TVS is a good practice to protect USB ports. A USB (at least 2.0) connection has the power path supplied with a 5 V source (Vbus=5V0) at the host side and the data path supplied with a 3.3 V sources (3V3) at both ends of the connection.

A TVS protecting the data lines (D+, D-), is connected like the following: 

![enter image description here](https://i.stack.imgur.com/aCvu7.png)

**Should it be connected to 5V0 (Vbus) or 3V3?** Why so? Arguments? References?

(This is especially interesting in the case of FTDI and similar ICs whose transceiver logic is supplied from the internal 3.3 V LDO.)