LTC4292 datasheet(16/40 Pages) AD

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LTC4291-1/LTC4292

Rev 0

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VEE and port current measurements are performed simul-

taneously, providing fully coherent port power calcula-

tions. The reported port power calculations enable coher-

ent and precise per-port power monitoring.

PoE BASICS

Common Ethernet data connections consist of two or

four twisted pairs of copper wire (commonly known

as Ethernet cable), transformer-coupled at each end to

avoid ground loops. PoE systems take advantage of this

coupling arrangement by applying voltage between the

center-taps of the data transformers to transmit power

from the PSE to the PD without affecting data transmis-

sion. Figures 10 and 11 show high level PoE system

schematics.

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 current 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 draft 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

APPLICATIONS INFORMATION

PD presents the same valid signature resistor to both

pairsets simultaneously. A dual-signature PD pres-

ents two fully independent valid detection signatures,

one to each pairset.

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.

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 lev-

els 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 appli-

cation and losses in the Ethernet cabling (Table 1).

See Autoclass section and LTC4291 Software

Programming documentation for details.

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