Improving the FAU200 power supply

[bloguetronica]

Hi,

I've decided to improve the transient response of the FAU200 power supply, essentially by adding a 2.2uF capacitor in parallel with R7. I didn't solved the overshoot issue, although I did solve the ringing. Probably, a 100nF capacitor would do, but a 2.2uF capacitor prevents the OPA705 from hitting the rails when the DAC output voltage changes.

So, I've included some scope screenshots from before the mod and after. The measured points are the supply output, the OPA705 output before R7, and the net between R7 and the Q2 base. Also did some load tests by setting the FAU200 supply to 2V and by using a 10 resistor as a load. You can see that the performance is much better.

I think that the extra capacitor increases the phase margin and speeds up the response. Note that R7 is necessary for adequate short circuit current limiting. It prevents the OPA705 from being shorted in case of short circuit at the output.

Kind regards, Samuel Lourenço