DC/DC Converter Selection Guide for Car Navigation Systems[Copy link]
The automotive environment presents harsh conditions for electronic systems, with wide operating voltage requirements, large transient voltages, and large temperature drifts. This article describes how to design multiple supply voltages to meet the requirements of different parts of automotive electronic systems as performance requirements become increasingly stringent.
Most mid- to high-end cars produced today are equipped with DVD-based GPS navigation systems as standard equipment (Figure 1). However, it can be proven that designing a power supply to control the different voltage rails in such a system is no less complex than designing a power supply system for a laptop computer. A standard automotive navigation system may have six or more power supplies, including 8V, 5V, 3.3V, 2.5V, 1.5V, and 1.2V. The 8V supply is used to power the DVD motor that rotates the disc; this often requires up to 2A of peak current. The 5V and 3.3V rails are usually system buses and are generally required to provide 2A to 3A of current each. The 2.5V rail is used for memory and I/O, so it is sufficient to deliver 1A to 2A of current. The 1.5V and 1.2V power rails are used to provide the CPU core and DSP core voltages respectively. The power levels of these two power rails are generally between 3W and 5W.
At the same time, as the number of components in these systems increases, available space becomes increasingly limited. Therefore, the importance of conversion efficiency becomes more prominent due to space constraints and operating temperature range requirements, as all practical heat sinks are too large to be easily installed. At low output voltages and moderate current levels above a few hundred mA, it is no longer feasible to simply use a linear regulator to generate these system voltages. Therefore, over the past few years, switching regulators have been gradually replacing linear regulators, mainly due to thermal limitations. The advantages of switching power supplies include higher efficiency and a smaller footprint, which makes the increase in complexity and EMI issues less important.
There are many applications in automotive systems that require continuous power even when the vehicle is at a standstill. A key requirement for these applications is low quiescent current. The device will operate in normal continuous switching mode until the output current drops below approximately 100mA. Below this current level, the switching regulator must skip several pulses to maintain regulation. The regulator can enter sleep mode between pulses, where only some of the internal circuitry is powered. Under light load current conditions, the switching regulator needs to automatically switch to Burst Mode operation. In this mode, the quiescent current should drop below 100μA for a 12V to 3.3V converter. In sleep mode, the internal reference and power good circuits remain operational to monitor the output voltage. In shutdown mode, the quiescent current should be less than 1μA.
4. Low Thermal Resistance Ideally, the junction-to-case thermal resistance should be low. If the back of the device is exposed copper and soldered to the surface of the PC board, the PC board can be used to conduct heat away from the device. A common four-layer board with an internal power plane can achieve a thermal resistance of about 40°C/W. High ambient temperature applications with good thermal conduction to the metal case can achieve typical junction-to-case thermal resistance values close to 10°C/W. This helps to extend the operating temperature range.
5. Considerations on Noise and EMI
Although switching regulators generate more noise than linear regulators, they are much more efficient than the latter. It has been proven in many sensitive applications that noise and EMI levels can be controlled as long as the switching power supply operates in a predictable manner. If the switching regulator switches at a constant frequency in normal mode and the switching pulse edges are clean and predictable (no overshoot or high-frequency ringing), EMI will be minimized. The use of small packages and high operating frequencies allows for a compact layout, which can minimize EMI radiation. In addition, if the regulator can be used with low-ESR ceramic capacitors, input and output ripple (which are additional noise sources in the system) can be minimized.
Obviously, the design and development of such switching regulators is not simple. However, Linear Technology has been working on this high-voltage DC/DC converter over the past few years and has a growing portfolio of products designed to meet these requirements (Table 1).
The LT3434 is a recent example of such a DC/DC converter, part of a growing family of monolithic step-down switching regulators that can handle 60V. This device solves many of the problems faced by the above-mentioned automotive navigation applications. The LT3434 operates over a wide input voltage range of 3.3V to 60V (Figure 2). It provides high efficiency at load currents up to 2.5A. The reference accuracy is ±2% over all line, load, and temperature conditions.
Because of the device's Burst Mode operation, its quiescent current is less than 100μA for 12V to 3.3V applications. The device is packaged in a small, flat TSSOP package with very low thermal resistance to achieve a small footprint. Finally, it uses a current mode topology for good transient response and easy compensation, and employs a patented circuit for maintaining a constant peak switch current at all duty cycle conditions. The switching frequency is a constant 200kHz, and the device can be synchronized to a higher frequency. It provides tight voltage regulation over the automotive temperature range, and features Power Good/Reset, soft start, and UVLO (undervoltage lockout). At current levels up to 2.5A, the circuit provides a rugged, efficient, small footprint solution.
Summary
Although the design of automotive navigation systems is complex and requires high performance from their analog DC/DC converters, there is no need to panic. Companies such as Linear Technology are introducing a "new wave" of regulators that combine many of the key features necessary to reduce the workload and harshness of power supply design for system engineers.
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