The proliferation of advanced driver-assistance systems (ADAS) and in-cabin infotainment has turned vehicles into complex electronic systems on wheels that require multi-level, noise-free DC rails. However, the typical vehicle’s battery is far from stable across its operating environment, requiring designers to pay close attention to power system design.
The range of ADAS systems includes adaptive cruise control, collision avoidance, GPS, backup cameras, lane departure warning, stability control, and connectivity, while infotainment includes displays and multimedia players. Regulating the DC power for these functions from the car’s 12 volt battery (and even 24 or 48 volts in some cases) is a challenge due to battery output noise, voltage spikes, load-dump transients, thermal extremes and cycling, via electronics in cramped, hot locations, having to endure vibration and impact.
Also, the DC/DC converter ICs which regulate the battery output to provide the multiple DC rails needed for the various ADAS functions must operate in hostile electrical and environmental conditions. They must also provide tight regulation at high efficiency, with low quiescent currents and minimal EMI generation.
This article will describe the operating environment and conditions and introduce automotive standards developed to help mitigate issues. It will then describe power regulators and DC/DC converters that will help meet automotive power distribution requirements, and how to go about using them.
Under the hood is not a nice place
The automobile is a challenging and harsh environment for electronics (and mechanical components, as well) with respect to four areas: electrical, thermal, shock/vibration, and available space. Looking at them briefly:
Electrical: The unconditioned rail from the battery is not a simple, steady source of DC current as it is with most batteries; instead, it is subject to cold cranking voltage drops (Figure 1), high-voltage surges due to “load dumps” (when the load connected to the alternator is suddenly disconnected) (Figure 2 and Table 1), and noise and EMI/RFI.
Figure 1: The typical battery voltage profile under a cold cranking condition bears little resemblance to that of battery output in more benign applications. (Image source: Texas Instruments)
Figure 2: The typical load-dump pulse is characterized by fast rise, slower fall, and variable timing. (Image source: Texas Instruments)
Table 1: The typical values of an unsuppressed load-dump pulse (here, defined by ISO7637-2:2004-5) for a 12 volt and 24 volt battery system. (Image source: Texas Instruments)
Thus, the local DC/DC regulators must deal with these realities, function over a wide input voltage (VIN) range, and tolerate reverse polarity connection of the battery. Further, these regulators must have very low quiescent current to minimize battery drain when the car is supposedly “off.”
The reason is that many of these ADAS (and other) functions are not physically disconnected from the battery but instead use “soft” on/off, and so are actually in their quiescent state when “off.” In aggregate, the so-called “vampire power” which they dissipate can drain the battery if the car is left unused for weeks.
Thermal: Under hood temperatures can range from sub-zero (parked in winter) to well above 150°C to 200°C, depending on operating conditions and probe placement (Figure 3). While other areas of the car such as the cabin do not get as hot, they can still see fairly high temperatures if the car is parked in the sun. When temperatures outside range from 25°C (77°F) to 40°F (104°F) the temperature inside a car parked in direct sunlight can climb to between 50°C (122°F) to 75°C (167°F).
Figure 3: Vehicle temperature measured at various locations for a Chevrolet Silverado climbing a hill at 40 mph; many spots exceed 150°C. (Image source: Pelican Parts)
Shock/vibration: Mechanical shock and vibration are a constant presence; basic mechanical analysis shows that smaller, lighter components are less prone to being affected by those disturbances, and they are also easier to cushion and “shock mount” if needed. Further, such components result in smaller circuit boards, which have the corresponding virtues.
Size: Small size has another major benefit, unrelated to the shock/vibration aspects. Given the fixed physical “envelope” of the car as an enclosure, it’s a challenge to find places to locate the ADAS function circuitry, and in many cases, their associated sensors. While some of this circuitry can be located in almost any vacant spot, many of the ADAS sensors and front-end signal conditioning circuitry need to be situated in specific places, even if the supporting electronics can be located elsewhere.
Auto standards define challenge
Automobiles have three main power sources; electric (EV), hybrid EV (HEV), and of course the internal combustion engine. They also come in a wide variety of sizes, styles, capabilities, and cost points. The industry has defined standards for levels of risk and performance for electronic components, software, and subsystems. By qualifying basic integrated circuits (ICs) to various levels, designers know they are getting building blocks which can be used to “build up” circuit boards, assemblies, subsystems, and complete functions with defined performance.
The dominant standard for this ability to define performance is Automotive Safety Integrity Level (ASIL) scheme, a multilevel risk classification approach defined by ISO 26262, the Functional Safety for Road Vehicles standard. At the top level is ASIL-D, representing the highest degree of automotive hazard, which therefore requires the highest degree of assurance in meeting safety requirements (Figure 4). In descending order below the top level ASIL-D ranking are levels -C, -B, and -A, which define intermediate degrees of hazard and of required assurance, finally concluding with a ranking of ASIL QM for applications with no automotive hazards, and therefore have no safety requirements to manage (radio, for example).
Figure 4: ASIL-D through ASIL-A classify automotive functions in terms of criticality to vehicle safety, operation, control, and other factors, with ASIL-D being the most stringent. (Image source: Mentor Graphics)
Vendors of components designed for ADAS functions, including the DC/DC regulators, test and certify that their devices meet and exceed specific levels of ASIL performance requirements. These include but are not limited to temperature, vibration, and failure modes.
Another relevant standard, AEC-Q100, is a set of qualification test sequences for ICs developed by the Automotive Electronics Council (AEC). It sets standards for part qualification and quality systems for both new and upgraded products. AEC-Q100 also establishes temperature ratings with defined grade designation classes for the components, with Grade 0 being the most wide ranging (Table 2).
Table 2: The AEC-Q100 ratings for temperature establish basic operating ranges, with corresponding suffixes. (Image source: Cypress Semiconductor Corp.)
DC/DC regulators that address ADAS requirements
The challenging requirements of ADAS functions mandate ICs, including DC/DC regulators, which address the demands of this application with respect to electrical, thermal, and size considerations. These components strive to meet multiple (if not all) automotive ASIL objectives related to electrical, thermal, shock/vibration, and available space aspects.
For example, the MAX16930 from Maxim Integrated is a 36 volt, DC/DC regulator drawing just 20 microamps (µA) of quiescent current (Figure 5). This automotive rated triple-output switching device integrates two synchronous step-down controllers and an asynchronous step-up “preboost” controller, providing up to three independently controlled power rails: a preboost with adjustable output voltage; a buck controller with a fixed 5 volt output or an adjustable 1 to 10 volt output; and a buck controller with a fixed 3.3 volt output or an adjustable 1 to 10 volt output.
Figure 5: The preboost feature of the MAX16930 multiple output buck regulator allows it to operate during cold cranking periods, when the battery voltage drops to low single-digit values (yellow). (Image source: Maxim Integrated)
The MAX16930 operates from a wide range power rail of 3.5 volts to 36 volts, while the preboost extends operation down to 2 volts (in bootstrap mode), needed for cold crank performance (Figure 5, again). The buck controllers and the preboost can each provide up to 10 amps of output current and are independently controllable. The user-adjustable switching frequency, from 200 kHz up to 2.2 MHz and with optional spread-spectrum operation, guarantees no AM band interference.
The MAX16930 includes a choice of clocking setup which allows designers to minimize issues associated with interference due to IC clocks, as well as beat frequencies which result from mixing between multiple system clocks. Users must decide among three frequency function modes:
- Basic fixed frequency operation, at a user-defined frequency.
- Skip mode, which disables the clock when the load is light and is invoked only as needed to maintain output voltage regulation.
- Synchronization to an external clock. The IC can be switched “on the fly” among these modes, but that requires more software for IC management.
Another option that this IC offers is to invoke spread-spectrum clocking to minimize clock sourced EMI occurring at a single frequency by dithering the clock randomly around a nominal frequency value; the undesired EMI energy is spread over a wider spectrum, but with lower peak amplitude at any single frequency.
Users must also decide during the system design phase on the “value” of the internal linear regulator (LDO), which can be bypassed by connecting it to an external rail.
On one hand, the LDO output is ultra-quiet and useful for supplying a small, localized load which needs minimum rail noise; on the other hand, it is less efficient than the switching regulators within the MAX16930.
To address footprint, a common technique is to increase the number of distinct outputs from a single IC. The LT8603 from Analog Devices is a quad output device which combines two high input voltage step-down switching regulators, one low input voltage step-down regulator, and a boost controller, all in a 6 × 6 mm package.
With the boost controller configured to supply the VIN supply, the IC develops three regulated outputs even when the boost input voltage falls below the regulated output voltages, such as during cold crank situations (Figure 6).
Figure 6: The LT8603 can be configured to operate to specifications and provide full DC output despite cold cranking conditions. (Image source: Analog Devices)
The IC operates from supply rails as high as 42 volts, switching at user-selected frequencies ranging from 250 kHz to 2.2 MHz to minimize EMI. The radiated EMI (for CISPR 25 Radiated Emission Tests with Class 5 Peak Limits) is below the allowed limits (short horizontal segments) (Figure 7).
Figure 7: The LT8603 radiated EMI for CISPR 25 Radiated Emission Tests with Class 5 Peak Limits using a 14 volt supply and switching at 2 MHz shows that its emissions are below the allowed limits (short horizontal segments). (Image source: Analog Devices)
The four channels of this IC are independently powered and designers must decide which way to connect them to meet system and circuit objectives. For example, the boost output can be configured to supply the input voltage to the buck converters, which yields three tightly regulated outputs even if the boost input voltage falls below the regulated buck outputs, as will likely occur during a cold crank situation. However, the boost mode controller can instead be driven from a buck controller output or configured as a SEPIC converter, in which case the IC provides up to four tightly regulated outputs.
The switching frequency range of the four channels is another factor which designers must determine, and this must be done before selecting the oscillator frequency, which can be set via a single resistor from 250 kHz to 2.2 MHz. Lower frequencies generally offer better efficiency and a wider input voltage operating range due to lower switching losses and less sensitivity to timing constraints, such as minimum on and off times.
However, higher switching frequencies allow use of smaller components and moves the switching related noise away from sensitive frequency bands, such as AM radio. The downside is reduced efficiency.
Powering high performance ADAS sensors
There are some ADAS functions having high performance sensor front ends, and so require lower noise or faster transient response than most switching buck regulators can provide. The Maxim MAX15027 low-dropout linear regulator (qualified to AEC-100 Grade-1) is designed for these situations. It operates from input voltages as low as 1.425 volts and delivers up to 1 amp of continuous output current with a maximum dropout voltage of only 225 millivolts (mV). Its wide bandwidth supports fast transient response, thus limiting the output voltage deviation to 15 mV with a 500 mA load step, using only a 4.7 microfarad (μF) ceramic capacitor on the output.
Some precautions for LDO optimum performance
Even though the MAX15027 is an LDO, and is one of the simplest power regulator topologies to use, a few precautions are necessary. First, both the 1 μF ceramic input capacitor and 4.7 μF ceramic output capacitor must be high quality, with a low ESR in the milliohm range; if ESR is on the order of ohms or higher, the line and load transient response of the LDO will be compromised, and there may be problems with internal LDO loop stability, and possible self-oscillation.
Second, the printed circuit board layout must accommodate heatsinking and thermal concerns, as LDOs have a relatively high ratio of dissipation to their package size compared to switching regulators. For this reason, the TDFN package of the MAX15027 has an exposed thermal pad on its underside to ensure a low thermal resistance path into the pc board. This path carries a majority of the heat away from the IC, allowing the pc board to be an effective heatsink. The exposed pad should be connected to a large ground plane for best thermal and electrical performance.
However, that approach considered in isolation is necessary, but is not sufficient. Use of thermal modeling is critical to assure that nearby ICs and other components are not also assuming they can use the same copper layer of the pc board for their own heatsink needs, making the aggregate heat load beyond what the chosen cooling strategy can provide.
This strategy usually begins with thermal conduction away from the IC through the pad and into the pc board layer, followed in most cases by convection at a remotely located heat sink or cold plate. Such thermal source “crowding” can negate the basic cooling plan which begins at the IC’s underside thermal pad.
The use of ADAS and infotainment means that their unique and often challenging DC power needs must be addressed. This is driving development and availability of ICs and other components which can function despite extremes of temperature and DC input rail voltage range, while having very low quiescent current drain. These ICs must also be small to reduce their sensitivity to vibration and shock, which fortunately also supports compact ADAS function circuit designs.
Vendors of power regulators now offer a wide array of ADAS optimized switching and LDO DC/DC devices which meet stringent industry standards, easing the design-in challenge and simplifying BOM decisions.