JAJSES8B October 2017 – November 2018 TPS2372
RTN pin provides the negative power return path for the load. 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 200 mA and 335 mA for the TPS2372-3 and TPS2372-4 respectively until the bulk capacitance (CBULK in Figure 22) 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 1.85 A for TPS2372-3 and 2.2 A for TPS2372-4, while the second one, if IRSHDL_EN is open, is to ensure a minimum inrush delay period of ~81.5 ms (tINR_DEL) from beginning of the inrush phase. The PG output becomes high impedance to signal the downstream load that the bulk capacitance is fully charged and the inrush period has been completed. Connecting IRSHDL_EN input to RTN pin disables the inrush delay, in which case only the first condition applies to reach the end of inrush phase.
If RTN ever exceeds about 14.5 V for longer than ~1.65 ms, then the TPS2372 returns to inrush phase; note that in this particular case, the second condition described above about inrush phase duration (81.5 ms) is not applicable.