USB PD: How to define the VBus voltage.

Thread Starter


Joined Feb 21, 2018
Hello there.

I have done quite some research over the internet about USB PD, trying to understand how the voltage and current levels to be sourced or sunk are decided. I have come to the conclusion that the DFP advertises the maximum available current it can source through the pull-up resistors it applies on the CC lines, while the UFP is identified by the pull-down resistors it applies on its CC lines.

There are several power profiles defined by USB PD standard specifications. What I am not able to figure out yet how the DFP decides what voltage it should source?

For example, If I have to design a USB powered device that draws 3A. So, I would simply pull both the CC lines down through a 5K1 resistor. But, my device operates at 5V. So, how am I going to instruct the adaptor (DFP/Source) to put a constant 5V on the VBus, and not anything higher? Or, say, if my device operates at 9V?