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LTC4290/LTC4271

429071f

APPLICATIONS INFORMATION

If classification is enabled, the port will classify the PD

immediatelyafterasuccessfuldetectioncycleinsemi-auto

or AUTO pin modes, or when commanded to in manual

mode. It measures the PD classification signature by ap-

plying 18V for 12ms (both values typical) to the port via

the OUTn pin and measuring the resulting current; it then

reports the discovered class in the Port Status register.

If the LTC4290/LTC4271 is in AUTO pin mode, it will ad-

ditionally use the classification result to set the ICUT and

ILIM thresholds. See the Reset and the AUTO/MID Pin

section for more information.

The classification circuitry is disabled when the port is

initially powered up with the AUTO pin low, in shutdown

mode, or when the corresponding Class Enable bit is

cleared.

Figure 13. PD Classification

path. LLDP classification requires the PSE to power the

PD as a standard 802.3af (Type 1) device. It then waits for

the host to perform LLDP communication with the PD and

update the PSE port data. The LTC4290/LTC4271 supports

changing the ILIM and ICUT levels on the fly, allowing the

host to complete LLDP classification.

The second 802.3at classification method, known as

2-event classification or ping-pong, is supported by

the LTC4290/LTC4271. A Type 2 PD that is requesting

more than 13W will indicate Class 4 during normal

802.3af classification. If the LTC4290/LTC4271 sees

Class 4, it forces the port to a specified lower voltage

(called the mark voltage, typically 9V), pauses briefly, and

then re-runs classification to verify the Class 4 reading

(Figure1).ItalsosetsabitintheHighPowerStatusregister

to indicate that it ran the second classification cycle. The

second cycle alerts the PD that it is connected to a Type

2 PSE which can supply Type 2 power levels.

2-event ping-pong classification is enabled by setting a bit

in the port’s High Power Mode register. Note that a ping-

pongenabledportonlyrunsthesecondclassificationcycle

when it detects a Class 4 device; if the first cycle returns

Class 0 to 3, the port determines it is connected to a Type 1

PD and does not run the second classification cycle.

Invalid Type 2 Class Combinations

The 802.3at specification defines a Type 2 PD class signa-

ture as two consecutive Class 4 results; a Class 4 followed

by a Class 0-3 is not a valid signature. In AUTO pin mode,

the LTC4290/LTC4271 will power a detected PD regardless

of the classification results, with one exception: if the PD

presents an invalid Type 2 signature (Class 4 followed by

Class 0 to 3), the LTC4290/LTC4271 will not provide power

and will restart the detection process. To aid in diagnosis,

the Port Status register will always report the results of the

last class pulse, so an invalid Class 4–Class 2 combination

would report a second class pulse was run in the High

Power Status register (which implies that the first cycle

found class 4), and Class 2 in the Port Status register.

VOLTAGE (VCLASS)

429071 F13

TYPICAL

CLASS 3

PD LOAD

LINE

48mA

33mA

PSE LOAD LINE

23mA

14.5mA

6.5mA

CLASS 4

CLASS 2

CLASS 1

CLASS 0

CLASS 3

OVER

CURRENT

802.3at 2-Event Classification

The 802.3at specification defines two methods of classify-

ing a Type 2 PD. LTC4290/LTC4271 parts support 802.3at

2-event classification.

One method adds extra fields to the Ethernet LLDP data

protocol; although the LTC4290/LTC4271 is compatible

with this classification method, it cannot perform clas-

sification directly since it doesn’t have access to the data

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