Table 3. Design Parameters

8.2.2.1 Input Bridges and Schottky Diodes

Using Schottky diodes instead of PN junction diodes for the PoE input bridges will reduce the power dissipation in these devices by about 30%. There are, however, some things to consider when using them. The IEEE standard specifies a maximum backfeed voltage of 2.8 V. A 100-kΩ resistor is placed between the unpowered pairs and the voltage is measured across the resistor. Schottky diodes often have a higher reverse leakage current than PN diodes, making this a harder requirement to meet. To compensate, use conservative design for diode operating temperature, select lower-leakage devices where possible, and match leakage and temperatures by using packaged bridges.

Schottky diode leakage currents and lower dynamic resistances can impact the detection signature. Setting reasonable expectations for the temperature range over which the detection signature is accurate is the simplest solution. Increasing R_DEN slightly may also help meet the requirement.

Schottky diodes have proven less robust to the stresses of ESD transients than PN junction diodes. After exposure to ESD, Schottky diodes may become shorted or leak. Take care to provide adequate protection in line with the exposure levels. This protection may be as simple as ferrite beads and capacitors.

As a general recommendation, use 1 A or 2 A, 100 V rated discrete or bridge diodes for the input rectifiers.

8.2.2.2 Protection, D₁

A TVS, D₁, across the rectified PoE voltage per Figure 24 must be used. TI recommends a SMAJ58A, or equivalent, is recommended for general indoor applications. If an adapter is connected from V_DD to RTN, as in ORing option 2 above, then voltage transients caused by the input cable inductance ringing with the internal PD capacitance can occur. Adequate capacitive filtering or a TVS must limit this voltage to within the absolute maximum ratings. Outdoor transient levels or special applications require additional protection.

8.2.2.3 Capacitor, C₁

The IEEE 802.3at standard specifies an input bypass capacitor (from V_DD to V_SS) of 0.05 μF to 0.12 μF. Typically a 0.1 μF, 100 V, 10% ceramic capacitor is used.

8.2.2.4 Detection Resistor, R_DEN

The IEEE 802.3at standard specifies a detection signature resistance, R_DEN between 23.7 kΩ and 26.3 kΩ, or 25 kΩ ± 5%. A resistor of 24.9 kΩ ± 1% is recommended for R_DEN.

8.2.2.5 Classification Resistor, R_CLS

Connect a resistor from CLS to V_SS to program the classification current according to the IEEE 802.3at standard. The class power assigned should correspond to the maximum average power drawn by the PD during operation. Select R_CLS according to Table 1.

Choose class 4 and R_CLS = 63.4 Ω.

8.2.2.6 APD Pin Divider Network RAPD1, RAPD2

For an adapter voltage threshold to switch from PoE to adapter at 37 V, choose 10 kΩ for RAPD2.

Equation 3.

Equation 4.

Solving for RAPD1: