Hardware classification allows a PSE to determine the power requirements of a PD before starting, and helps with power management once power is applied. The maximum power entries in Table 8-1 determine the class the PD must advertise. A Type 1 PD may not advertise Class 4. The PSE may disconnect a PD if it draws more than its stated Class power. The standard permits the PD to draw limited current peaks; however, the average power requirement always applies.

Voltage from 14.5 V to 20.5 V is applied to the PD for up to 75 ms during hardware classification. A fixed output voltage is sourced by the CLS pin, causing a fixed current to be drawn from V_DD through R_CLS. The total current drawn from the PSE during classification is the sum of bias and R_CLS currents. PD current is measured and decoded by the PSE to determine which of the five available classes is advertised (see Table 8-1). The TPS23753A disables classification above V_{CU_OFF} to avoid excessive power dissipation. CLS voltage is turned off during PD thermal limit or when APD or DEN are active. The CLS output is inherently current-limited, but should not be shorted to V_SS for long periods of time.