8.3.2 Programmable Inrush Current Limit and Fixed Operational Current Limit

Inrush limiting has several benefits. First, it maintains the cable voltage above the UVLO turn-off threshold as the bulk capacitor charges. Second, it keeps the PSE from going into current limit. This reduces stress on the PSE and allows an arbitrarily large bulk capacitor to be charged. Third, the inrush limit is used as the foldback current during a hard overload.

The TPS2376-H operational current limit protects the internal power switch from sudden output faults and current surges. The minimum operational current limit level of 625 mA lies above the minimum TPS23841 output current limit of 600 mA. This current limit enables the PD to draw the maximum available power.

The TPS2376-H incorporates a state machine that controls the inrush and operational current limit states. When V_(VDD) is below the lower UVLO threshold, the current limit state machine is reset. In this condition, the RTN pin is high impedance, and floats to V_(VDD) once the output capacitor is discharged. When V_(VDD) rises above the UVLO turn on threshold, the TPS2376-H enables the internal power MOSFET with the current limit set to the inrush value programmed by R_(ILIM). The load capacitor charges and the RTN pin voltage falls from V_(VDD) to nearly V_(VSS). Once the inrush current falls about 10% below the programmed limit for 150-μs, the current limit switches to the 765-mA operational level and PG goes open-drain. The internal power MOSFET is disabled if the input voltage drops below the lower UVLO threshold and the state machine is reset.

An output overload, or increasing input voltage step, may cause the operational current limit to become active. The MOSFET voltage will then start to rise, causing high power dissipation. Current-limit foldback controls this MOSFET power dissipation to a manageable level. Foldback is achieved by switching the current limit state machine from the operational level to inrush when the MOSFET voltage exceeds 10 V for 150-μs. An additional layer of protection is provided by thermal shutdown if the overload persists long enough.

Practical values of R_(ILIM) lie between 125 kΩ and 1 MΩ; however, selecting lower inrush current values reduces peak stresses under output-short circuit conditions. An inrush level of 140 mA, set by an R_(ILIM) of 287 kΩ, is recommended for most applications.