# High current in the presence of high frequency digital logic circuits

I'm in the process of designing an ARM single-board computer that would also have high current (18A) running through the same board. Think of a BeagleBoard, but powered by a 6V/2800mAh battery. So the raw battery voltage at a possible peak current of 18A is fed to motor outputs, and about 1-2A is fed through a 5V regulator to the logic part of the board.

How close could I route the high-current traces to the logic traces? Would these high-current traces need to be on their own layer? What other design/layout considerations would I need to take into account for something like this?

• 18A isn't the problem (unless your tracks are too thin). The problem is how is the 18A changing - changes in current (${di}/{dt}$) induce voltages into other tracks. Jul 30 '13 at 7:52

## 1 Answer

Distance between noisy power lines and sensitive signal tracks depend on the strength and spectrum of the emitted noise (not the DC current) as well as the impedance and noise margin (difference between 0 and 1 voltages) of the target signals. There is no exact solution, it is an empirical problem. I can give you some general rules: 1 - keep noisy and sensitive traces as apart as possible and avoid running them parallel for long distances (ideally crossing directions) 2 - Have at least one ground plane between power, noisy signals and sensitive ones. If this is not possible, have a ground guard as wide as possible between them 3 - improve your design to minimize HF currents and minimize emissions 4 - strengthen your sensitive lines with lower impedance drivers and higher noise margin.

An additional design consideration is the thermal one. 18 amps needs already some mm wide traces. Google for "trace width calculator" or similar and use one of them. When changing layers, use more then 1 via. It is different for inner and outer layers and it depends on the ambient temperature (the hotter the wider the trace). To be in the safe side, be generous with the result and increase it by a 10 - 20%. Your PCB life will be longer.