JAJSES8B October 2017 – November 2018 TPS2372
The TPH, TPL and BT pin are active-low, open-drain outputs, which give an indication about the PSE allocated power along with its Type. Optocouplers can interface these pins to circuitry on the secondary side of the converter. A high-gain optocoupler and a high-impedance (for example, CMOS) receiver are recommended. Design of the optocoupler interface can be accomplished as follows:
Most applications require that only the PSE’s allocated power information (TPH and TPL) is needed for the MCU or PD load. In this case, the circuitry needed to drive the BT signal is not necessary and the BT pin can be left floating.
Some applications such as PoE lighting can benefit from the BT signal to show that the power consumption in standby operation may not meet regulatory requirements. In non-standard PoE applications, BT is used with TPH and TPL to indicate a PoE++ PSE.