Electronic – What trace length is considered short in case of buck voltage regulator


I'm working on LM2596 based voltage regulator circuit that will be printed on PCB.

According to LM2596 datasheet some traces should be kept as short as possible. However no numbers are given.

This is my design (with ground plane hidden), from left to right:

  • LM2596S voltage regulator
  • SS54 Diode
  • 12.5mm 100uH Inductor
  • 2x Low impedance 300uF capacitor

First attempt:


I'm a little bit worried about C In (capacitor on the right between input voltage and ground) connection with regulator. The trace length is about 35mm.

Next attempt:


Best Answer

Grouping of the LM2596, catch diode, inductor, and Cout aren't too bad. But, Cin is too far from the LM2596 (as you were worried).

With a buck, there are two main signals that need to be minimized:

  • The current loop from Cin through the LM2596 and catch diode needs to be as short as possible. This is a pulsed current, most likely the highest current, and richest in harmonics on the board.

  • The switching node: pin 2 of LM2596 and it's connection to the inductor and catch diode, also needs to be minimal. This voltage signal is very rich in harmonics, and is the primary source of common mode noise.

The easiest way to minimize the input current loop is probably to take the LM2596, catch diode, and Cout as a group and rotate them 180 degrees. This will allow Cin to be much closer to the LM2596.

Minimizing the switching node could be done by rotating the inductor by 90 degrees, ending with the inductor input very close to the cathode of the catch diode.

Currents through the inductor and out to the load aren't such a concern. It's mostly DC with some ripple current (contained by Cout), and not that harmonically rich.

Additionally, it will be desirable to have some input filter inductance to reduce EMI for the rest of your system. Probably a common mode choke, which if you choose well will have enough leakage inductance to also use differentially. (You may already have this, but couldn't see any in the board section shown)