Next Generation Automotive Exterior Lighting LED Driver Technology Evolution

在电动汽车、ADAS和对现代风格的渴望的推动下，汽车照明正在快速发展。汽车工业继续向电气化转变，导致汽车外部照明发生显著变化。过去，大型单一功能灯控制着车辆的前部、后部或侧面。然而，围绕着汽车的更新、更薄的设计 取代了他们的位置。汽车制造商正在使用灯光作为标志性的造型特征，并加入动画来象征品牌身份。这些新设计不仅美观，而且实用。

Animated taillights are not new as they first appeared on 1960 muscle cars such as the 1965 Chevrolet Impala SS. It featured a unique taillight design called the "Impala Super Sport Sequential Taillights" that included a sequential lighting pattern that activated the turn signal indicators. Since then, advances in automotive LEDs, MCUs, and the rise of electric vehicles have increased the popularity of animated taillights. Automotive designers now have the flexibility to design animated lights that display information or add a unique, personalized signature style. For example, some cars are now equipped with LED lighting that makes it easier for other drivers or pedestrians to predict the car's movements or provide a "welcome" lighting sequence to the owner.

While animated taillights are not legally required to be installed, when installed on a vehicle, they must comply with lighting safety standards for a specific region. In the United States, these regulations are set under Federal Motor Vehicle Safety Standard (FMVSS) No. 108. It outlines technical specifications for lighting equipment, such as the color, intensity, positioning, and alignment of lights on vehicles sold on public roads in the United States. In Europe and parts of Asia, similar standards are found in United Nations Economic Commission for Europe (UNECE) Regulation No. 48. The goal of all lighting regulations is to improve visibility, promote consistent, reliable signaling, and increase the overall safety of vehicles on public roads.

Although there are minor regional differences, the commonality is that all passenger vehicles must have at least two red taillights symmetrically located on either side of its longitudinal centerline, visible from a certain distance under normal daylight conditions (e.g., 100 meters UN ECE R48). Both FMVSS108 and UN ECE R48 have recently been updated for LED lamps, setting minimum luminous intensity, color, and PWM-derived flashing rate. All three of these new requirements will have an impact on LED-based animated taillights.

To create LED lights that are both functional and aesthetically pleasing, engineers and designers face new challenges in controlling a large number of LED pixels rather than a few LED strings. As automotive designs become more streamlined and aerodynamic, the shape and position of the lights become more important. The lights must fit perfectly with the overall design of the car while still providing adequate visibility, safety, reliability, energy efficiency, and complying with government regulations.

Animated exterior lights help improve road safety by providing additional information to other drivers. As these lights become more sophisticated, they can use complex patterns and animations to display additional safety information. This will require a shift from controlling strings of LEDs or blocks of incandescent bulbs to driving individual LED pixels. A great deal of engineering and design innovation is required to achieve this. Special drivers are needed to form a matrix, or cluster of individually controlled LEDs, that spans the entire rear of the car. Only LED drivers with digital interfaces can control the large number of LEDs required for animated lighting.

Figure 1 shows the evolution of lighting control architecture from traditional to newer animated pixel lighting.

Figure 1. Automotive lighting control architecture

The vehicle's lighting system is managed by a body control module (BCM) that contains electronics and a microcontroller (MCU). The BCM manages various electrical and electronic systems in the vehicle body, including lighting, windows, mirrors, seats, and doors.

In the past, lighting animation was not a requirement, so the BCM could easily enable FET or relay switches to control LED strings or incandescent bulbs using the MCU's general-purpose input/output (GPIO) pins. Today, the BCM's MCU needs to control a variety of lights with different LED arrangements and animation patterns. With the increasing demand for animated lighting features, the BCM now interacts with a lighting-specific electronic control module (ECU) to send commands to multiple LED drivers that control a single LED group.

Design Challenges of Animation Lighting

Automotive engineers tasked with the design of animated lighting systems face some daunting challenges, such as controlling heat, meeting functional safety targets, and keeping costs to a minimum. While LEDs offer many advantages over incandescent bulbs, there are challenges in making them comply with automotive requirements. Challenges associated with compliance, optics, enclosures, and electrical design can be simplified by carefully selecting the appropriate LED type and its driver. The selected LED driver should have the ability to fine-tune the brightness of individual LEDs and enable fast and accurate communication between the ECU and the LED lighting system.

Example Split RCL Design

A wide variety of rear combination lamp (RCL) designs give vehicles a distinctive and unique look. The transition from incandescent bulbs to LED taillights opens up a wide range of design possibilities, such as animated turn signals, animated welcome lighting, etc. Achieving split light design lighting effects requires multiple LED drivers to control various LEDs with different lumens/watt and different colors from white, amber, red and super red. These lighting components can be distributed on the vehicle and connected to each other using wiring harnesses.

The rear combination lamp is shown in Figure 2; it is a split design with half the lamp on the body and the other half on the trunk. Achieving seamless animation requires communication through a noisy wiring harness, which affects the reliability of the animation. To address these issues, a controller area network (CAN) bus is needed to ensure fast, reliable communication. The CAN bus connects the BCM to the ECU, and the ECU to the LED driver.

Figure 2. RCL with CAN bus

Lumissil Microsystems offers a wide range of LED drivers designed to meet the needs of animated lamp designs while addressing the challenges mentioned previously.

Lumissil's IS32LT3138A LED driver is designed to establish reliable off-board communication between a microcontroller (MCU) control board and the driver (see Figure 3). It provides a high-speed 1MHz UART interface compatible with the CANFD physical layer. The IS32LT3138A achieves reliable long-distance communication by using the industrial standard UART to CAN physical layer at both the transmit and receive ends of the harness. Each driver has 18 high-voltage (16V) constant-current channels to facilitate the use of LED light strings. By using a single resistor, all OUTx channels can be configured to sink up to 100mA.

Figure 3. IS32LT3138A 18-channel LED driver with UART-to-CAN interface

Lumissil offers many unique, cost-effective solutions for automotive LED lighting in the high brightness, matrix, and animation categories. One of the new LED drivers for automotive animation is the IS32FL3749. It is a matrix LED driver with a fast 33MHz serial shift daisy-chain bus that can interconnect multiple IS32FL3749 devices. Each driver has 24 high-voltage (16V) constant-current channels. This high-voltage capability facilitates replacing each pixel with a string of LEDs with a forward voltage of up to 16V (Figure 4). By using a single resistor, all row channels can be configured to sink up to 60mA. The driver also includes four column controls, allowing it to drive a 24 (row) × 4 (column) LED array with an array size of 96 LEDs. Each LED in the array can also be mapped as a red, green, or blue group to achieve RGB group control, thereby reducing data programming traffic.

Figure 4. IS32FL3749 24×4 matrix LED driver with 33MHz serial shift bus

Fault Diagnosis and Functional Safety

LED driver ICs need to monitor the taillights of cars because their reliability directly affects road safety. Even a small lamp failure can lead to an accident. Fortunately, the expected life of LEDs is longer than the life of the car. However, they are still susceptible to random failures. Both the IS32LT3138A and IS31FL3749 integrate LED fault detection and reporting functions to meet functional safety goals. Fault events are recorded in registers that can be accessed by the MCU through the bus interface.

Conclusion and Summary

Exterior automotive lighting serves a greater purpose than simply ensuring optimal vehicle visibility. It has evolved into a major driving force behind innovative designs in automotive taillights. A new generation of lighting designs using animated LEDs has emerged and replaced traditional incandescent bulbs and LED light strings. Today’s cars require innovative lighting technology that meets both functional needs and reflects brand image. This next level of automotive lighting effects can only be achieved by independently controlling a large number of light-emitting diodes.

Advanced LED drivers such as the IS32LT3138A and IS32FL3749 enable lighting designers to manage and display automotive lighting effects while enhancing safety and enriching the overall user experience. Lumissil Microsystems offers other animation-focused LED drivers such as the IS32LT3146 standalone sequential lighting driver and the IS32FL3248 48-channel serial shift driver. These and other LED drivers play a vital role in enabling contemporary lighting technology while improving traffic safety for everyone.