Hi,
SOT153 package, SMD code CO... comes up as possibly: SY6280. But that's not an exhaustive search and it would be wise to research this a lot further yourself to see if the part remotely fits with the circuit/application. I am assuming it is SOT153 from the photo and having done a little Googling to get what looks like the correct package name.
5.5V, 2A Low Loss Power Distribution Switch (page 2) "Top mark: COxyz for SY6280 (Device code: CO, x=year code, y=week code, z= lot number code)"
Good luck!