The SL811HS demonstration board has both a 10uF capacitor and a 120uF capacitor on the Vbus power.   Why are both of ...

Question: The SL811HS demonstration board has both a 10uF capacitor and a 120uF capacitor on the Vbus power. Why are both of these capacitors used?

 

Answer:

This board has the Vcc of both the upstream and down stream connectors tied together.  Under normal designs the 120 uF capacitor is for supplying power to the downstream port.  It will maintain the volt level during the in-rush current period of the downstream device.  If it is too low the voltage will drop below acceptable levels.   The upstream requirements are in reality different.  The upstream port should have no more than 10 uF attached to it and the bus should be separated from the down stream port.  If you attach more than 10 uF, you will exceed the inrush current requirement of the upstream port.  This is why the 10 uF part is needed. The attempt is to limit the inrush  current when the part is being connected as a device.