Over a 200ms period of which this circuit regulates a 500mA current, the current drift downwards for about 0.4%.
At first I thought it was due to R202 temperature coefficient, but from my calculation the drift related to R202 should only be in the range of 0.05%.
R202 is a 2ohm, 2W, 2 point, 100ppm/°C resistor and from the touch it barely heats at all.
Any other idea where could be the source of the drift ? Solder of the resistor pads ?
The input at R198 is stable.
To my surprise, this morning I restarted the circuit, it hasn't moved a bit overnight, and measured the drift, before touching anything. It was down to 0.1%, the night before it was about 0.4%.
Now as time pass, the drift is increasing little by little and I guess in 1h it will be back to 0.4%.
For my tests, I do a 200ms pulse every 1s and is constantly running. The circuit does heats up a little bit.
Here is a graph with 3 measurements done at different times. As one can see, the first two have about 0.15% drift, the last one has 0.7% drift.
The drift seems to vary over time, and so far I've failed to understand the reason nor to find a way to influence the drift.
The circuit hasn't change between those measurements, I've played around with power supply, voltage rails, I tried to heat the circuit up, but no changes.
Removing C121 and C122, affect the stability, but the drift remains the same.