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Working on Adj Current/Voltage SMPS Lab PSU. Questions...


Hi everyone,

I'm kind of a newbie so if I sound like a moron, that's probably why.

I'd like to design my own switchmode bench supply that can be configured to have an adjustable current limit and adjustable output voltage.

I like National's Simple Switcher chips, as I've used an LM2576-5.0V once before and it worked very well.

However, I need a simple way to do this and I don't want to use an L200c for that would mean huge transformer, huge heatsinks, and a big pricetag. At which point a commercial PSU is probably the best option. I figure that I can build my own for quite cheap if I use a switching design and have something that I can trust.

So I was going to try to use a LM2576-Adj which National has a nice 1-30V adjustable schematic for in the datasheet. But I'm too stupid to figure out how to do adjustable current limit. I know you'd probably need some op-amps or something, but really don't have much of an idea of how to do it correctly.

Then I saw the LM2673 which is called "3A Step-Down Voltage Regulator with Adjustable Current Limit", but it SEEMS that in the datasheet, that you can't have a very wide output voltage range due to something with inductors and capacitors and resonance they have with eachother. However, I don't know that for sure. LM2673 also switches at 260KHz which means I have more places to screw up my layout.

So I have a few questions. Does anyone have a decent solution to adding adjustable current limit to the LM2576? If not, how about getting a full range of voltage adjustment on the LM2673? If not, do you have any other ideas for this? Other chips/designs etc?

Thank you.

Generally, adjustable current limit is done independently of the chip. Voltage too.

You just measure both voltage and current using opamp-based circuits, compare them to references and provide a feedback to the regulator. The regulator may be SMPS or linear, doesn't matter.

Take a look at power converters intended for charging batteries, since those need to control both voltage and current.


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