Low EMI/EMC Switching Converter Simplifies ADAS Design

ADAS is an acronym for Advanced Driver Assistance System, and it’s common in many of today’s newer cars and trucks. Such systems often facilitate safe driving; when it detects that surrounding objects (such as pedestrians not obeying traffic rules, cyclists, or even other vehicles on unsafe driving paths) pose a risk, the system can provide drivers alarm! In addition, these systems typically offer dynamic features such as adaptive cruise control, blind spot detection, lane departure warning, driver drowsiness monitoring, automatic braking, traction control and night vision. Therefore, consumers’ increasing emphasis on safety, requirements for driving comfort, and increasing government safety regulations are the main growth drivers for automotive ADAS in the second half of the next decade.

This growth has not been without challenges for the industry, including price pressures, inflation, complexity and difficulty in system testing. In addition, the European automotive industry is one of the most innovative automotive markets, not surprisingly, with significant breakthroughs in market penetration and customer acceptance of ADAS. Still, U.S. and Japanese automakers are not far behind. The ultimate goal is to achieve autonomous driving without human intervention behind the wheel!

system problems

Generally speaking, ADAS integrates some microprocessors to collect all the input provided by the numerous sensors in the vehicle and then process it so that it can be presented to the driver in a convenient and understandable manner. Additionally, these systems are typically powered directly from the vehicle’s main battery, which is nominally 9 V to 18 V, but can be as high as 42 V due to voltage transients inside the system, and as low as 3.4 V during a cold crank . Therefore, any DC-DC converter in these systems must be able to handle at least a wide input voltage range of 3.4 V to 42 V. Also, many dual-battery systems, such as those commonly found in trucks, require a wider input range, pushing the upper limit to 65 V. As a result, some ADAS manufacturers have designed their systems to cover the 3.4 V to 65 V input range, making them usable in cars or trucks while gaining the benefits of economies of scale in the manufacturing process.

Most ADAS use 5 V and 3.3 V rails to power their various analog and digital IC devices. Accordingly, manufacturers of such systems prefer to use a single converter to address both single-battery and dual-battery configurations. In addition, the system is usually installed in parts of the vehicle where space and heat dissipation are limited, which imposes limitations on the radiator used for heat dissipation purposes. While it is commonplace to use high voltage DC-DC converters to generate 5 V and 3.3 V rails directly from the battery, in today’s ADAS switching regulators must also achieve switching frequencies of 2 MHz or higher, rather than previous ones Switching frequencies below 500 kHz. The key driver behind this change is the need for a smaller form factor solution while also staying above the AM band to avoid any potential interference.

Plus, as if the designer’s task wasn’t complicated enough, they also had to ensure that the ADAS complied with various in-vehicle noise immunity standards. In the automotive environment, switching regulators are replacing linear regulators in areas where low heat generation and high efficiency are important. Also, the switching regulator is usually the first active component on the input power bus and therefore has a significant impact on the EMI performance of the entire converter circuit.

There are two types of EMI emissions: conducted and radiated. Conducted emissions are on wires and traces connected to the product. Since this noise is localized to a specific terminal or connector in the design, compliance with conducted emissions requirements can often be assured relatively easily with good layout or filter design during development.

Radiated emissions are another matter entirely, though. Anything on a circuit board that carries current will radiate electromagnetic fields. Every trace on the board is an antenna, and every copper layer is a resonator. Anything other than a pure sine wave or DC voltage will create noise across the entire signal spectrum. Even with careful design, power supply designers don’t really know how bad radiated emissions will be until the system is tested—and radiated emissions testing can’t be formally performed until the design is largely complete.

Filters are often used to attenuate signal strength at a specific frequency or a range of frequencies, thereby reducing EMI. This part of the energy propagating (radiating) through space can be attenuated by adding metallic and magnetic shielding. The portion of the energy located in the PCB traces (conducted) can be suppressed by adding ferrite beads and other filters. EMI cannot be eliminated, but it can be attenuated to levels acceptable to other communications and digital devices. In addition, multiple regulatory agencies ensure product compliance by implementing relevant standards.

Figure 1. Schematic of LT8645S with 5 V, 8 A, 2 MHz Output

Modern input filters using surface mount technology have better performance than through-hole devices. However, this improvement has not kept pace with the increasing operating frequency of switching regulators. Higher efficiency, shorter on/off times and faster switching transitions result in higher harmonic content. With all other parameters (such as switching capacity and transition time) held constant, EMI deteriorates by 6 dB for every doubling of switching frequency. If the switching frequency is increased by a factor of 10, broadband EMI behaves like a first-order high-pass filter with 20 dB more radiation.

An experienced PCB designer will keep the hot loop small and keep the shield ground plane as close to the active layer as possible. Nonetheless, the device pinout, package construction, thermal design requirements, and package size required to store sufficient energy in the decoupling components all require a thermal loop of a certain minimum size. To complicate things further, in a typical planar printed circuit board, magnetic or transformer-like coupling between traces above 30 MHz can weaken all filters, because the higher the harmonic frequencies, the more pronounced the bad magnetic coupling.

High Voltage DC-DC Converters with Low EMI Emissions

Given the application constraints described above, Analog Devices’ Power by Linear™ division developed the LT8645S, a high input voltage, single chip, low EMI radiated synchronous buck converter. Its input voltage range of 3.4 V to 65 V makes it suitable for both automotive and truck applications, including ADAS, which must be capable of regulation in cold-crank and start-stop scenarios, with a minimum input voltage as low as 3.4 V, with instantaneous power cut-off. becomes more than 60 V. As shown in Figure 1, the device is a single-channel design with a 5 V, 8 A output. Its synchronous rectification topology achieves up to 94% efficiency at a switching frequency of 2 MHz, while Burst Mode® keeps quiescent current below 2.5 μA during no-load standby conditions, making it ideal for always-on system use.

The switching frequency of the LT8645S is programmable from 200 kHz to 2.2 MHz, and synchronization is supported across the entire frequency range. Its unique Silent Switcher® 2 architecture integrates internal input capacitors as well as internal BST and INTVCC capacitors to reduce solution size. Combining tightly controlled switching edges and an internal structure with an integrated ground plane, and replacing bond wires with copper posts, the LT8645S is designed to greatly reduce EMI emissions. In addition, its Silent Switcher 2 design provides robust EMI performance on any printed circuit board (PCB, including 2-layer PCB). Also, it is much less sensitive to PCB layout than other similar converters. This is because the LT8645S’s internal dual input, BST, and INTVCC capacitors minimize hot loop area, enabling new levels of performance. It still requires two external input capacitors, but it is no longer strictly required to place these capacitors as close as possible to the input pins. Combined with the internal capacitance, which minimizes the hot loop area, the integrated ground plane of the BT substrate provides a significant improvement in EMI performance (see Figure 2). The multilayer BT substrate also enables the I/O pins to use the exact same pattern as the QFN package, while enabling the realization of large ground pads. This laminated QFN (LQFN) package is more flexible and flexible than standard QFN, and its solder joint reliability exhibits much better performance during board-level temperature cycling, allowing customers to use lead-based devices previously only LQFN can be used below.

The LT8645S easily meets automotive CISPR25, Class 5 peak EMI limits over the entire load range. Spread spectrum frequency modulation can also be used to further reduce EMI levels (Figure 2). The LT8645S incorporates high-efficiency top and bottom power switches and integrates the necessary boost diodes, oscillators, and control and logic circuits into a single chip. The low-ripple burst mode of operation maintains high efficiency at low output currents while keeping output ripple below 10 mV pp. Finally, the LT8645S is available in a small thermally enhanced 4 mm × 6 mm, 32-pin LQFN package.

Figure 2. LT8640S Radiated EMI Performance Plot

in conclusion

The rollout of ADAS in the car and truck market won’t end anytime soon. It is also clear that finding the right power conversion device to meet all the necessary performance metrics so as not to interfere with ADAS is not a simple task. Fortunately, designers of such automotive systems now have access to the robust performance and capabilities offered by Analog Devices’ Silent Switcher 2 DC-DC converters. These devices not only greatly simplify the work of the power supply designer, but also provide all the performance they need without requiring complex layout or design techniques.