9.4.1 Board Plug-In

Figure 31 and Figure 32 illustrate the inrush current that flows when a hot swap board under the control of the TPS2475x is plugged into a system bus. Only the bypass capacitor charge current and small bias currents are evident when a board is first plugged in. The TPS2475x is held inactive for a short period while internal voltages stabilize. In this short period, GATE, PROG, and TIMER are held low and PGb, and FLTb, are held open-drain. When the voltage on the internal VCC rail exceeds approximately 1.5 V, the power-on reset (POR) circuit initializes the TPS2475x and a start-up cycle is ready to take place.

GATE, PROG, TIMER, PGb, and FLTb are released after the internal voltages have stabilized and the external EN (enable) thresholds have been exceeded. The part begins sourcing current from the GATE pin to turnon internal FET. The TPS2475x monitors both the drain-to-source voltage across internal MOSFET and the drain current passing through it. Based on these measurements, the TPS2475x limits the drain current by controlling the gate voltage so that the power dissipation of the internal FET does not exceed the power limit programmed by the user. The current increases as the voltage across the FET decreases until finally the current reaches the current limit I_LIM.

Figure 31. Inrush Mode at Hot-Swap Circuit Insertion