LT4293 datasheet(25/44 Pages) AD

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LTC9101-2/LTC9102

Rev. 0

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APPLICATIONS INFORMATION

Class Reset

An issue arises when a Class 4 or Class 5 dual-signature

PD is connected. In order to determine PD Type, three

class events are issued. Based on the class event count,

the PD has been allocated 25.5W. If the PSE desires to

both determine PD Type (3 events) and demote to 13W

(1 event), a class reset event must be issued as shown

in Figure 21.

Figure 21.

9101-2 F21

VCLASSMIN

VMARKMAX

VRESET

DETECT

1ST

CLASS

1ST

MARK

1ST

MARK

2ND

MARK

2ND

CLASS

3RD

CLASS

1ST

CLASS

POWER ON

CLASS

RESET

Class Reset Event Between Class Sequences

A class reset event is issued by maintaining the channel

voltage below 2.8V for at least tCLASS_RESET. The subse-

quent single event classification is used to demote the

PD to 13W.

In auto mode, the 4PID information is used to automati-

cally determine the number of powered channels.

LLDP signaling may, at some time later, determine the

pre-bt PD is actually four pair capable and the LTC9101-2/

LTC9102 may be instructed to deliver 4-pair power.

Invalid Multi-Event Classification Combinations

The 802.3bt specification defines a set of valid class sig-

nature combinations. All PDs return the same classifica-

tion signature on the first two class events. Type 3 and 4

PDs modify the classification signature on all subsequent

class events. For example, a single-signature Class 5 PD

will respond to the class events 1, 2, 3, and 4 with a class

signature of 4, 4, 0, and 0, respectively.

Any individual class signature that exceeds the class cur-

rent limit is flagged as an invalid classification result. Any

sequence of class signatures that does not represent a

legal sequence based on PD configuration will likewise

be flagged as an invalid classification result.

Type 4 PSEs present five classification events (see

Figure 20) to Class 7 and 8 SS PDs if sufficient power is

available. Class 7 and 8 PDs present class signature 4 on

the first two events. Class 7 and 8 SS PDs present class

signature 2 or 3, respectively, on the subsequent events.

802.3bt Classification of Dual-Signature PDs

Classification and power allocations to each pairset of a

dual-signature (DS) PD are fully independent. For exam-

ple, a DS PD may request Class 1 (3.84W) on one pairset

and a Class 4 (25.5W) on the second pairset for a total

PD requested power of 29.3W. As such, all classification

is performed to the pairset entity as opposed to the PD.

The terms should be considered interchangeable for the

remainder of this section.

Type 3 and Type 4 PSEs issue three classification events

(see Figure 18) to all Class 1 through 4 DS PDs.

Power limited Type 3 and Type 4 PSEs may issue a class

reset to Class 4 and 5 DS PDs in order to demote those

PDs to 13W (see Understanding 4PID section).

Power limited Type 3 and Type 4 PSEs may issue only

three events to Class 5 DS PDs in order to demote those

PDs to 25.5W.

Type 4 PSEs present four classification events (see

Figure 19) to Class 5 DS PDs if sufficient power is avail-

able. Class 5 DS PDs present class signature 4 on the first

two events and class signature 3 on subsequent events.

Understanding 4PID

4-pair identification (4PID) refers to a set of conditions for

determining whether a PD is capable of receiving power

over both pairsets simultaneously.

The PSE may apply 4-pair power if the PD presents a valid

detection signature on both pairsets and one or more of

the following conditions are met:

• The PD is single-signature configuration.

• The PD is Type 3 or Type 4.

• The PD presents a valid detection signature on an

unpowered pairset when power is applied over the

other pairset.

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