In my circuit, I use a R-78E3.3-0.5 to generate the 3.3V supply voltage for a microcontroller from a 24V rail. The total load draws about 10mA. I'm using a 47uF decoupling capacitor at the regulator output and some smaller decoupling capacitors at the supply pins of the uC which already reduce the noise quite a bit.
There is still a significant amount of noise left which I would like to further reduce and thought about using a second stage LC filter for that purpose (a first stage LC filter is included in the R-78E3.3-0.5 if I understand correctly.)
I'm getting a switching noise at 570kHz (consistent with the R-78E3.3-0.5 datasheet) and a ripple noise at around 7.7kHz as can be seen in my oscilloscope measurement. Channel 1 (yellow) is the same signal measured at full range.
All the instructions I can find talk about attenuating the switching noise with a LC filter set at a cutoff frequency of around 1/10 the switching frequency which would be 57kHz in my case. That would surely help with some of the noise but the ripple at 7.7kHz would be largely untouched, right? An LC filter with a cutoff frequency of 1/10 * 7.7kHz would require a really large inductor.
So my questions are:
- Am I overall on the right track to tackle this problem?
- Is perhaps the R-78E3.3-0.5 a bad choice in my application because I only draw 10mA of its potential 500mA? Would the R-78E3.3-0.5 produce a higher (and thus easier to attenuate) ripple frequency in an application with a higher load?
- Is it common to attenuate low ripple frequencies like my 7.7kHz from a switching regulator?