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Recently I came across several 4-pin LM25xx DC-DC adjustable power converter modules such as the ones below.

They look like this:

DC-DC adjustable step up power converter module from HobbyKing.com

I'm only familiar with similar 3-pin LM25xx modules that only have one ground connection, such as these.

They look like this:

Murata's 78xxSR Series

I searched the Net trying to find a datasheet for any of the 4 pin modules, and found none. Unlike the 3-pin versions that have plenty of good quality technical information in their datasheets.

So, my questions are:

  1. What are those IN- and OUT- pins for?

  2. Can I just short IN- and OUT- to ground and use the 4 pin modules like the 3-pin versions?

The context for my question is this other question (which is currently on-hold) and the fact that I want to design one of such modules myself and wanted to broaden my options.

Also, I realize that the 4-pin modules are adjustable while the 3-pin modules are fixed output. But I don't understand what that has to do with the differences in the number of negative pins. From the application notes from the adjustable versions of LM25XX, such as below, I cannot imagine how IN- and OUT- pins would be wired up. To me, the only option is to provide positive input and output and a connection to ground.

LM2577-ADJ Application Note schematics

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up vote 4 down vote accepted

Can I just short IN- and OUT- to ground and use the 4 pin modules like the 3-pin versions?

It's 100% that it's already shorted on the PCB. So you don't have to short it again.

What are those IN- and OUT- pins for?

Well, probably it's there so that you can connect the input and the output wires there. In many cases, the input circuit and the output circuit are separate circuits so their negative poles should be connected somewhere. Those pins can be used for this.

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Note in Isolated DC-DC converters the In- and OUT- are not connected together. Very useful for isolated circuits or stacking voltage rails . – Spoon Feb 28 '14 at 12:35

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