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LTC4291-1/LTC4292

Rev 0

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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 sees 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. Type 2 PSE, 2-Event Class Sequence

42911 F16

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1ST

CLASS

1ST

MARK

2ND

MARK

2ND

CLASS

POWER ON

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 in all situations.

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 LTC4291-1/LTC4292 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)

classification supersede Type 1 and Type 2 classifica-

tion. 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the preceding sections as a historical reference and to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 9.

Classification treatment of single-signature and dual-

signature PDs differs. The following sections explain the

Physical Layer classification of each PD configuration

separately.

Table 9. Type 3 and Type 4 PD Classifications by PD Configuration

SINGLE-SIGNATURE PDs

DUAL-SIGNATURE PDs

CLASS

PD AVAILABLE

POWER

CLASS

CHANNEL AVAILABLE

POWER*

Class 1

3.84W

Class 1

3.84W

Class 2

6.49W

Class 2

6.49W

Class 3

13W

Class 3

13W

Class 4

25.5W

Class 4

25.5W

Class 5

40W

Class 5

35.6W

Class 6

51W

Class 7

62W

Class 8

71.3W

*Dual-signature PD total available power is the sum of both channels

available power. Class signatures may differ between channels of a port,

e.g., Class 3 + Class 4 = 13W + 25.5W = 38.5W.

802.3bt Classification of Single-Signature PDs

Type 3 and Type 4 PSEs issue a single classification event

(see Figure 17) to Class 0 through 3 single-signature (SS)

PDs. A Class 0 through 3 SS PD presents its class signa-

ture to the PSE and is then powered on if sufficient power

is available. Power limited 802.3bt PSEs may also issue a

single classification event to Class 4 and higher SS PDs in

order to demote those PDs to 13W. See Figure 17.

Type 3 and 4 PSEs present three classification events

to Class 4 SS PDs (see Figure 18) if sufficient power is

available. Class 4 SS PDs present class signature 4 on

all events. The third event differentiates a Class 4 SS PD

from a higher Class SS PD. Power limited IEEE 802.3bt

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