LTC4292 datasheet(24/39 Pages) AD

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Data Sheet

LTC9101-2B/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 24 of 39

CLASSIFICATION

802.3af (Type 1) 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 signature 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 40, with the current level indicating

one of five possible PD signatures. Figure 39 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 12 shows the possible

classification values.

Figure 39. PD Classification

Figure 40. Type 1 or Type 2 PSE, 1-Event Class Sequence

Table 12. 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 (802.3at, 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 OUTn

and measuring the resulting current; it then reports the PD request-

ed Class in the appropriate Port Status register. In Auto mode

the LTC9101-2B/LTC9102 sets 2P Police, Current Limit, and DC

Disconnect thresholds accordingly.

Classification is disabled for a port when the port is in shutdown

mode or when the corresponding Class Enable bit is cleared.

LLDP Classification

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

tion defines a Link Layer Discovery Protocol (LLDP) method of

classification. The LLDP method adds extra fields to the Ethernet

LLDP data protocol.

Although the LTC9101-2B/LTC9102 is compatible with this classifi-

cation method, it cannot perform LLDP classification directly since it

does not have access to the data path. LLDP classification allows

the host to perform LLDP communication with the PD and update

the PD’s power allocation. The LTC9101-2B/LTC9102 supports

changing the 2P Police levels dynamically, enabling system-level

LLDP support.

802.3at 2-Event Classification

In 802.3at, 802.3af classification is named Type 1 classification.

The 802.3at standard introduces an extension of Type 1 classifica-

tion: Type 2 (2-event) classification. Type 2 PSEs are required to

perform classification.

A Type 2 PD requesting 25.5W presents class signature 4 during

all class events. If a Type 2 PSE with 25.5W of available power

measures class signature 4 during the first class event, it forces the

PD to VMARK (9V typical), pauses briefly, and issues a second class

event as shown in Figure 41. The second class event informs the

PD that the PSE has allocated 25.5W.

Figure 41. Type 2 PSE, 2-Event Class Sequence

Note that the second classification event only runs if required by the

IEEE classification procedure. For example, a Class 0 to 3 PD will

only be issued a single class event as shown in Figure 40.

The concept of demotion is introduced in 802.3at. A Type 2 PD may

be connected to a PSE only capable of delivering 13W, perhaps

due to power management limitations. In this case, the PSE will

perform a single classification event as shown in Figure 40, and

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