This is a basic positive hot swap controller – the ADM4210.  The hot swap controller is used in conjunction with a number of components. The most significant being the sense resistor, MOSFET and timer capacitor. The sense resistor is used to measure the current on the main power path and is usually a very small resistance, in the order of milliohms. The N-Channel MOSFET is the main pass switch used to isolate the load, limit the current during a fault or during power-up, and, when fully enhanced, it will provide the main power for the module. The TIMER capacitor is critical to ensuring a maximum limit time is defined. When a fault occurs and the controller is limiting the current, the TIMER defines when to shutdown the MOSFET.