Buck converter not outputting correctly unless input dips below ~7V

[cbc02009]

Hello,

I got my board in for the MPPT controller I'm building, and I got all the parts on it, and for the most part it works perfectly, but I am having a very strange issue with it.

For any input voltage higher than around 7V, the buck converter initially outputs .6V no matter what the input is, but then if I dip the input below 7V, even for just a fraction of a second, something kicks in and it just starts working like it's supposed to, even if I raise the voltage back up above 7V.

My gut is telling me that it's a problem with high-side mosfet not getting boosted correctly. Maybe the boost capacitor can't charge quick enough?

The picture of my schematic is attached, as well as the datasheets for the mosfets and buck controller.


Any advice on how to isolate the problem would be greatly appreciated.

Thanks!

[killcode]

Your bootstrap cap is lower than the datasheet typical application - did you calculate the required size for your particular switching freq and fet gate charge?

Maybe try some decoupling on vcc as well.

[cbc02009]

Your bootstrap cap is lower than the datasheet typical application - did you calculate the required size for your particular switching freq and fet gate charge?

Maybe try some decoupling on vcc as well.

So I finally got into school to use the scope, and you are absolutely correct with your second suggestion. This is what my clean Vcc looks like after I hook it up to my circuit:

Now I just need to figure out how to modify my board to add decoupling. I can't believe I missed it on the original schematic.

[cbc02009]

And one bodged ceramic capacitor later, everything is working as it should. Thanks!

[killcode]

Glad I could help.
When in doubt, sprinkle a little 100nF pixie dust around.