Detection

Classification

PD Powered

Idle

1.4V

DETECTION

CLASSIFICATION

data sheet

TPS2375-1

TPS2377-1

Figure 17. Threshold Voltages

This feature of IEEE 802.3af eliminates powering and potentially damaging Ethernet devices not intended for

application of 48 V. When a voltage in the range of 2.7 V to 10.1 V is applied to the PI, an incremental resistance

of 25 k

Ω signals the PSE that the PD is capable of accepting power. A PD that is capable of accepting power,

but is not ready, may present an incorrect signature intentionally. The incremental resistance is measured by

applying two different voltages to the PI and measuring the current it draws. These two test voltages must be

within the specified range and be at least 1 V apart. The incremental resistance equals the difference between

the voltages divided by the difference between the currents. The allowed range of resistance is 23.75 k

Ω to

26.25 k

Ω.

The TPS2375-1 is in detection mode whenever the supply voltage is below the lower classification threshold. The

TPS2375-1 draws a minimum of bias power in this condition, while PG and RTN are high impedance and the

circuits associated with ILIM and CLASS are disabled. The DET pin is pulled to ground during detection. Current

When the input voltage rises above the 11.3 V lower classification comparator threshold, the DET pin goes to an

open-drain condition to conserve power.

The input diode bridge incremental resistance can be hundreds of ohms at the low currents seen at 2.7 V on the

with the type of diode selected by the designer, and it is not usually specified on the diode data sheet. The value

Once the PSE has detected a PD, it may optionally classify the PD. This process allows a PSE to determine the

PD power requirements in order to allot only as much power as necessary from its fixed input power source. This

allows the PSE to power the maximum number of PDs from a particular power budget. This step is optional

because some PSEs can afford to allot the full power to every powered port.

The classification process applies a voltage between 14.5 V and 20.5 V to the input of the PD, which in turn

classes (Table 1) that the PD is signalling. The total current drawn from the PSE during classification is the sum

classification range to avoid excessive power dissipation (Figure 16 and Figure 17).

requirements of the PD. The power rating of this resistor should be chosen so that it is not overstressed for the

required 75-ms classification period, during which 10 V is applied. The PD could be in classification for extended

periods during bench test conditions, or if an auxiliary power source with voltage within the classification range is

connected to the PD front end. Thermal protection may activate and turn classification off if it continues for more

than 75 ms, but the design must not rely on this function to protect the resistor.

Copyright © 2005–2008, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Link(s): TPS2375-1 TPS2377-1