8.2.2.6 Opto-isolators for TPH, TPL and BT

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:

See Table 2 to decode PSE Type

Figure 23. TPH, TPL, and BT Interface

1. As shown in Figure 23, let V_DD = 48 V, V_OUT = 5 V, R_TPx–OUT = 10 kΩ, V_TPx = 260 mV, V_TPx-OUT = 400 mV.
Equation 1.
2. The optocoupler current transfer ratio, CTR, is needed to determine R_TPx. A device with a minimum CTR of 100% at 1 mA LED bias current, I_TPx, is selected. In practice, CTR will vary with temperature, LED bias current, and aging, These variations may require some iteration using the CTR-versus-IDIODE curve on the optocoupler data sheet.
   1. The approximate forward voltage of the optocoupler diode, V_FWLED, is 1.1 V from the data sheet.
   2. Use .
   Equation 2.
   3. Choose a 46.4 kΩ resistor.

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.