I have this IC and I don't know what it does.. It just says ST WTF405 plus the place it was made at.
I've been searching at the ST product page and google but I just can't find the description.. The IC has only 3 legs (pretty much like a transistor) but it is much bigger and doesn't have the looks of a transistor.
Sorry Could not get any part specified by any manufacturer marked with that code
Is there any other information on the device (can you post the picture?)
If you are sure its an ST component then just email (find email id on thier website) the ST and ask them regarding the datasheet or any other information you want.