# SMPS Constant Current Source only works with low resistance loads

I'm looking for any advice that can be given to solve this issue. I followed the design idea from here: buck regulator to CC source and it appears to work pretty well until I started raising the load resistance. 1Ω-2Ω works great, above 2Ω the current starts to fall off.

I first thought I was running out of head room, but with a 45V input the output voltage is only reaching about 30V. I would think that the output voltage should be able to get much closer to the input voltage.

The target Vref is 0.97V for this chip, and it's doing a pretty good job of keeping that until the load resistance goes up.

Any ideas on what to check? Thanks!

If you try to push 4.5 A into a 4 ohm load, you'll have the output voltage at 18 V , so the U1 inputs will be at about $$\\frac{21}{22}(18\ V)\$$ and U1 will stop working correctly.