Visual data processing is an integral part of automotive lighting applications, such as adaptive lighting, ground projection and animation. Advancements in image processing and artificial intelligence are supercharging these systems to decipher data in real time at faster rates. However, traditional digital signaling interfaces have created a bottleneck for these quickly evolving systems. This article explores how low-voltage differential signaling (LVDS) interface circuits can help designers overcome automotive lighting challenges related to bandwidth, signal integrity and power consumption.
To create safer driving environments, original equipment manufacturers (OEMs) and Tier 1 suppliers are developing adaptive headlights that dynamically adjust to varying road and weather conditions. The most advanced headlight systems incorporate animated control of exterior lights or ground-projectable warnings, but one of the most popular applications is glare-free high beams, which automatically adjust the distribution of light upon the detection of a pedestrian or car.
There is more than one way to implement glare-free headlights. Some architectures follow a purely mechanical approach, while others, such as adaptive driving beam (ADB) systems, control an array of LEDs.
In ADB headlights, a forward-facing camera captures real-time road-condition data and automatically adapts the lighting profile of the headlights. Figure 1 depicts a simplified design of an ADB system using an LED matrix controller. The primary blocks include a camera, an electronic control unit (ECU) and an LED driving module. A high-speed camera transports data through a serialized-deserialize interface to a microprocessor or microcontroller (MCU), which resides on the ECU board. The MCU calculates the pixel configuration and transports control data to the LED driving module at the headlights.
The LED module could use LED matrix controllers (shown in Figure 1) or high-density microLEDs, depending on the targeted resolution. Adding LEDs provides a greater degree of resolution to adaptive control. Systems can range from tens to tens of thousands of LEDs. As OEMs continue to pack in higher densities of LEDs into headlamps, the required signaling rates increase from a few megabits per second to gigabits per second. The resulting ECU-to-LED board interface must adapt to enable this drastic increase in bandwidth.
Aside from high speeds, this interface also requires robust and long-range transmission. The following three qualities of differential signaling make it an excellent candidate for reliable automotive communication:
● Differential signaling methods carry data in two complementary paths from driver to receiver. The receiver is designed to extract data from the voltage difference between the two signals, known as the differential voltage. This allows the receiver to reject common-mode noise that may be present on the transmission media.
● Differential signaling reduces the effects of ground shifts between the driver and receiver because data is not referenced to a common ground.
● The balanced transmission of data in equal and opposite magnitude helps minimize electromagnetic interference (EMI).
Four common differential interfaces will be considered: Controller Local Area Network (CAN), RS-422, RS-485 and LVDS. The standard physical electrical characteristics determine factors such as the supported transmission rate, length, common mode tolerances, and power. Table 1 summarizes these trade-offs.
The CAN bus has a strong heritage in the automotive industry because of its low cost, reliability and tremendous flexibility. The CAN protocol is standardized by International Organization for Standardization (ISO) 11898, which defines both the data link and physical layer of the Open Systems Interconnection (OSI) model. Considering just the physical layer, it uses balanced differential signaling. CAN High and CAN Low make up the differential pair, where a logic high is signaled at 3.5 V and a logic low is 1.5 V. The resulting differential voltage swing is 2 V.
The common-mode voltage range dictates the allowable voltage difference between the transmitter and receiver ground. ISO 11898 requires a minimum of –2 V to 7 V of common-mode tolerance. Given these electrical parameters, the CAN bus supports up to 40-meter bus lengths and a maximum of 30 nodes. The data rate is limited to a maximum of 1 Mbps.
Although CAN is a predecessor to newer standards, such as CAN-Flexible Data Rate, which support speeds as high as 10 Mbps, vision-based networks like ADB systems still require much higher throughput.
RS-422 (Telecommunications Industry Association/Electronics Industries Alliance [TIA/EIA] 422) and RS-485 (TIA/EIA-485) transceivers are also subject to constrained data rates. These specifications define only the physical layer. Both standards use a large differential voltage swing - up to 5 V - to achieve 1,200 meters of reach. It is important to note that data rate and transmission distance have an inverse relationship. As frequency increases, the maximum allowable distance decreases. These standards also permit a large common-mode voltage range, which makes them a good choice in industrial applications, such as factory automation and control and building automation, but not for a high-speed vision-based network.
As the name suggests, LVDS has a very small differential voltage swing. This and other fundamental electrical characteristics allow it to achieve over 3-Gbps signaling rates, up to 10 meters of transmission distance and very low power consumption, which are all advantages in designs using adaptive LED control. Let’s take a closer look at the LVDS standard to better understand its operation (Figure 2).
The most fundamental LVDS link consists of a driver, the transmission media, a 100-Ω termination resistor and a receiver. LVDS drivers accept single-ended complementary metal-oxide semiconductor (CMOS) input signals and translate them to an LVDS output. The driver contains a 3.5-mA constant-current source that is responsible for creating a very small 350-mV differential across the termination resistor. An input logic level low or a logic level high controls the polarity of the driver current. It is possible to achieve very fast rise and fall times because of this small voltage swing, also contributing very little power dissipation.
LVDS receivers read the ±350-mV signal across the termination resistor and translate it back to a single-ended CMOS output. The input of the receiver is high-impedance, ensuring that the current passes through the termination resistor. TIA/EIA-644A also specifies a minimum common-mode voltage range of ±1 V; however, many available LVDS receivers support extended common-mode ranges. LVDS receivers have a differential threshold voltage of 100 mV, providing a good margin relative to the differential input.
As shown in Figure 3, adding an LVDS driver and receiver pair on the ECU and LED module can resolve the bottleneck in adaptive lighting. Because LVDS is protocol-agnostic, it gives engineers the flexibility to define the data-link layer. In lighting, it is common to pass protocols like universal asynchronous receiver transceiver (UART) over the LVDS physical layer. LVDS is a very well-established standard with many different product offerings and device functions. Texas Instruments’ (TI’s) portfolio consists of hundreds of LVDS devices encompassing different channel counts, product ratings, voltages and data rates. For adaptive lighting applications, the DS90LV011AQ-Q1 and DS90LT012AQ-Q1 are a cost-effective automotive-grade driver and receiver pair. TI’s DS90LVRA2-Q1, an automotive-rated, dual-channel LVDS receiver, supports 3.3-V, 2.5-V and 1.8-V logic voltage for interoperability with low-voltage processors.
Adaptive headlights are quickly becoming commonplace throughout the automotive industry. These systems are disrupting the automotive lighting ecosystem, demanding higher-performance interface standards. LVDS empowers these systems to control LED lighting profiles in real time with reliable, low-latency communication.
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