The primary objective of this article will be to investigate the new challenges that face the designers of power-supply ICs for automotive applications and how these new "automotive specific" power-supply ICs can meet all of these new demands. The automotive environment is both electrically and thermally caustic to any ICs, thus requiring new designs to meet the demands of this rapidly accelerating market.
Why is this topic increasingly important? Each year automobiles continue to incorporate increasingly complex electronic systems. Market research firm IC Insights predicts the automotive semiconductor market will grow to more than $18.1 billion (USD) in 2008, up from the $12.5 billion in 2005. Strategy Analytics, another research firm, offers an equally positive view: Today, electronic systems account for more than 20% of a typical car's cost, but that figure will jump to more than 30% by 2008.
Examples include infotainment systems (such as telematics), safety systems, engine management, satellite radio and TV, hands-free cellular phones and other wireless connectivity. Five years ago, these systems were only found in "high-end" European luxury cars. But now, they are integrated into midrange automobiles from every manufacturer, accelerating automotive IC growth even faster.
At the same time, the electronic-component count in these systems increases while the available space continues to shrink, greatly increasing the electronic density of each system. All of these systems require power-conversion ICs, often with multiple voltage rails for each subsystem. Because of the very high power density and relatively high ambient temperatures, any practical heat sinking is too large to be accommodated.
Thus, power-conversion efficiency becomes critical due to space limitations and operating-temperature-range requirements. At low output voltages and even with moderate current levels above a few hundred milliamps, it is no longer practical to simply use a linear regulator to generate these system voltages, because they dissipate too much heat. As a result of these constraints, switching regulators are replacing linear regulators. The benefits of a switching regulator, including the increased efficiency and smaller footprint, outweigh the additional design complexity and EMI considerations.
Load Dump Condition & Cold Crank Conditions
Load Dump is a condition where the battery cables are disconnected while the alternator is charging the battery. This can occur whether a battery cable is loose while the car is operating, or a battery cable breaks while the car is running. An abrupt disconnection of a battery cable can produce transient voltage spikes up to 80V, as the alternator is attempting a full charge of an absent battery (Figure 1).
Transient absorption devices (commonly known as transorbs) on the alternator usually clamp the bus voltage somewhere between 36 V and 60 V, and absorb the majority of the surge current; however dc/dc converters which are down stream of the alternator are subjected to these 36 to 60 V transient spikes. As these converters, and the subsystems they power, are expected to survive (and in some instances regulate an output voltage through this transient event) it is critical that they converters are capable of dealing with these high-voltage transients. There are various protection circuits, usually transorbs, which can be implemented externally, but they add cost and take up valuable space.
Cold Crank is a condition which occurs when a car's engine is subjected to cold or freezing temperatures for a period of time. The engine oil becomes very viscous and requires the starter motor to deliver more torque, which in turn, needs more current from the battery. This large-current load can pull the battery/primary bus voltage below 4.0 V upon ignition, after which it typically returns to a nominal 13.8 V (Figure 1). It is imperative for some applications such as engine control, safety and navigation systems to require a well-regulated output voltage (usually 3.3 V) through the cold crank period, in order to operate seamlessly through this scenario.
Figure 1. 80 V Load-dump condition and cold-crank conditions
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Multiple solutions are available
Depending on output current and the level of transient protection required, vendors such as Linear Technology offer various switching regulators which can operate through both cold crank and load dump scenarios. For example, the LT3508 is a dual-output switching regulator that can withstand transients up to 40 V and cold-crank voltages as low as 3.7 V, yet still regulate output voltages on both channels through these events. This IC delivers up to 1.4 A per channel, either in a thermally enhanced 4 x 4 mm QFN-24 or TSSOP-16 package, offering a very compact and thermally efficient solution; see Figure 2a for schematic and Figure 2b for the efficiency graph. It also offers other features such as user-programmable operating frequency, low VCESAT (300mV @1.4A), a 0.8 V internal reference suitable for low-voltage processors.
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Figures 2a and 2b. LT3508 schematic (upper) and efficiency graph (lower)
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Low supply currents for "always-on" systems
In addition to the previously mentioned Load Dump and Cold Crank requirements, many electronic subsystems also are required to operate in "standby mode", where they draw minimal quiescent current. These circuits are found in most navigation, safety, security, and engine management electronic power systems. Each such subsystem can each use several microprocessors and microcontrollers. (Most luxury cars have between 60 and 100 of these DSPs onboard.)
Most of these systems operate in two different modes. First, when the car is running they generally operate at full current, fed by the battery and charging system. However, when the car ignition is turned off, many of these microprocessors must remain "alive", thereby drawing current from the battery. Since there may be 30 of these always-on processors required for the navigation, security, climate control and engine management systems, there is an ever-increasing power demand on the battery even when the ignition is turned off.
Collectively, several hundreds of milliamps of supply current can be required to power these always-on processor voltages, and this current draw could completely drain a battery in a matter of days. For example, after an extended two-to-three week business trip, a luxury automobile's battery would be unable to crank-over the engine.
Quiescent currents of the supplies for these "not on" units need to be drastically reduced in order to preserve battery life without greatly increasing the size or complexity of the electronic systems. Until recently, the requirement of high input-voltage capability and low quiescent currents were mutually exclusive parameters for a dc/dc converter. If a car's high-voltage step-down converters each require 2-10 milliamps of supply current, and combining several of these with other mandatory, always-on systems such as ABS brakes, leakage current from electronically actuated windows, and a host of other always-on systems can create a significant drain on the battery.
A better solution
In order to better manage these requirements, several automotive manufacturers created a low quiescent current target of 100 μA for each always-on dc/dc converter. Until recently, systems manufacturers were required to connect a low-quiescent-current LDO in parallel with a step-down converter, and switch from this converter to a much-lower-current LDO each time the car was turned off. This created expensive, bulky and relatively inefficient solutions. Another approach is to use a dc/dc converter such as one from Linear Technology's 36-to-60 V input-capable, <100 μa="" step-down="" dc/dc="" converters="" family,="" which="" provides="" a="" much="" more="" compact="" and="" efficient="" solution="" to="" the="" always-on="" problem.="">100>
For example, the LT3480 is a 2 A, 38 V step-down switching regulator with Burst Mode' operation to keep quiescent current under 70 μA (Figure 3).
Figure 3: LT3480 supply current vs. input voltage
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In a typical automotive application, Figure 4, its 3 A internal switch can deliver up to 2 A of continuous output current to voltages as low as 0.79 V.
Figure 4: LT3480: 4.5 V to 38 V to 3.3 V (Transients to 60 V) @ 2 A step-down converter with 70 μA no-load quiescent current
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The LT3480's Burst Mode operation offers ultra-low quiescent current that is well suited for applications such as automotive systems, which demand always-on operation and optimum battery life. It uses a high-efficiency 3A, 0.5 mVCESAT switch, with the necessary boost diode, oscillator, control and logic circuitry integrated into a single die. It can deliver efficiencies as high as 86% with a 12 V input and a 3.3 V output (Figure 5).
Figure 5: LT3480 efficiency vs. load current
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Thermal constraints in an automotive environment
In addition to the caustic electrical environment in automotive applications, the thermal environment can be equally challenging. As more electronics share the same prime real estate in cars, thermal management becomes critical. Under-the-hood applications typically require ambient temperatures of 125 °C and higher, while prime electronics "real estate" such as the navigation/infotainment system and gauges experience thermal challenges since they are both close to the cars firewall with high ambient temperatures, and also have a very high density of electronics.
All electronics dissipate some amount of electrical power through heat. The key in managing heat in power converters starts with maximizing each converter's efficiency thereby minimizing the power lost to heat. This has been one of the driving forces in replacing LDOs with switching regulators in the past few years.
In addition to the efficiency of specific devices, it is also important that each power-conversion device have a very thermally efficient package, to better conduct heat away from the IC. Linear Technology and other vendors achieve this by packaging automotive parts in the most thermally efficient packages available. Leadless packages, such as DFN, MSOP, and TSSOP packages, all use a thermally enhanced design which incorporates a thermal pad at the bottom of the package to reduce thermal resistance by more than a factor of two.
In order to meet the most demanding high temperature applications, such as under-the-hood applications, vendors such Linear Technology offer converters that can operate with junction temperatures of either 140°C or 150°C (depending on the part).
For example; an application running from 12 V and regulating to 5 V, while delivering 1 A of output current, a typical LDO offers only 41% efficiency, dissipating 7 W. This wasted power requires substantial heat sinking to prevent thermal failure, at even 80°C. Conversely, a switching regulator such as the LT3480 would operate at 90% efficiency, dissipating only 0.5 W externally. With thermal resistance (θJA) of 45°C/W for its TSSOP-16E package; this represents a 22.5°C temperature rise, which enables a 102.5°C ambient temperature for an industrial-grade device (125°C) and a 137.5°C ambient temperature for an "H" Grade (extended temperature) device.
The rapid growth of very specialized electronic subsystems in automobiles has created stringent performance requirements for power ICs in automotive applications. Depending on where the power supplies operate on the automotive power bus, they may be subjected to load-dump and cold-crank conditions, as well as high ambient temperatures. Additionally, some of these systems will operate in an always-on, standby mode, driving the need for minimal supply current.
As more electronic systems are added to each vehicle, minimizing the solution footprint while maximizing thermal efficiency is also critical. Fortunately, some power IC designers have created solutions to meet these requirements paving the way for even higher electronic content in future cars.
About the author
Jeff Gruetter is a Product Marketing Engineer at Linear Technology Corporation, Milpitas, CA.