PM8800ATR datasheet(17/35 Pages) STMICROELECTRONICS

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PM8800A

PD interface

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5.2

Classification

Classification process in the IEEE 802.3af standard is optional for the powered device. This

feature allows PSE to plan and allocate the available power to the appliances connected to

the PoE network. IEEE 802.3af specification groups the need for power of the PD in 5

classes, one is reserved for future use. After successful detection, the PSE sets a constant

voltage between 15.5 V and 20.5 V for a maximum duration of 75 ms and senses the current

flowing through the cable to determine the PD's class.

The relevant thresholds in PM8800A are 11.5 V and 23 V, with a turn off hysteresis of 1.4 V.

To support the classification function, an equivalent programmable constant current

generator has been implemented. The following figure depicts a principle schematic of the

classification circuit. Just after the detection phase has been successfully completed, the

voltage of the RCLASS pin is set to the 1.4 V voltage reference and a pass transistor

connects the VIN pin to RCLASS pin.

Figure 8.

PM8800A: reference schematic of the PoE classification logic

Classification resistor can be detached by three main causes:

–

An auxiliary power source (front or rear) has been connected (see Section 7),

–

The device is in thermal protection

–

The classification has been successfully completed.

Designers can set the current by changing the value of the external resistor according to the

following table.

VIN

VSS

1.4V

RCLASS

Thermal_alarm

Class_enable

Aux_enabled

PM8800A

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