Low EMI automotive power designs: Page 6 of 7

February 05, 2015 //By Jeff Gruetter
Low EMI automotive power designs
Jeff Gruetter, Product Marketing Engineer, Linear Technology Corporation considers low EMI automotive power design issues.
rectification eliminates the need for any external diodes, improving efficiency while simplifying the solution footprint. This particular schematic switches at a 1MHz switching frequency utilizing a 3.3µH inductor, delivering efficiency of 96%. However, as can be seen in Figure 5, 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 unique design, which minimizes switching losses enabling it to deliver this high efficiency at switching frequencies of 2MHz or higher.

Figure 4. LT8640 Typical Automotive Schematic for a 5V Output
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Figure 5. 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 unique 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 optimize efficiency while avoiding critical noise-sensitive frequency bands. The LT8640’s 20-lead 3mm x 4mm QFN package and high switching frequency keeps external inductors and capacitors small, providing a compact, thermally efficient footprint.


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. However, the LT8640’s unique design offers

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