Electrical – Filtering for USB VBus to 3.3V


I have a circuit taken from an FTDI app note that takes USB +5V VBus and filters it into 5V Vcc. This circuit works well. I now want to modify this circuit to output +3.3V and I am unsure exactly what to do about the filtering. Should the filtering be placed before or after the 3.3V regulator? Is it best to filtering the 3.3V after the regulator (bottom drawing)? Or is it better to filter the 5V and then run that through the 3.3V regulator (top drawing)? Below are circuits for both configurations.
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Best Answer

You likely can do away with the filtering all together, as the linear regulator will filter out the noise sufficiently. If you want to be conservative, though, you should do it as shown in your first circuit. (Probably with a small electrolytic on the output for stability).