9.1.3 Classification

The classification process applies a voltage between 14.5 V and 20.5 V, for a maximum of 75 ms, to the input of the PD, which in turn draws a fixed current set by R_(CLASS). An 802.3af PSE measures the PD current to determine which of the five available classes shown in Table 1 that the PD is signaling. The total current drawn from the PSE during classification is the sum of bias currents and current through R_(CLASS). The TPS2376-H disconnects R_(CLASS) at voltages above the classification range to avoid excessive power dissipation see Figure 11 and Figure 12.

A high power end-to-end system may choose to not implement classification, or redefine the power associated with each class. Low-voltage systems, for example 24 V, may not be able to use CLASS because the operational voltage may lie within the classification voltage range. This would cause the TPS2376-H classification circuits to dissipate power continuously.

The power rating of the class resistor should be chosen so that it is not overstressed for the required 75-ms classification period, during which 10 V is applied. A higher wattage resistor might be required to withstand testing over longer time periods.