SLVSER7 October 2020 TPS23731
PRODUCTION DATA
RTN pin provides the negative power return path for the load. It is internally connected to the drain of the PoE hotswap MOSFET, and the DC-DC controller return. RTN must be treated as a local reference plane (ground plane) for the DC-DC controller and converter primary to maintain signal integrity.
Once VVDD exceeds the UVLO threshold, the internal pass MOSFET pulls RTN to VSS. Inrush limiting prevents the RTN current from exceeding a nominal value of about 140 mA until the bulk capacitance (CBULK in Figure 9-1) is fully charged. Two conditions must be met to reach the end of inrush phase. The first one is when the RTN current drops below about 90% of nominal inrush current at which point the current limit is changed to 0.925 A, while the second one is to ensure a minimum inrush delay period of 80 ms (tINR_DEL) from beginning of the inrush phase. DC-DC converter switching is permitted once both inrush conditions are met, meaning that the bulk capacitance is fully charged and the inrush period has been completed.
If VRTN - VVSS ever exceeds about 14.8 V for longer than 1.8 ms, then the PD returns to inrush limiting; note that in this particular case, the second condition described above about inrush phase duration (80 ms) is not applicable