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. enter image description here

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.

enter image description here

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. enter image description here

Any ideas on what to check? Thanks!


Your feedback op-amp U1 is only going to work correctly if its input voltages are between its supply rails. That is, 0 to 12 V.

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.

Try using a higher voltage to power U1 (which might require choosing an op-amp that allows higher voltage power supplies if you want to build this circuit in the real world).

  • 3
    \$\begingroup\$ Alternatively, move to low side current sensing, if the application permits. \$\endgroup\$ Feb 26 '20 at 17:47
  • \$\begingroup\$ Replace the opamp too because you’ve majorly exceeded its absolute maximum ratings. \$\endgroup\$
    – Andy aka
    Feb 26 '20 at 17:51
  • \$\begingroup\$ @Andyaka, A simulated op-amp usually doesn't fail just from that. But they are cheap to replace. \$\endgroup\$
    – The Photon
    Feb 26 '20 at 17:51
  • 2
    \$\begingroup\$ Trouble is I have a really good sim and have to download a new model each time this occurs. \$\endgroup\$
    – Andy aka
    Feb 26 '20 at 17:53
  • \$\begingroup\$ @ThePhoton I was so focused on the differential input value, I didn't consider that the common mode of the input was above the supply rail. Thanks! \$\endgroup\$
    – Aaron
    Feb 26 '20 at 17:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.