Four design considerations for telematics hardware in connected cars

At its most basic definition, eCall is simply a basic cellular phone in a car that automatically dials for help in an emergency and has been available since the 1990s. Looking ahead, consumers are demanding more advanced integration, which is why the Telematics Control Unit (TCU) was introduced.

The TCU provides the connected car with all the functionality of eCall and additional features including sending and receiving data such as location, over-the-air updates, or phone calls. Without the TCU, eCall is limited to making phone calls. Figure 1 provides an overview of the TCU with the ability to make an emergency call.

Figure 1: Emergency call function integrated into the Hyundai TCU

Requirements for a typical TCU with integrated eCall system

Designing a TCU has many hardware variables because original equipment manufacturers (OEMs) and Tier 1 suppliers have their own design specifications for placing the TCU in different locations within the vehicle.

The EU mandates that eCall systems in new cars must be able to:

It can automatically work during and after a car crash, even without a working car battery.

Withstands extreme temperatures, such as -20°C or -40°C.

With a 10-year battery life, each call lasts 8 to 10 minutes.

Emergency services are available with 60 minutes of call back over the cellular network.

Complies with the International Organization for Standardization (ISO) 26262 Automotive Safety Integrity Level (ASIL) A standard.

Start with a backup battery

When designing a TCU, a good place to start is with the backup battery, which must support 6W to 20W of audio power and the peak current of about 2A (350mA nominal) generated by the Global System for Mobile Communications (GSM) module, according to EU requirements.

The choice of backup battery, which is determined by the battery chemistry (common types include lithium-ion, lithium-ion phosphate, and nickel-metal hydride), the number of cells, and the current capacity, determines the rest of the system. Where the battery is placed in the power path also determines the type of charger or low-dropout regulator you use, and whether a boost regulator is required.

Figures 2 and 3 show two variations of the power path based on different power supply schemes. Each path uses the same number of components but in different configurations to accomplish the same task, depending on the backup battery selection and battery charger capabilities.

The first power path is a simple design with low cost, but you have to use multiple boost regulators to trade off size and redundancy. The second power path uses lithium-ion batteries that require more protection, but fewer batteries are required. Both are viable options, but cost, size, and reliability all play a role in choosing one over the other.

Figure 2: The first TCU power supply path

Figure 3: Second TCU power supply path

Selecting a Power Conditioner

Once you consider the use of a backup battery, another design consideration that you need to consider separately is the power regulator. As with automotive applications, the on-board battery power supply must withstand harsh temperatures, wide input voltages, and reduce electromagnetic interference (EMI). Telematics systems can be installed in locations in the car that can withstand high temperatures (such as the windshield, passenger compartment, trunk, and engine), which requires integrated circuits (ICs) that can withstand junction temperatures as high as 150°C, giving the car excellent thermal performance and efficiency.

The input voltage varies based on OEM load dump, polarity reversal and cold crank conditions, and is typically 4.5V initially with peak voltages up to 42V. Any switching regulator must not interfere with the AM and FM bands of the car radio, so the switching frequency must be around 2.1MHz (above the AM band and below the FM band) or around 400kHz (below the AM band). Selecting a switching regulator with appropriate switching frequency, dithering/spread spectrum and optimized layout are key factors in ensuring good electromagnetic interference (EMI) performance.

Enrich your sound design

Speaker power varies widely. Some designers may choose a low power system of 4-6W, while others may go up to 20W. In addition to power consumption and variation, speaker diagnostics and protection are key functions for audio, including open and shorted output loads, output/power switching, and shorts to ground in addition to typical automotive short circuit protection, load dump, temperature protection and monitoring.

Consider the data rate

As the integration of data, modems and internal storage in telematics systems increases, the data rates connected to the head unit or central gateway are also increasing. Gone are the days of just CAN, LAN or even USB, replaced by 10/100Mbps and even 1Gbps.

Predicting the future of telematics

A variety of intertwined forces are influencing the development of telematics, whether it is the legislative factors mentioned above, infrastructure requirements, user experience, driver expectations, or the fact that telematics is still a fragmented market. It is worth mentioning the advancement of small aftermarket telematics products, such as on-board diagnostic electronic dongles, and the emergence of more advanced systems, such as the surrounding Internet of Things (V2X) modules that facilitate communication between the vehicle and other vehicles and the driving environment. These devices may have similar modem, processing and data communication functions as modern TCUs.

From what I have observed, the reality is that the future of connected cars will depend on innovations in telematics and how well automotive design engineers can rise to the design challenges and keep pace with the trends.