, even six when using multiplexing, allowing it to help satisfy strict installation space requirements. Developed in accordance with ISO 26262 for ASIL-B applications, it can also be used for functional safety systems. Its DFN18 package with wettable flanks help make inspections easier. The differential digital interface makes it the leading choice for dynamic ambient lighting, because—unlike other bus systems and large bus arrangements it doesn’t cause lag effects resulting from insufficient data transfer rates. This means that consistent color displays and movements are guaranteed.
Smaller LEDs Move Closer Together
For smaller LED modules, the E521.39 LIN-RGB/W driver is ideal. It is designed for ambient lighting, which, at present, is usually static in nature. Unlike its predecessor, it has flash memory to enable the module to receive software updates in the vehicle. It can withstand a supply voltage of up to 40V and has a low maximum idle current draw of 30µA. The robustness and low consumption of the module make it perfect for use in battery powered vehicles. Thanks to the 12 bit ADC, it also offers high resolution temperature measurement by reading the internal temperature sensors and using a differential measurement of the forward voltage. The LED driver can use this to have the software compensate for aging and temperature effects.
The E521.38 multi-channel driver uses an integrated pulse width modulation generator to generate a (PWM) frequency of 1,000 Hz with full 16bit pulse width resolution. The E521.39 provides up to 700Hz with the same resolution. This puts them well above the requirements of the automotive OEMs, which specify 500Hz in their respective standards to ensure an absence of flickering in interior lighting. This provides reserves to meet even stricter requirements.
The LED driver’s PWM system enables not only flicker-free light but also considerable capacity for soft color transitions. A further benefit is that, given that OEMs keep specifying increasingly strict color spaces for LEDs, this allows for more LED binning and reduces costs when selecting LEDs.
The Elmos LED drivers will soon be launched on the market and will then be available from Rutronik, as will other ICs from the manufacturer that can be used to create optical applications for HMIs, such as non-contact gesture control systems. Rutronik can also supply all other components needed for innovative ambient lighting, from white and RGB LEDs with top or side LED form factors to power supply chips, to passive components such as capacitors and inductors. The Rutronik Automotive Business Unit (ABU) provides manufacturer-agnostic technical support specifically for automotive applications.
Find components at www.rutronik24.com.
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