7.4.4 Detection

The TPS2373 pulls DEN to V_SS whenever V_(VDD-VSS) is below the lower classification threshold. When the input voltage rises above V_{CL_ON}, the DEN pin goes to an open-drain condition to conserve power. While in detection, RTN is high impedance, and almost all the internal circuits are disabled. An R_DEN of 24.9 kΩ (±1%), presents the correct signature. It may be a small, low-power resistor because it only sees a stress of about 5 mW. A valid PD detection signature is an incremental resistance ( ΔV / ΔI ) from 23.7 kΩ to 26.3 kΩ at the PI.

The detection resistance seen by the PSE at the PI is the result of the input bridge resistance in series with the parallel combination of R_DEN and internal V_DD loading. The input diode bridge’s incremental resistance may be hundreds of Ω at the low currents drawn when 2.7 V is applied to the PI. The input bridge resistance is partially compensated by the TPS2373 effective resistance during detection.

The hardware classification protocol of IEEE 802.3bt specifies that a Type 2, 3 or 4 PSE drops its output voltage into the detection range during the classification sequence. The PD is required to have an incorrect detection signature in this condition, which is referred to as a mark event (see Figure 24). After the first mark event, the TPS2373 will present a signature less than 12 kΩ until it has experienced a V_(VDD-VSS) voltage below the mark reset threshold (V_MSR). This is explained more fully under Hardware Classification.