LT8614 silent switcher reduces EMI without drawbacks

Article By : Christian Kueck

Linear Technology's LT8614 has low minimum on-time of 30ns that enables large step-down ratios even at high switching frequencies.

Switching regulators replace linear regulators in areas where low heat dissipation and efficiency are valued. The switching regulator is typically the first active component on the input power bus line, thus has a significant impact on the EMI performance of the complete converter circuit.

Modern input filter components in surface mount technology have better performance than through-hole parts. However, this improvement is outpaced by the increase in operating frequencies of switching regulators. Higher efficiency, low minimum on- and off-times result in higher harmonic content due to the faster switch transitions.

For every doubling in switching frequency the EMI becomes 6dB worse when all other parameters, such as switch capacity and transition times, remain constant. Savvy PCB designers will make the hot loops small and use shielding GND layers as close to the active layer as possible; nevertheless pinout, package construction, thermal design requirements and package sizes needed for adequate energy storage in decoupling components dictate a certain minimum hot loop size.

To make layout even more challenging, on a typical planar PCB the magnetic or transformer style coupling between traces above 30MHz will diminish all filter efforts, since the higher the harmonic frequencies are, the more effective unwanted magnetic coupling becomes.

The tried and true solution is to use a shielding box for the complete circuit. Of course, this adds costs, increases required board space, makes thermal management and testing more difficult, and introduces additional assembly costs. Another frequently used method is to slow down the switching edges. This has the undesired effect of reducing the efficiency, and increasing minimum on-, off-times, as well as the required dead times and compromises the potential current control loop speed.

With Linear’s LT8614 silent switcher regulator, you have the effect of a shielded box without using a shield and also eliminate the drawbacks. The LT8614’s low minimum on-time of 30ns enables large step-down ratios even at high switching frequencies. As a result, it can supply logic core voltages with a single step-down from inputs up to 42V.

In conclusion, the LT8614 silent switcher regulator reduces EMI from current state-of-the-art switching regulators by more than 20dB, while increasing conversion efficiencies with no drawbacks. A 10x improvement of EMI in the frequency range above 30MHz is attained without compromising minimum on- and off-times or efficiency in the same board area. This is accomplished with no special components or shielding, representing a significant breakthrough in switching regulator design.

Learn more: Download the full application note

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