XC2787X Power Supply Filtering

[Caar]

Hello,

I have been asked to assess a customer's PCB design for the XC2787X. During this discussion, a question has arisen is regarding a recommendation in the applicaiton note AP16116 titled "Design Guidelines for XC2000 & XE166 Microcontroller Board Layout.

This application note recommends the use of a 5~10uF inductor in the supply path at the regulator. In the PCB design, my customer is only able to place an 0603 inductor which is too small to handle all of the current from the regulator to the circuit on the 5V rail. It would be very difficult for them to place the inductor to supply the microcontroller only.

What is the main purpose of this inductor? Is it for conducted emissions or is it for immunity/susceptibility? If the inductor is for immunity/susceptibiility, what is the problem which this inductor addresses?

"Inductance/ferrite beads in the range L ~5-10µH should be inserted in the supply paths at the regulator output." - Page 7 of AP16116.


Regards,

Chris

[Juergen]

Inductors are good low pass filters and are used to suppress conducted high frequency interference. This is mainly for immunity/susceptibility and helps to prevent wrong operation of the microcontroller. In a noisy environment this is essential because high frequency spikes can travel along the supply lines and have unpredictable effects on any logic devices that are on these lines, including microcontrollers. It is not unheard of that high frequency noise can corrupt program counters and cause microcontrollers to execute from random code memory. This is a problem with all microcontrollers. Unfortunately, the more current the circuit needs, the larger the inductors become. For that reason it becomes important to evaluate what part of the circuit needs to have a clean supply, and place the inductor strategically so that the minimum size can be used. The part of the circuit that is not noise sensitive can be supplied separately from the other side of the inductor. Best is still to make sure that any noise generating switching is suppressed at source to prevent propagation of interference.