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LTC9101-1/

LTC9102/LTC9103

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

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 17. The second class event informs the PD that

the PSE has allocated 25.5W.

Figure 17.

9101123 F17

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 16.

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 16, 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 LTC9101-1/LTC9102/LTC9103 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)

Table 10. 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 OUTn and measuring the result-

ing current; it then reports the discovered class in the

Port Status or Channel Status register, as appropriate. If

the LTC9101-1/LTC9102/LTC9103 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 LTC9101-1/

LTC9102/LTC9103 is initially powered up with the AUTO

pin low, 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

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 LTC9101-1/LTC9102/LTC9103 is compat-

ible with this classification 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 LTC9101-1/LTC9102/LTC9103

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.

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