Auxiliary Power Option B (Rear Aux or RAUX Pin Method)
In the following discussion, pay close attention to Fig. 14.6. Application of Rear Aux can be very useful, but also confusing.
FIGURE 14.6 Rear Aux method in more detail.
Rear Aux has the following advantage from a heuristic viewpoint: it introduces the ability to cause the PSE to disconnect power if desired, by just turning the pass-FET of the front end (“Q1”) OFF. If the PSE current then falls below 5 mA, the PSE will typically disconnect, though it will likely keep attempting detection. ...