TPS25751: power source 5V/3A only issue

Part Number: TPS25751
Other Parts Discussed in Thread: LM51772,

Tool/software:

I am using TPS25751 for USB PD application, and now I just want it to be 5V/3A source. I designed a buck LM51772 to output 5V, which connected to TPS25751 PP3.

Is it feasible that I config PP3 to be the source for this application?

It is strange after I config as I described, and I connect it to a cell phone, I measured the output voltage is 0V~3V transient voltage. 

How can I locate where the problem is ? Thank you