LT4293 datasheet(18/44 Pages) AD

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LTC9101-2/LTC9102

Rev. 0

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To avoid damaging legacy data equipment that does not

expect to see DC voltage, the PoE standard defines a

protocol that determines when the PSE may apply and

remove power. Valid PDs are required to have a specific

25k common-mode resistance at their input. When such

a PD is connected to the cable, the PSE detects this sig-

nature resistance and applies power. When the PD is

later disconnected, the PSE senses the open circuit and

removes power. The PSE also removes power in the event

of a current fault or short circuit.

When a PD is detected, the PSE looks for a classification

signature that tells the PSE the maximum power the PD

will draw. The PSE can use this information to allocate

power among several ports, to police the power con-

sumption of the PD, or to reject a PD that will draw more

power than the PSE has available.

New in 802.3bt

The 802.3bt specification introduces several new features:

Type 3 and Type 4 PSEs may provide power over all

four pairs (both pairsets), depending on connected PD

characteristics.

Type 3 and Type 4 PDs are required to be capable of

receiving power over all four pairs (both pairsets).

Type 3 and 4 PDs can be formed as either a single-

signature PD or dual-signature PD. A single-signature

PD presents the same valid signature resistor to both

pairsets simultaneously (see Figure 10). A dual-signa-

ture PD presents two fully independent valid detection

signatures, one to each pairset (see Figure 11).

Type 3 single-signature PDs request exactly one of six

possible power levels: 3.84W, 6.49W, 13W, 25.5W,

40W, or 51W.

Type 3 dual-signature PDs request exactly one of four

possible power levels on each pairset: 3.84W, 6.49W,

13W, or 25.5W. The total PD requested power is the

sum of the requested power on both pairsets.

APPLICATIONS INFORMATION

Type 3 PD Classes overlap with Type 1 and 2 Classes in

order to provide additional Type 3 feature sets at lower

power levels.

Type 4 single-signature PDs request exactly one of two

possible power levels: 62W or 71.3W.

Type 4 dual-signature PDs request exactly 35.6W on at

least one pairset and one of five possible power levels

on the other pairset: 3.84W, 6.49W, 13W, 25.5W, or

35.6W. The total PD requested power is the sum of the

requested power on both pairsets.

Classification is extended to a possible maximum of

five class events. The additional events allow for unique

identification of existing and new PD Classes.

Type 3 and 4 PSEs issue a long first class event to

advertise Type 3 and 4 feature support to attached PDs.

Lower standby power is enabled by shortening the

length of the maintain power signature pulse (short

MPS). The PD duty cycle drops from ~23% to ~2%. A

PD is allowed to present short MPS if the PSE issues

a long first class event.

Power management is augmented by Autoclass, an

optional feature for 802.3bt PSEs and PDs. In an

Autoclass system the maximum PD power is mea-

sured and reported to the PSE host, enabling the PSE

to reclaim output power not used by the PD applica-

tion and losses in the Ethernet cabling (Table 3). See

Autoclass section and LTC9101-2NDASI Software

Programming documentation for details.

Table 3. IEEE-Specified Power Allocations, Single-Signature PD

PD CLASS

PSE OUTPUT

POWER

ALLOCATED

CABLING LOSS

PD INPUT

POWER

0.16W

3.84W

6.7W

0.21W

6.49W

14W

13W

30W

4.5W

25.5W

45W

40W

60W

51W

75W

13W

62W

90W

18.7W

71.3W

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