LM25149-Q1: Why does PGOOD is tied to VCC on the LM25149 reference design?

Hi Li,

PGOOD is open drain output, it can be pulled up to any logic voltage level. 

It will only pull down PG pin when out of regulation.

You don't have to use VCC, you can use any external voltage within the 6.5V ABSMAX. 

Hope this helps,

-Orlando