LTC4292 datasheet(22/40 Pages) AD

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LTC4291-1/LTC4292

Rev 0

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APPLICATIONS INFORMATION

Table 8. Type 1 and Type 2 PD Classification Values

CLASS

RESULT

Class 0

No Class Signature Present; Treat Like Class 3

Class 1

3.84W

Class 2

6.49W

Class 3

13W

Class 4

25.5W (Type 2)

If classification is enabled, the PSE will classify the PD

immediately after a successful detection cycle. The PSE

measures the PD classification signature by applying

VCLASS to the port via OUTnM and measuring the result-

ing current; it then reports the discovered class in the Port

Status or Channel Status register, as appropriate. If the

LTC4291-1/LTC4292 is in auto mode, it will additionally

use the classification result to set the ICUT-2P, ILIM-2P, and

PCUT-4P thresholds.

Classification is disabled for a port when the LTC4291-1/

LTC4292 is initially powered up with the AUTO pin low,

when the port is in shutdown mode, or when the corre-

sponding Class Enable bit is cleared.

LLDP Classification

Introduced in 802.3at and extended by 802.3bt, the PoE

specification defines a Link Layer Discovery Protocol

(LLDP) method of classification. The LLDP method adds

extra fields to the Ethernet LLDP data protocol.

Although the LTC4291-1/LTC4292 is compatible with this

classification method, it cannot perform LLDP classifica-

tion directly since it does not have access to the data path.

LLDP classification allows the host to perform LLDP com-

munication with the PD and update the PD’s power allo-

cation. The LTC4291-1/LTC4292 supports changing the

ILIM-2P, ICUT-2P, and PCUT-4P levels dynamically, enabling

system-level LLDP support.

802.3at 2-Event Classification

In 802.3at, 802.3af classification is named Type 1 clas-

sification. The 802.3at standard introduces an extension

of Type 1 classification: Type 2 (2-event) classification.

Type 2 PSEs are required to perform classification.

Classification

802.3af Classification

A PD may optionally present a classification signature

to the PSE to indicate the maximum power it will draw

while operating. The IEEE specification defines this sig-

nature as a constant current draw when the PSE port

voltage is in the VCLASS range (between 15.5V and 20.5V)

as shown in Figure 15, with the current level indicating

one of five possible PD signatures. Figure 14 shows a

typical PD load line, starting with the slope of the 25k

signature resistor below 10V, then transitioning to the

classification signature current (in this case, Class 3)

in the VCLASS range. Table 8 shows the possible clas-

sification values.

Figure 14. PD Classification

VOLTAGE (VCLASS)

42911 F14

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

Figure 15. Type 1 PSE, 1-Event Class Sequence

42911 F15

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

CLASS

POWER ON

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