Low EMI/EMC switching converters simplify ADAS design

Generally speaking, ADAS integrates some microprocessor to collect all the inputs provided by the numerous sensors in the car, and then processes them so that they can be presented to the driver in a convenient and understandable way. In addition, these systems are usually powered directly by the vehicle's main battery, which has a nominal voltage of 9 V to 18 V, but due to voltage transients within the system, the voltage can be as high as 42 V, and during cold start, the voltage can be as low as 3.4 V. Therefore, any DC-DC converter in these systems must be able to handle a wide input voltage range of at least 3.4 V to 42 V. In addition, many dual-battery systems, such as those commonly found in trucks, require a wider input range, pushing the upper limit to 65 V. Therefore, some ADAS manufacturers design their systems to cover an input range of 3.4 V to 65 V, allowing them to be used 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 a space- and thermally constrained part of the vehicle, which puts limitations on the heat sink used for heat dissipation purposes. While it is common 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, instead of the previous switching frequencies of less than 500 kHz. The key driving force behind this change is the need for a smaller solution size while also staying above the AM band to avoid any potential interference.

Designers must also ensure that ADAS meets various noise immunity standards within the vehicle. In the automotive environment, switching regulators are replacing linear regulators in areas where low heat and high efficiency are important. Moreover, the switching regulator is usually the first active component on the input power bus, so it has a significant impact on the EMI performance of the entire converter circuit.

There are two categories of EMI emissions: conducted and radiated. Conducted emissions are located on the wires and traces that connect to the product. Because this noise is localized to specific terminals or connectors in the design, it is usually relatively easy to meet conducted emissions requirements during development with good layout or filter design. Radiated emissions, however, are another matter entirely. Anything on the board that carries current will radiate electromagnetic fields. Every trace on the board is an antenna, and every copper plane is a resonator. Anything other than a pure sine wave or DC voltage will generate noise across the entire signal spectrum. Even with careful design, a power supply designer doesn't really know how bad radiated emissions will be until the system is tested—and radiated emissions testing can only be formally performed after the design is essentially complete.

Filters are often used to reduce EMI by attenuating the signal strength of a specific frequency or a range of frequencies. The portion of energy that propagates through space (radiation) can be attenuated by adding metal and magnetic shielding. The portion of energy that resides in PCB traces (conduction) can be suppressed by adding ferrite beads and other filters. EMI cannot be eliminated, but it can be attenuated to a level that is acceptable to other communications and digital devices. In addition, multiple regulatory agencies ensure product compliance by implementing relevant standards.

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

Experienced PCB designers will make thermal loops smaller and keep the shield ground plane as close as possible to the active plane. Nonetheless, device pinout, package construction, thermal design requirements, and package size required to store sufficient energy in the decoupling components all require a certain minimum size of the thermal loop. To further complicate matters, in a typical planar printed circuit board, magnetic or transformer-like coupling between traces above 30 MHz will weaken the effectiveness of any filter because the higher the harmonic frequency, the more significant the negative magnetic coupling will be.

In view of the application limitations mentioned above, ADI developed the LT8645S, a synchronous buck converter that supports high input voltage, single chip, and low EMI radiation. Its input voltage range is 3.4 V to 65 V, making it suitable for both automotive and truck applications, including ADAS, which must be capable of regulating cold crank and start-stop scenarios with minimum input voltages as low as 3.4 V and instantaneous power loss. becomes more than 60 V. As shown in Figure 1, the device is a single-channel design that provides a 5 V, 8 A output. At a switching frequency of 2 MHz, its synchronous rectification topology can achieve up to 94% efficiency, and under no-load standby conditions, Burst Mode ^® keeps quiescent current below 2.5 μA, making it ideal for always-on System usage.

Figure 1. Schematic of the LT8645S providing a 5 V, 8 A, 2 MHz output.

The LT8645S's switching frequency is programmable from 200 kHz to 2.2 MHz, and synchronization is supported throughout the 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 with an internal structure that integrates the ground plane and uses copper pillars instead of bond wires, the LT8645S is designed to significantly reduce EMI emissions. Additionally, its Silent Switcher 2 design provides robust EMI performance on any printed circuit board (PCB), including 2-layer PCBs. 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 the hot loop area, enabling new levels of performance. It still requires two external input capacitors, but there is no longer a strict requirement to place these capacitors as close as possible to the input pins. Combined with internal capacitance, which minimizes the thermal loop area, the integrated ground plane of the BT substrate results in significantly improved 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 implementation 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-containing devices in situations where they were previously the only option. LQFN can be used below.

Across the entire load range, the LT8645S can easily comply with automotive CISPR25, Class 5 peak EMI limits. Spread spectrum frequency modulation can also be used to further reduce EMI levels (Figure 2). The LT8645S features high-efficiency top and bottom power switches and integrates the necessary boost diode, oscillator, and control and logic circuitry into a single chip. Low-ripple burst mode operation maintains high efficiency at low output currents while keeping the 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 graph

Finding the right power conversion device to meet all the necessary performance indicators without interfering with ADAS is not a simple task. Fortunately, designers of such automotive systems now have access to the powerful performance and capabilities provided by ADI Silent Switcher 2 DC-DC converters. These devices not only greatly simplify the work of power supply designers, but also provide all the performance they need without requiring complex layout or design techniques.