Modern cars are constantly adding increasingly complex electronic systems. Market research firm Allied Business Intelligence predicts that by 2007, the automotive semiconductor market will grow to more than $17 billion a year, compared to $12.3 billion last year. Another market research firm, Strategy Analytics, also holds the same optimistic view that in general cars, electronic systems cost more than 20% of total costs, but by 2008, this proportion will grow to over 30%. Anti-collision radars, adaptive cruise control, tire pressure monitoring, navigation systems, hands-free cellular telephones and other wireless connections, and biometric access systems are all examples of these electronic systems.
However, one area that has grown tremendously is the DVD/HDD-based navigation system. This type of system was launched in 1997 and is expected to sell more than 13 million units worldwide in 2005. For any type of electronic product, the automotive application environment has always been very harsh, which is not surprising. Wide operating voltage range requirements coupled with high transient voltages and large temperature variations present a significant problem for electronic systems. More seriously, performance requirements continue to increase, and different parts of the system require multiple different supply voltages.
Most mid- to high-end cars produced today offer DVD-based GPS navigation systems as standard equipment. However, designing a power supply that handles all of the different voltage rails in such systems can be as complex as designing a power system for a notebook PC. A typical navigation system may have six or more different power supplies, including 8V, 5V, 3.3V, 2.5V, 1.5V, and 1.2V power supplies. The 8V is used to power a DVD motor that spins a disc, which typically requires up to 2A of peak current. The 5V and 3.3V rails are typically used on the system bus and typically require 2A to 3A of current, respectively. Both the memory and I/O require a 2.5V rail, so 1A ~ 2A is sufficient. The 1.5V and 1.2V power rails are used to power the CPU core and DSP core, respectively. The power of these two rails is usually 3W~5W. Figure 1 shows the interior of a car with a built-in navigation system.
At the same time, as the number of components in these systems increases, the available space is getting smaller and smaller. Therefore, since any actual cooling system is too large to fit easily into an electronic system, conversion efficiency becomes more critical under the preconditions of space limitations and operating temperature range. It is no longer practical to simply generate these system voltages with a linear regulator at low output voltages and even medium currents above a few hundred milliamps. Therefore, in the past few years, switching regulators have been continually replacing linear regulators primarily due to thermal limitations. The benefits of switching include increased efficiency and reduced footprint, which is more beneficial than the additional complexity and EMI issues.
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With these constraints in mind, when a switching regulator is used in a car navigation system, it needs to have the following characteristics:
â— Wide input working range
â— High efficiency over a wide load range
â— Low quiescent current during normal operation, standby and shutdown
â— Low thermal resistance
â— Lowest noise and EMI emissions
â— Load dump damage resistance
Let's consider these important features one by one in more detail.
Wide input working range
Any switching regulators are required to operate over a wide input voltage range of 3V to 60V to ensure that the requirements of "cold car start" and "load dump" are met. The wide input voltage range has an added benefit: allowing these automotive systems to operate at 14V or 42V. Moreover, the 60V rated 14V system provides a good margin, and this system is usually clamped in the range of 36V~40V.
effectiveness
In most automotive systems, high efficiency power conversion over a wide load range is extremely important. For example, in the load range of 10 mA to 2.5 A, the desired power conversion efficiency of the 5V output is approximately 85%. At high currents, the internal switch needs to have good saturation, typically 0.1Ω at 3A. To increase efficiency at light loads, reduce the drive current or let the drive current be proportional to the load current. The internal control circuitry can also be powered by the bias pin, which can be powered by the output. This takes advantage of the power conversion efficiency of the buck converter. In fact, this bias current is obtained from the output rather than the input, which reduces the input supply current required by the control circuit in terms of output to input voltage ratio. For example, an output current of 100mA at 3.3V requires only an average of 30mA of input current at 12V. This minimizes the input current required by the control circuitry and increases efficiency at light loads.
Low quiescent current
There are many automotive system applications that require continuous power, even after the car is parked. A key requirement for these applications is low quiescent current. This application will always operate in the normal continuous switching mode until the output current drops below approximately 100 mA. Below this current value, the switching regulator must pulse jump to maintain a stable voltage. The regulator can enter sleep mode between pulses, in which only some of the internal circuitry is powered. At light load currents, the switching regulator needs to automatically switch to burst mode operation. For a 12V to 3.3V converter, the quiescent current should be reduced to less than 100mA in this mode. The internal reference and power good circuits will remain operational during sleep mode to monitor the output voltage. The quiescent current should be less than 1 mA at shutdown.
Low thermal resistance
Ideally, the node to case thermal resistance should be low. If the underside of the device is exposed to copper and soldered to the surface of the PC board, the PC board conducts heat away from the device. A 4-layer board with an internal power plane is now used to achieve thermal resistance in the 40°C/W range. High ambient temperature applications that have good conduction to metal casing heat can achieve thermal resistances approaching typical node-to-case thermal resistance of 10 °C/W. This helps to expand the useful operating temperature range.
Noise and EMI radiation problems
Although switching regulators generate more noise than linear regulators, switching regulators are much more efficient. It has now proven that noise and EMI are manageable in many sensitive applications, as long as the switch works as expected. If a switching regulator is switched at a constant frequency during normal operation, the switching edges are clean and predictable, and there is no overshoot or high frequency ringing to minimize EMI. The small package size and high operating frequency allow for a small and compact layout, which also minimizes EMI emissions. In addition, if the regulator can use low-ESR ceramic capacitors, both input and output voltage ripple can be minimized, and this ripple is an additional source of noise in the system.
Ability to not be damaged when the load is abrupt
"Load dump" refers to the situation in which the battery cable is disconnected while the alternator is charging the battery. Figure 2 shows a typical current pulse caused by a load dump. This sudden disconnection can produce transient voltage spikes of up to 60V because the alternator attempts to fully charge the battery. The transient voltage suppressor on the alternator typically "clamps" the bus voltage between 36V and 60V, thus allowing the downstream mains DC/DC converter of the alternator to withstand a 60V transient "spike." Since it is desirable for these converters and the subsystems powered by these converters to function properly during and after transient events, it is critical that the DC/DC converter be able to handle such high transient voltages. There are a variety of "protective" circuits (usually transient voltage suppressors) that can be implemented externally, but these circuits add cost and require valuable board space. Linear Technology's high voltage switching regulators are capable of withstanding transient voltages up to 60V while maintaining output stability without compromising system performance or reliability. In general, these regulators are buck or buck-boost converters that can withstand voltage replacement of automotive and alternators caused by charge/battery system interruptions.
â— The load dump transient is a voltage spike caused by an accidental disconnection of the battery being charged.
â— The load dump transient specification is 36V ~ 60V, but it depends on which generation of power supply is used and the control method used.
â— If unchecked, load dump transients can damage many electronic components on the power bus.
Automotive subsystems with inherent high voltage transients and high efficiency requirements place ever-increasing demands on power supply design. These power supplies must provide high power, high efficiency, and low noise over a very compact footprint, and must maintain high efficiency over a wide input voltage range. Some DC/DC converter solutions meet these requirements at high input voltages, but they do not maintain high efficiency at low input voltages. Many of these converters have frequency compensation circuitry that requires bulky input and output capacitors, which not only increases the overall size of the solution, but also results in high output ripple voltage.
Linear Technology's recently introduced LT3434 DC/DC converter solves many of the key issues in automotive navigation applications described above. It is part of an expanding family of 60V monolithic step-down switching regulators. The LT3434 operates over a wide input voltage range of 3.3V to 60V, see Figure 3. It is still highly efficient at load currents up to 2.5A. The baseline accuracy is ±2% for all voltage, load and temperature conditions.
Due to its ability to operate in Burst Mode, its quiescent current is less than 100uA for 12V to 3.3V applications. The device is available in a small, flat TSSOP package with very low thermal resistance, allowing for a small footprint. Finally, it uses a current mode topology to achieve good transient response and easy compensation, and a patented circuit to maintain a constant peak switching current at all duty cycles. The switching frequency is constant at 200kHz and the device can also be synchronized to higher frequencies. It provides a tight voltage across the automotive temperature range with power good/reset, soft start and UVLO capabilities. The IC can be used for automotive step-down applications with currents up to 2.5A and a rugged, efficient and small footprint solution.
in conclusion
Although car navigation systems are very complex in design and require high performance analog DC/DC converters, there is no need to panic. Vendors like Linear Technology are introducing a number of "new" regulators that combine a number of key features that make power supply design less of a headache for system engineers.
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