Electronic – Cleaning up a 5V Switching DC-DC converter for low noise 5V output

powerpower supplyswitch-mode-power-supply

I have a project that I have to hook to a 5V bus coming from this DC-DC switching converter.

I need to clean this up to use with a logamp (LOG114), photodiode, and ADC (ADS8519). I would also like this circuit to work on future projects with similar power supplies.

Best Answer

You cannot avoid ripple on the output of a switching regulator. You can somewhat minimize it, and you can shift it to a different frequency, but you cannot eliminate it.

A linear regulator has no ripple on the output. There is noise on the output, but not ripple in the same sense as a switching regulator. Of course, linear regulators have their own issues like terrible efficiency.

When I need something like relatively high efficiency AND super low noise I will use both a switching regulator and a low-noise, low-dropout linear regulator. For a +5v output I will set up the switcher for a +6v output, and then use the linear regulator to bring that down to a clean 5v.

Note: not all LDO linear regulators are created equal. Some have lower noise than others and some have lower voltage drops. Pick wisely. Sorry, I don't have part numbers handy.

Some people would use additional filtering on the output of the switcher instead of a linear regulator, but I find that such a filter isn't worth it. Those filters are based on an LC topology and require large inductors and caps to be effective. A linear regulator can be smaller and cheaper than a filter (but somewhat less power efficient).

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