PM8801 datasheet(20/37 Pages) STMICROELECTRONICS

20 / 37 page

PD interface

PM8801

20/37

DocID018560 Rev 2

Figure 6. PM8801: reference schematic of the PoE classification logic

The classification resistor can be disconnected for the following reasons:

–

the classification has been successfully completed

–

an auxiliary power source has been connected

–

the device is in thermal protection.

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

following table.

4.3

Indication of successful 2-event classification

PM8801 is capable of recognizing whether it is connected to a PSE performing physical

layer classification by asserting the T2P signal.

T2P is an open-drain, active-low signal which is pulled down in case a successful 2-event

classification event is completed.

T2P will be asserted as soon as the high-voltage startup regulator output is stable (see

Figure 7 and Figure 8 for timing sequences). If PM8801 sees a 1-event classification or no

classification, T2P is pulled up and the main circuit in the PD can try to establish an LLDP

connection to negotiate the power. No LLDP response from the PSE means that the PD is

connected to a Type1 PSE, and only 13 W input power will be available.

Table 7. Value of the external classification resistor for the different PD classes of

power

Class

PD max average power (W)

RCLS (Ω)

013

2 k

13.84

150

6.49

80.6

313

51.1

25.5

35.6

AM045038v1

PM8801

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