Acting as an interface between the Power Source Equipment (PSE) and the Powered Device (PD), the TPS2371 performs all detection, class-ification, inrush current limiting, and switch FET control that is necessary for compatibility withLegacy/IEEE 802.3af Standard. The TPS2371 incorporates precision UVLO thresholds and hysteresis as well as a UVLO off-time delay to enable Legacy IEEE802.3af PoE compatibilty. An internal 0.3- \Omega FET provides maximum power delivery. As an additional feature, the TPS2371interfaces with the enable/soft-start signal of a dc-to-dc converter, eliminating the need to havean accurate UVLO in the dc-to-dc converter. At low input voltages (1.8 V to 10 V), the TPS2371draws less than 12 t,A, allowing accurate sensing of the external 24.9-k\Omega discovery resistor. At input voltages between 15 V and 20 V, an external resistor sets the level of current to be drawn during classification mode. TPS2371 is compatible with current as well as voltage measurement schemes for classification. Above 20-V input, the classification current is shut off, reducing internal power dissipation.