It can be tough to find a power-conversion device that meets the broad voltage range in today’s automotive ADAS. A novel low-EMI buck converter simplifies that search.
By Tony Armstrong, Director of Marketing, Power Products, Analog Devices Inc.; firstname.lastname@example.org
The automotive market has grown significantly during the last year and continues to show consistent signs of growth as we move forward. Some of the key growth drivers are the needs for improved safety, fuel efficiency, and advanced driver-assistance systems (ADAS). Furthermore, the proliferation of hybrid and all-electric vehicles continues to drive the need for innovative analog power-conversion products. In addition to increased vehicle electronics, the world automotive market is also expected to grow steadily, thereby increasing demand for more vehicle production.
Many applications in automotive electronic systems require continuous power even when the car is parked, such as remote keyless entry, security, and even personal infotainment systems, which usually incorporate navigation, GPS location, and e-call functionality. It may be hard to understand why these systems must remain on, even when the vehicle is not moving. However, the GPS aspect of these systems needs to be “always-on” for emergency communications and security purposes (to pinpoint its location in the event of an accident, for example). This is also a necessary requirement so that rudimentary control can be employed by an external operator, if needed. Accordingly, a key requirement for these applications is low quiescent current from its electronic systems to extend battery life.
Therefore, it should come as no surprise that ADAS systems are commonly found in many of today’s new automobiles and trucks, where they can facilitate safe driving and provide the driver with an alert if the system detects risks from surrounding objects, such as errant pedestrians, cyclists, or even other vehicles on an unsafe trajectory.
Furthermore, these systems typically provide dynamic features such as adaptive cruise control, blind-spot detection, lane-departure warning, driver drowsiness monitoring, automatic braking, traction control, and night vision. As a result, it’s the increasing focus of consumers on safety, demands for comfort while driving, and the continued increase of government safety regulations that are main growth drivers of ADAS in cars and trucks.
Nevertheless, this growth doesn’t come without challenges for the industry, which include pricing pressure, inflation, complexity and difficulty in testing these systems. The European automotive industry is one of the most innovative automotive markets, and as such, it’s seen major market penetration and adoption of ADAS from its customers. Nevertheless, both American and Japanese auto makers are not far behind.
It’s very common for an ADAS system to incorporate a microprocessor, or a custom ASIC, that can gather all of the input from the vehicle’s numerous sensors and then process it so it’s easily presented to the driver in an easily understandable way. Moreover, these systems are usually powered directly from the vehicle’s main battery that’s a nominal 9 V to 18 V in a car, but could be as high as 42 V due to voltage transients within the system, and as low as 3.4 V during a cold-crank condition. Therefore, any dc-dc converters within these systems must be able to handle the wide input voltage range of 3.4 to 42 V, at a minimum.
Furthermore, many dual-battery systems, such as those commonly found in trucks, require an even broader input range, pushing the upper limit as high as 65 V. Since many manufactures want to standardize their designs to be capable of use in either an automobile or truck, they’re designing their systems to cover a 3.4- to 65-V input range specifically for this purpose. This not only gives them economies of scale when purchasing components needed to build them, but also allows for more effective cost matrices during their manufacturing processes.
Many ADAS systems use a 5-V and 3.3-V rail to power their various analog and digital IC content. Correspondingly, the manufacturers of such systems prefer to use a single converter to address both the single- and double-battery configurations simultaneously. Furthermore, the system is usually mounted in a part of the vehicle that’s space and thermally constrained, thereby limiting the heat sinking available for cooling purposes.
While it’s commonplace to use a high-voltage dc-dc converter to generate a 5-V and 3.3-V rail directly from the battery, in today’s ADAS systems a switching regulator must also switch at 2 MHz, or greater, rather than the historical switching frequency of sub-500-kHz. 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.
Furthermore, as if the designers task isn’t already complicated enough, they must also ensure that the ADAS system complies 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 electromagnetic-interference (EMI) performance of the complete converter circuit.
What Emissions Are We Dealing With?
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 localized 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. Even with careful design, a power-supply designer never really knows how the bad the radiated emissions are going to be are until the system gets tested. And radiated emissions testing can’t be formally performed until the design is essentially complete.
Filters are often used to reduce EMI by attenuating the strength at a certain 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 PCB traces (conducted) is tamed by adding ferrite beads and other filters. EMI can’t be eliminated, but can be attenuated to a level that’s 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 6 dB 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 20-dB higher emissions if the switching frequency increases by 10 times.
Savvy PCB designers will make the hot loops small and use shielding ground layers as close to the active layer as possible. Nevertheless, 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 PCBs, the magnetic or transformer-style coupling between traces above 30 MHz will diminish all filter efforts—the higher the harmonic frequencies, the more effective the unwanted magnetic coupling.
High-Voltage Dual DC-DC Converter with Low EMI Emissions
It was because of the application constraints described above that Linear Technology (which was recently acquired by Analog Devices) developed the LT8645S—a high-input-voltage-capable monolithic synchronous buck converter that also has low EMI emissions. Its 3.4- to 65-V input voltage range suits it for both automotive and truck applications, including ADAS, which must regulate through cold-crank and stop-start scenarios with minimum input voltages as low as 3.4 V and load-dump transients more than 60 V.
As can be seen in Figure 1, it’s a single-channel design, delivering an 8-A output at 5 V. Its synchronous-rectification topology delivers up to 94% efficiency at a switching frequency of 2 MHz, while Burst Mode operation keeps quiescent current under 2.5 µA in no-load standby conditions, making it ideal for always-on systems.
The LT8645S’s switching frequency can be programmed from 200 kHz to 2.2 MHz and synchronized throughout this range. Its Silent Switcher 2 architecture uses two internal input capacitors as well as internal BST and INTVCC capacitors to minimize the area of the hot loops.
Combined with very-well-controlled switching edges and an internal construction with an integral ground plane and use of copper pillars in lieu of bond wires, the LT8645s’ design dramatically reduces EMI/EMC emissions (Figure 2). This improved EMI/EMC performance isn’t sensitive to board layout, simplifying design and reducing risk even when using two-layer PCBs. The LT8645S can easily pass the automotive CISPR25, Class 5 peak EMI limits over its entire load range. Spread-spectrum frequency modulation is also available to lower EMI levels further.
The LT8645S utilizes internal top and bottom high-efficiency power switches with the necessary boost diode, oscillator, and control and logic circuitry integrated into a single die. Low-ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 10 mV p-p. Finally, the LT8645S is packages in a small thermally enhanced 4- × 6-mm 32-pin LQFN package.
The proliferation of ADAS systems in both cars and trucks continues to gain momentum and is going to further increase its proliferation and momentum as it also adds “autonomous” vehicles to its base from the current crop of vehicles. In addition, it’s evident from the issues outlined in this article, especially a broad input range of 3.4 V to 65 V for power-conversion ICs, that finding a power-conversion device that meets all necessary performance metrics isn’t a simple task.
Fortunately for the designers of these systems, power converters developed by Power by Linear, part of Analog Devices, greatly simplify the designer’s task while simultaneously delivering all the performance they need. Moreover, they minimize the need for sophisticated layout or design techniques.