To get a stable current control loop, two circuit modification are necessary
- add a dominant pole, e.g. low-pass between U3.1 and U8A.3
- add Riso of a few 100 ohms between OP output and MOSFET gate, as required according to OP datasheet with large capacitive load
Similar circuits often use a N-channel FET and an inverting error amplifier. In this case the OP gain changes from "1 + lowpass" to just lowpass, allowing a simple compensation of the loop gain.Is there a better way to design a circuit like this by the way?
To ease the design, I recommend
* don't switch to bjt, because with a mosfet you gain accuracy.
* make all voltages and the regulation 12V referenced. (No need to jump from 12V referenced to GND referenced)
* use a higher value shunt. (This additionally generates a local feedback for the Mosfet.
* omit the current measurement amplifier
... these are not typical values.vcc-0.2V and gnd+0.2V
.. weird stuff?it starts doing weird stuff if I exceed these limits.
The OPAMP regulates shunt voltage. Shunt voltage = shunt_current * R_shuntHow will using a mosfet gain accuracy as opposed to a bjt?
I usually use local HF feedback at the OPAMP. This makes it stable. Get some ideas here: https://www.edaboard.com/showthread.php?376094-Design-of-high-side-current-sourceBy using a base resistor of correct value I can drastically reduce the loop gain which is a good thing, right?
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