Legacy/IEEE802.3af Compatible Power Interface Switch for PoE PDs

Acting as an interface between the Power Source Equipment (PSE) and the Powered Device (PD), the TPS2371 performs all detection, classification, inrush current limiting, and switch FET control that is necessary for compatibility with Legacy/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- FET provides maximum power delivery. As an additional feature, the TPS2371 interfaces with the enable/soft-start signal of a dc-to-dc converter, eliminating the need to have an accurate UVLO in the dc-to-dc converter.

At low input voltages (1.8 V to 10 V), the TPS2371 draws less than 12 µA, allowing accurate sensing of the external 24.9-k 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.

The TPS2371 drives an internal low-side FET for control of the return side of the power path. The internal FET is turned on when the input voltage reaches 36 V and above. When the input voltage decreases, the FET remains on until the input voltage drops to below 30 V.

During initial turn-on of the switch (inrush mode), an external resistor is used to program the inrush current, allowing a wide range of capacitor values to be used at the load. According to IEEE 802.3af specification, inrush current of 400 mA is allowed only for 50 ms, limiting the load capacitor to approximately 180 µF. A programmable inrush current limit removes this limitation, allowing a larger capacitor to be used with a lower inrush current limit.