7.4.1.7 Startup and Converter Operation

The internal PoE UVLO (Under Voltage Lock Out) circuit holds the hotswap switch off before the PSE provides full voltage to the PD. This prevents the converter circuits from loading the PoE input during detection and classification. The converter circuits discharge C_IN, C_vc, and C_vb while the PD is unpowered. Thus V_(VDD-RTN) is a small voltage just after applying full voltage to the PD, as seen in Figure 24. The PSE drives the PI voltage to the operating range once the PSE has decided to power up the PD. When V_VDD rises above the UVLO turn-on threshold (V_UVLO-R, approximately 38 V) with RTN high, the TPS23751 and TPS23752 enable the hotswap MOSFET with approximately 140 mA (inrush) current limit as seen in Figure 26. Converter switching is disabled while C_IN charges and V_RTN falls from V_VDD to nearly V_VSS, however the converter startup circuit is allowed to charge C_VC (the bootstrap startup capacitor). Converter switching is allowed if the PD is not in inrush, OTSD is not active, and the V_C UVLO permits it. Once the inrush current falls about 10% below the inrush current limit, the PD current limit switches to the operational level (approximately 1000 mA). Continuing the startup sequence shown in Figure 26, V_VC continues to rise until the startup threshold (V_CUV approximately 8.9 V) is exceeded, turning the startup source off and enabling switching. The V_B regulator is always active, powering the internal converter circuits as V_VC rises. There is a slight delay between the removal of charge current and the start of switching as the softstart ramp sweeps above the V_ZDC threshold. V_VC falls as it powers both the internal circuits and the switching MOSFET gates. If the converter control bias output rises to support V_VC before it falls to V_CUV – V_CUVH (approximately 5.7 V), a successful startup occurs. In Figure 26, T2P is active if a type 2 PSE is plugged in.

Figure 26. Power Up and Start

If V_VDD- V_VSS drops below the lower PoE UVLO (V_UVLO-R - V_UVLO-H, approximately 32 V), the hotswap MOSFET is turned off, but the converter still runs. The converter stops if V_VC falls below the converter UVLO (V_CUV – V_CUVH, approximately 5.7 V), the hotswap is in inrush current limit, 0% duty cycle is demanded by V_CTL (V_CTL< V_ZDC, approximately 1.75 V), or the converter is in thermal shutdown.