INDUSTRY FOCUS AUTOMOTIVE ELECTRONICS
KEEPING POWER CONVERSION SIMPLE IN AUTOMOTIVE ADAS
Automotive ADAS need low EMI/EMC emission switching converters. Here Tony Armstrong, director of product marketing, power products group at Analog Devices, Inc. looks at the latest solution
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any ADAS systems use a 5V and 3.3V rail to power their various
analogue and digital IC content; however, the processor I/O and Core voltages that are typically used will have operating requirements in the sub-2V realm, and could be as low as 0.8V. Furthermore, the system is usually mounted in a part of the vehicle which is both space and thermally constrained, thereby limiting the heat sinking available for cooling purposes. While it is commonplace to use a high voltage DC/DC converter to generate a 5V and 3.3V rail directly from the battery, in today’s ADAS systems a switching regulator must also switch at 2MHz, or greater, rather than the historical switching frequency of sub- 500kHz. The key driving force behind this change is the need for smaller solution footprints while also staying above the AM frequency band to avoid any potential interference. The ADAS system must also comply
with the various noise immunity standards within the vehicle. In an automotive environment, switching regulators are replacing linear regulators in areas where low heat dissipation and efficiency are valued. Moreover, the switching regulator is typically the first active component on the input power bus line, and therefore has a significant impact on the EMI performance of the complete converter circuit. There are two types of EMI emissions; conducted and radiated. Conducted emissions ride on the wires and traces that connect up to a product. Since the noise is localised to a specific terminal or connector in the design, compliance with conducted emissions requirements can often be assured relatively early in the development process with a good layout or filter design as already stated. However, radiated emissions are another story altogether. Everything on the board that carries current radiates an electromagnetic field. 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, generates noise all over the signal spectrum. Filters are often used to reduce EMI by attenuating the strength at a certain
28 JUNE 2018 | ELECTRONICS
switching frequency increases by 10 times. Device pin-outs, package construction, thermal design requirements and package sizes needed for adequate energy storage in decoupling components dictate a minimum hot loop size. To further complicate matters, in typical planar printed circuit boards, the magnetic or transformer style coupling between traces above 30MHz will diminish all filter efforts since the higher the harmonic frequencies are the more effective unwanted magnetic coupling becomes. Analog Devices’ Power by Linear Group
Figure 1:
The LT8650S is a high input voltage capable, dual output monolithic synchronous buck converter with low EMI/EMC emissions
Figure 2:
LT8650S schematic delivering 5V at 4A and 3.3V at 4A outputs, at 2MHz
frequency or over a range of frequencies. A portion of this energy that travels through space (radiated) is attenuated by adding metallic and magnetic shields. The part that rides on the PCB traces (conducted) is tamed by adding ferrite beads and other filters. EMI cannot be eliminated but can be attenuated to a level that is acceptable by other communication and digital components. Moreover, several regulatory bodies enforce standards to ensure compliance. Modern input filter components in surface mount technology have better performance than through-hole parts. However, this improvement is outpaced by the increase in operating switching frequencies of switching regulators. Higher efficiency, low minimum on- and off-times result in higher harmonic content due to the faster switch transitions. For every doubling in switching frequency, the EMI becomes 6dB worse while all other parameters, such as switch capacity and transition times, remain constant. The wideband EMI behaves like a first order high pass with 20dB higher emissions if the
developed the LT8650S – a high input voltage capable, dual output monolithic synchronous buck converter that has low EMI/EMC emissions. Its 3V to 42V input voltage range makes it ideal for automotive applications, including ADAS, which must regulate through cold-crank and stop-start scenarios with minimum input voltages as low as 3V and load dump transients in excess of 40V. As can be seen in Figure 2, it is a dual channel design consisting of two high voltage 4A channels, delivering voltages as low as 0.8V, enabling it to drive the lowest voltage microprocessor cores currently available. Its synchronous rectification topology delivers up to 94.4% efficiency at a switching frequency of 2MHz, while Burst Mode operation keeps quiescent current under 6.2µA (both channels on) in no-load standby conditions making it ideal for always-on systems. ADAS systems in automobiles will
continue to evolve. It is clear that finding a power conversion device that meets all the necessary performance metrics so as not to interfere with the ADAS system is not a simple task. Fortunately for the designers of these systems there are now “best-in-class” power converters from Analog Devices’ Power by Linear Group that greatly simplifies their task while simultaneously delivering all the performance they need without requiring sophisticated layout or design techniques.
Analog Devices Ltd.
www.analog.com 01628 477 066
e:
uksales@linear.com / ELECTRONICS
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