Power Supply Filtering

[notsam]

Hi,

We have a new design engineer who is complaining about our old designs - specifically power filtering.  We now have the 5V power going to a 10uh inductor followed by a 10uF, 0.1uf and 0.01uf cap.  He is concerned that there will be a surge of current from the microprocessor causing ringing and wants to add a 1 ohm resistor in series with a 39uF cap across the 5 volts after the inductor.  This  would be repeated for all three voltages - VDDX, VDDR, and VDDA.  Is this necessary?  Thanks

[bigmac]

Hello, and welcome to the forum.

The presence of the filter inductor is presumably to help reduce external high frequency noise from reaching the MCU.  What would seem a better arrangement would would be to connect the 10u "bulk" capacitor at the output of the 5 volt regulator, rather than at the MCU pin.  This would mean that the potential ringing frequency would increase from about 16kHz, to 160kHz.  For a given MCU load, the effective Q of the parallel resonant circuit will reduce by a factor of 10.

Any tendency to ring should occur when the MCU changes from normal operation, to a low power mode.  If an unacceptable level of ringing does occur under this circumstance you might try a moderately low value resistor, say 100 ohms, connected in parallel with the inductor.

Regards,

Mac

[notsam]

Thanks Mac,  appreciate the advice.  The 5 volts is distributed to this board and others from a power supply.  I have a 4.7 uf cap at the connector.  I'm concerned about the addition of the 1ohm resistor in series with 39uf.  I just don't see the necessity.  I have never seen any unacceptable level of ringing on these lines before the addition of this circuit.

[bigmac]

Hello,

The need for the series resistor, in theory at least, is to reduce the resonant circuit Q factor, so that ringing does not occur.  However, depending on the capacitor type for the 10u or 39u, the "equivalent series resistance" ESR intrinsic to the capacitor may well be more than sufficient.  Aluminium electrolytics tend to have higher ESR than tantalum types, but check the datasheets.

However, with a "bulk" capacitor already present at the 5 volt input to the board, it would seem that neither the 10u nor the 39u capacitors would achieve much at all, and could be omitted.  The one exception might be the reference input for the ADC (assuming this module is in use), retaining the 10u only, for reduction of lower frequency noise.  This input would not be subject to wide current fluctuations for ringing to ever be an issue.

Regards,

Mac