We are starting to see more and more products powered through the USB. This is because of the lower power consumption of integrated circuits plus a host of low power techniques.
But USB standard imposes constraints and limitations on the USB bus, which can be problematic for the electronic designers.
One of these limitations is the amount of capacitance on VBUS, which must be less than 10 micro-farads. Obviously, this limitation is because of the amount of inrush current. When we hook-up the USB cable into our device, the high value of capacitance causes a higher value in inrush current.
Suppose you want to connect the VBUS to the input of a DC/DC converter, such as a step-up converter, to push the voltage up to higher values. You will get into trouble, since 10 micro Farad capacitance on the input line of the DC/DC converter is not enough to reduce the noise induced by the DC/DC converter. On the other hand, the noise generated by the DC/DC converter will get into the 5V voltage rail and affect the circuit and/or the PC USB circuitry.
Here is the solution:
Use a P-MOSFET switch to keep the VBUS disconnected from the higher value capacitance and when the capacitor is charged enough through a leakage resistance, the switch will be connected.
To allow the C2 to be charged properly we have to keep the DC/DC converter off while it is charging. The circuit below does the job:

Mazi Hosseini M.A.Sci., P.Eng