Can I attenuate ripple noise from a R-78E3.3-0.5 with an LC filter?

Question

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:

1. Am I overall on the right track to tackle this problem?
2. 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?
3. Is it common to attenuate low ripple frequencies like my 7.7kHz from a switching regulator?

Answer 1

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?

You can see this from the waveform - it's going into burst mode and the burst repetition rate is 7 kHz. The little spikes on the rising ramp of the voltage are your actual switching waveforms then, when the output voltage reaches the regulation point, given the low current draw, the device shuts down into burst mode operation.

This is fundamentally a problem of that device and your low load current. You should be able to find many designs that can overcome this but, it's less likely you'll find an off-the-shelf device that avoids burst mode on such a low load current.

Is it common to attenuate low ripple frequencies like my 7.7kHz from a switching regulator?

In my opinion I would say not. You design the switching regulator (or choose the device) so that burst mode is avoided.

• Q and A on burst mode for your reference.
• An article on burst mode.