The scope should have a test point you can use which produces a 1khz square wave or something (marked on the front). Use that to get started. I know almost nothing about scopes and can hardly use my own surplus scope.
The buck controller senses current [by measuring voltage drop over the sense resistor bank] and turns the FET on and off accordingly (and rapidly). At high input voltages the FET will be off a lot of the time (think 50% if input is double output). This is exactly what we want for regulation; other parts of the buck circuit ensure that voltage never goes too high - we don’t need to talk about how but it’s related to the rapid switching of the FET. A PWM input will tell the buck controller to keep the FET off even more in order to give lower-than-maximum modes. Keeping the FET at 100% duty cycle (always on) is direct drive. If it’s not turning on and off rapidly then maximum voltage will be allowed on the output. In a buck circuit the FET should never be at 100% duty cycle. [100% duty cycle is fine on the PWM input of the buck controller though, since this just tells it “don’t turn off any more than you normally would to maintain the current you are supposed to”]
The point of the above is to explain that the buck controller itself is in charge of regulation. If you bypass it that is bad.
I also have no formal electronics training, so you can only wave that flag at me so much ;-). I’m still happy to help.
Let’s get down to business:
- You never posted a clear pic of what’s under the MCU (traces). That might be helpful or it might not.
- In post #29 you say ‘clockwise’, but we mean counter-clockwise, correct? (If not post a drawing to help please.)
- In post #29 what mode is the driver set to?
- Let’s re-check pad voltages without the MCU installed. Please record them so that we may compare with your results from post #29. This is what I really feel like we need to see right now.
- If you haven’t gotten a handle on your scope yet, maybe try this: If you have a working stock driver hooked up, please power it up and set it to medium. Do a quick check with your DMM and look for any pins which seem to have a significantly different voltage from the values recorded in post #29. [If the driver was set to medium for your results in post #29 then recheck with the driver set to High this time and look for pins that show a different voltage.]
FYI: In my last suggestion we are not looking for a true/accurate voltage. The DMM will probably show something inaccurate, but hopefully different, for different PWM duty cycles.
