MCU controllable switch regulator

[Philfreeze]

Hi guys
I am new here and I hope this is not a repost, if it is then you can just link to the old post and I will read it. I don't mind reading.

I am currently planning a project where the goal is to make a portable programmer and mini PSU for different other projects.
For this I would like to have a controllable output voltage which would allow me to use it for 5V and 3.3V projects (maybe 2.5 and 1.8V projects down the road, you never know...).
These are the specs I would like to achieve:
Vout from 1.8V to 12V regulated.
Iout @ 5V should be arround 1A
Vin can vary from arround 3V up to 5.5V

I have evaluated two differnt solutions. One is to directly control a buck-boost converter with the MCU and the other is to build a regulator with a normal chip and then bias the feedback pin with a DAC which is controlled by my MCU.
I really can't decide which of this solutions would be better. I tend to try the first one because it is cheaper and I have full control over the regulater rather than just passively changing the output voltage.

Which one would you choose and why?


If you need additional information just ask and I will provide it.

[mariush]

Another simple way would be to use a digital potentiometer instead of a resistor in the voltage regulator's feedback.

There's plenty of regulators that can be controlled through i2c protocol, just search google for "i2c controlled switching regulator".  Linear, Micrel, Semtech are just a few that have them.

To get such wide range of output voltages you'd probably be looking at SEPIC configuration (buck and boost) not just buck regulator, and that makes it a bit more complicated.

[ajb]

Using a dedicated controller and biasing its feedback would be simpler to get right.  Directly controlling the switching stage(s) of a buck-boost converter with an MCU is certainly doable, but will take some care to create a stable and robust solution.  A dedicated controller IC pretty much has that taken care of. 

So I'd say unless you're interested in building an MCU-based switching converter as a project unto itself, just use a controller IC.

[Philfreeze]

Thanks you both.
I have a SEPIC or a buck-boost configuration planed. SEPIC would be easier to control with a MCU but the buck-boost is more common, I think.
The digital potentiometer is a nice idea, I will have to look into the precision and stability of it but it sounds good.

Since you both recommend to change the feedback I will go for this solution first, thanks!

[0xPIT]

Maybe you can try a MAX1620, it's a programmable LCD bias controllers, which is basically a switch mode controller.