How to overcome the challenges of automotive front-end design

    We've all had the experience of starting a car and only hearing a clicking sound instead of the engine turning. This is caused by a dead battery, and while there are many reasons why this can happen, in most cases it's human error (has anyone left the interior lights on overnight?). Human error can also occur when starting a car.

Figure 1 – Battery correctly connected to jumper starter cable.

Automotive system designers must prepare for two possible scenarios that could arise when an error occurs during engine startup: reverse battery connection and dual battery configuration. Reverse battery connection refers to the battery jumper cable having its red terminal connected to ground and its black terminal connected to the positive terminal, opposite to the configuration shown in Figure 1. In this case, the diode will protect the system, but it will also introduce system losses during normal operation. Depending on the system's rated power, the forward voltage drop can range from 0.5V to 1.0V. This generates heat and increases the overall system efficiency.

    Trucks and buses use two batteries connected in series, while cars only have one. The only vehicle around that could start our car is likely a truck. This creates a dual-battery situation, where the voltage applied to the entire system is now twice as high. This puts a lot of stress on the system.

    Load sags are another condition that engineers must design systems to withstand and survive. This doesn't happen due to user errors like reverse battery connection, as it relates to problems inherent in the system itself.

Figure 2 – Load drop scenario.

A load sag occurs when the alternator or battery grounding is disconnected from the system due to corrosion or even improper installation (as shown in Figure 2). This disconnection generates a large voltage spike that can affect the entire system. The alternator plays a vital role in the operation of a vehicle's electrical system, and without it, our battery will run out of power.

All the conditions discussed present significant challenges to automotive front-end design: protecting electronic devices such as control units, sensors, and infotainment systems from destructive surges, voltage transients, electrostatic discharge (ESD), and noise present on power lines. Transient voltage suppressors (TVS) are a low-cost solution for automotive electronic protection, possessing several important parameters for these automotive applications, including rated power, shutdown voltage, breakdown voltage, and maximum breakdown voltage. We must also ensure the use of a downstream DC/DC converter with a higher V_IN to prevent system spikes, as TVS cannot suppress all of these.

In front-end power supplies, switching regulators are crucial for the overall power density and efficiency of the system when converting battery voltage to the voltage required by the processor. A problem with switching regulators is that they generate noise during operation. Switching requires rapid changes in electrical conditions: this includes the current flowing into the switch; changes in the switching node voltage; and output voltage ripple, as well as other factors related to the selection of external components. When controlling large amounts of power circulating in multiple directions, some of these factors cannot be changed due to the laws of physics. All of these conditions contribute to electromagnetic compatibility, or EMC for short.

The primary standard for automotive EMC is CISPR 25, which includes several levels based on the stringency of the end-device requirements and the level of EMC need. We can often hear EMC-generated noise in the AM band of a radio. This is highly undesirable, so we need to suppress this noise through filtering. Automotive-specific switching regulators are designed to minimize electromagnetic noise by combining architectural design (current-mode is the most common architecture in automobiles), component layout (as close to the switching regulator as possible and as few components as possible), and even device pin arrangement. It can never be zero, but it can be reduced.

At TI, we developed a reference design that enables front-end automotive power designers to meet all these automotive system standards and address the issues discussed in this blog.

Figure 3 – TI CISPR 25 Class 5 Multi-output Reference Design for Automotive Rear Cameras and ADAS Systems.

The TI Designs reference design shown in Figure 3 is a conducted electromagnetic interference (EMI) optimized multi-output design compliant with CISPR 25 Category 5 automotive standards. This 9W design is ideal for a wide range of V_IN automotive Advanced Driver Assistance Systems (ADAS) applications supporting cold start conditions. The reference design utilizes an LM53603-Q1 DC/DC regulator (used as a buck converter), an LM26420 DC/DC regulator (used as a dual buck converter), and a TPS60150 switched-capacitor voltage converter (used as a charge pump for the 5V output). The design accepts input voltages from 4.5V_IN to 20V_IN and provides outputs of 3.3V at 1.0A, 2.5V at 1.0A, 1.8V at 1.0A, and 5V at 100mA. It is small, inexpensive, efficient, and customizable for ADAS and other automotive-related applications. The four-layer printed circuit board (PCB) measures 65mm x 100mm.

The CISPR 25 Class 5 multi-output reference design for automotive rear-view cameras and ADAS systems ensures compliance with all relevant EMC, transient voltage, and even size limitations faced by automotive power design engineers. This design also protects against the aforementioned failure conditions, such as dual-battery and reverse-battery configurations, which are typically unrelated to the switching regulator but are part of the overall system design.