ZHCSMU0D July 2009 – December 2020 TPS23753A
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 VDD through RCLS. The total current drawn from the PSE during classification is the sum of bias and RCLS 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 VCU_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 VSS for long periods of time.
|CLASS||POWER AT PD PI||CLASS CURRENT REQUIREMENT||RESISTOR (Ω)||NOTES|
|MINIMUM (W)||MAXIMUM (W)||MINIMUM (mA)||MAXIMUM (mA)|
|4||12.95||25.5||36||44||63.4||Only permitted for type 2 devices|