3.3V power supplies

Question

Hi team, 
 i'm using the PLL ADF4382 for a new project. 
 I read in the datasheet and the user guide that the +3.3 V power supplies are divided into two separate groups, each powered by its own dedicated LDO. 
 Would it be possible to use a single LDO to supply both groups? 
 Would any phase noise degradation be expected if a single LDO were used to supply both groups? 
 Thank you in advance. 
 Best Regard, 
 Matteo Ricci

enash · Accepted Answer

Yes, it is possible to power both 3.3 V supply groups of the ADF4382 from a single LDO, but with some important caveats. 
 The two 3.3 V groups are separated to keep sensitive blocks (VCO, output dividers, RF outputs) isolated from potentially noisy digital/phase-detector related rails (e.g., V3_PFD, NDIV). This partitioning is a precaution to prevent switching noise from coupling into the VCO and degrading phase noise. It is not strictly proven that separate regulators are mandatory, but it is the recommended configuration for best performance. 
 If you use a single LDO, you should at minimum:

Use separate filtering/inductor or ferrite bead isolation for the &ldquo;sensitive&rdquo; group (VCO, OUTDIV, RFOUTx) versus the &ldquo;digital/PFD&rdquo; group to limit noise coupling.

Ensure the bead/filters do not introduce more than about 0.3 V drop or violation of rail tracking, since supply differences between rails should stay within about &plusmn;0.3 V of each other.

Phase-noise degradation is possible if noise from the PFD/digital section couples into the VCO supply when everything is tied directly together. As a result we often recommend separate LDOs (or at least well-isolated subrails) for analog, digital, and VCO supplies to minimize this risk. Using one high‑quality LDO plus proper LC/ferrite isolation is often an acceptable compromise, but strictly best-in-class phase noise is more likely with fully separate regulators.)

enash · Answer

There are some dual LDOs available. Take a look at LT3029 for example. It has a surface area of 12mm2. Another option would be MAX25307 . However, you can find some really small single LDOs. Take a look for example at MAX38902, a single LDO with a surface area of only 1mm2.

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3.3V power supplies

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