PM8801 datasheet(19/37 Pages) STMICROELECTRONICS

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DocID018560 Rev 2

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PM8801

PD interface

4.1

Detection

In Power over Ethernet systems, the Power Sourcing Equipment (PSE) senses the Ethernet

connection to detect whether the Powered Device (PD) is plugged to the cable termination

by applying a small voltage (2.7 to 10 V) on the Ethernet cable and measuring the

equivalent resistance in at least two successive steps. During this phase, the PD must

present a resistance between 23.75 k

Ω and 26.25 kΩ.

The signature resistor must be connected between the DET and VDD pins. This resistor is

in-series to a pass transistor (see Figure 3 ) enabled only during the detection phase. No

current is flowing through the signature resistor for the rest of the operative phases

(classification and turn-on).

The value of the detection resistance has to be selected also taking into account the typical

drop in voltage of the diode bridges. The typical value that can be used in most cases is

24.9 k

Ω.

During detection, most of the circuits inside the PM8801 are disabled to minimize the offset

current.

4.2

Classification

The classification phase in a PoE network is the feature that allows PSE to plan and allocate

the available power to the appliances connected to various Ethernet ports.

PM8801 complies with both IEEE802.3at 1-event and 2-event classification schemes.1-

event classification in IEEE802.3at is the same as specified in the IEEE 802.3af standard,

which divides the power levels below 12.95 W into 5 classes (Class 0 to Class 4).

While Class 4 was reserved in IEEE802.3af, in IEEE802.3at Class 4 identifies Type2 PDs

requiring up to 25.5 W.

A Type2 PD is a PD that provides a Class 4 signature during Physical Layer classification,

understands 2-event classification and is capable of Data Link Layer classification.

Figure 8 represents the voltage at the input of the PD when connected to a PSE performing

2-event classification. A Type2 PD will present in both classification events a Class 4

current, while during the so called “mark-event”, between the 2 classification fingers, the PD

will present an invalid signature resistance.

To support the classification function, an equivalent programmable constant current

generator has been implemented. Figure 6 depicts the primary schematic of the

classification circuit. Following the successful completion of the detection phase, the voltage

of the CLS pin is set to the 1.4 V voltage reference and a pass transistor connects the VIN

pin to the CLS pin.

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