you are correct, but the A320 PTU is slighlty different from most PTUs out there in that it is bi-directional, i.e. it can be used to pressurise both systems, rather than a uni-directional one (Boeing 737, A300 etc) that only pressurise one system.
It's a very complex beast and has gone through many design changes over the years. Fundamentally, the motor also acts a pump when required and the pump as a motor. It operates on the pressure diffferential between the two systems and will operate at around 500 psi delta P (however, airlines have complained at the PTU kicking in a lower delta Ps, during landing gear retraction for example, when by design it shouldn't).
I'll dig out some data I have later and see what I have.