What is hot-swapping?
During hot-swapping, a module can be plugged into or unplugged from a live supply bus without affecting the overall system operation. The idea probably comes from telephone line cards, whose edge connectors were designed with fingers of different lengths. When the cards were inserted into the backplane, ground was always connected first, followed by the power bus and the I/O pins. That was sufficient for the simple and robust electronics of the public-switched telephone network. But today's routers, servers, and embedded systems often need a hot-swap controller, a sense resistor, and a FET pass transistor as well to provide a more robust and intelligent solution.
What kind of module needs a hot-swap controller?
Modules with a significant amount of bypass capacitance across the input power line or a failure mode that could bring down the main power bus need a hot-swap controller. A closed-loop current-limiting capability that limits the initial charging current to the bypass capacitors keeps the main supply bus voltage from drooping when a new load is added. It also prevents the connector power contacts from being overloaded by short-term excess currents.