TPS54821 , TPS54020 / Current Share

Generally speaking you cannot operate devices such as TPS54821 in parallel. there is no provision for accurate current sharing. I have done it before with a similar device (TPS54620), but layout is very tricky and current share is not very accurate. It was more of an academic study than a recommended production. I think TPS54020 is the same although I have not worked much with it.

You could look at a controller such as TPS40170, TPS4030x or for an integrated solution you could try TPS56121.

I've done the same with TPS54020. It does "work," in that they will not "fight" each other for regulation, but as John mentioned, the current sharing is not accurate from part-to-part. These devices were not designed with this type of operation in mind, so the parameters which control current sharing are not necessarily tightly controlled. The issue with poor current sharing is that, theoretically one phase could trip OCP before another, or at the very least, the two devices would not be evenly sharing the thermal stress of increased current.

LMZ31710 is specifically designed for current share. It has an ISHARE pin. You may be able to design so that two TPS54020 are connected in parallel, but is not directly supported.