That is the mosfet. Your mosfet driver high side needs one to charge the bootstrap capacitor, to work. Read any of these application notes, doesn't matter which company they all do it similar. https://www.google.com/search?client=firefox-b-1-d&q=boot+strap+high+side+driverHello Mr. Shortbus,
Thanks for you reply.
CSD18540q5b datasheet show that it has built in Diode.
I still don't understand the circuit flow