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Addressing low EMI automotive power design issues

Posted: 16 Feb 2015     Print Version  Bookmark and Share

Keywords:EMI  power ICs  electromagnetic interference  LT8640  power conversion 

The schematic for the LT8640 is shown in figure 3. Synchronous rectification eliminates the need for any external diodes, improving efficiency while simplifying the solution footprint. This particular schematic switches at a 1MHz switching frequency utilising a 3.3µH inductor, delivering efficiency of 96%. However, as can be seen in figure 4, running the LT8640 at 2MHz avoids any interference concerns with the AM radio band, and enables the use of a smaller 2.2µH inductor while still delivering 95% efficiency. The LT8640 uses a design touted to minimise switching losses enabling it to deliver this high efficiency at switching frequencies of 2MHz or higher.

Figure 3: LT8640 Typical Automotive Schematic for a 5V Output Sponsor video, mouseover for sound.

Figure 4: LT8640 Efficiency Graph of Figure 4 Circuit at 1MHz, 2MHz & 3MHz.

The LT8640's 3.4V to 42V input voltage range makes it ideal for automotive and industrial applications. The internal high efficiency switches deliver up to 5A of continuous output current and peak loads of 7A to voltages as low as 0.97V. Its Burst Mode operation offers only 2.5µA of quiescent current, making it well suited for applications such as automotive always-on systems, which need to extend operating battery life.

The LT8640's design maintains a minimum dropout voltage of only 100mV (@1A) under all conditions, enabling it to excel in scenarios such as automotive cold-crank. Furthermore, a fast minimum on time of only 40ns enables 2MHz constant frequency switching from a 16V input to a 1.5V output, enabling designers to optimise efficiency while avoiding critical noise-sensitive frequency bands. The LT8640 comes in a 20-lead 3mm x 4mm QFN package.

Conclusion
The rapid growth of extremely complex electronic systems in automobiles has created even higher demands on power management ICs. In the past, high load currents, fast switching frequencies and high efficiency designs collectively created big EMI challenges. Fortunately, a new generation of synchronous power ICs is now available to pave the way for even higher electronic content in future vehicles.

About the author
Jeff Gruetter is Product Marketing Engineer at Linear Technology Corp.


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