LT4293 datasheet(23/44 Pages) AD

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LTC9101-2/LTC9102

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

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 LTC9101-2/LTC9102 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

communication with the PD and update the PD’s power

allocation. The LTC9101-2/LTC9102 supports changing

the 2P Police and 4P Police 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.

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 16. The second class event informs the PD that

the PSE has allocated 25.5W.

Figure 16.

9101-2 F16

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1ST

CLASS

1ST

MARK

2ND

MARK

2ND

CLASS

POWER ON

Type 2 PSE, 2-Event Class Sequence

Note that the second classification event only runs if

required by the IEEE classification procedure. For exam-

ple, a single-signature Class 0 to 3 PD will only be issued

a single class event as shown in Figure 15.

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 limi-

tations. In this case, the PSE will perform a single classi-

fication event as shown in Figure 15, and note that 25.5W

is requested. Due to the limited power availability, the

PSE will not issue a second event and proceeds directly

to power on the PD. The presence of a single class event

informs the Type 2 PD it has been demoted to 13W. If

demoted, the PD is subject to power limitations and may

operate in a reduced power mode.

802.3bt Multi-Event Classification

The LTC9101-2/LTC9102 implements Type 3 and Type 4

classification as required by 802.3bt. Type 3 and Type 4

classification are backwards-compatible with Type 1 and

Type 2 PDs.

While Type 2 (802.3at) classification extends Type 1

(802.3af) classification, Type 3 and Type 4 (802.3bt) clas-

sification supersede Type 1 and Type 2 classification. Type

1 and Type 2 classification are described in the preceding

sections as a historical reference and to define common

terminology such as power demotion, class events, mark

events, and electrical parameters.

IEEE 802.3bt defines eight PD Classes for single-signature

PDs and five PD Classes for dual-signature PDs, as shown

in Table 10.

Classification treatment of single-signature and

dual-signature PDs differs. The following sections

explain the Physical Layer classification of each PD

configuration separately.

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