LTC4270SLASH datasheet(17/32 Pages) LINER

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LTC4290/LTC4271

429071f

PoE BASICS

Common Ethernet data connections consist of two or four

twisted pairs of copper wire (commonly known as CAT-5

cable), transformer-coupled at each end to avoid ground

loops. PoE systems take advantage of this coupling ar-

rangement by applying voltage between the center-taps

of the data transformers to transmit power from the PSE

to the PD without affecting data transmission. Figure 10

shows a high level PoE system schematic.

To avoid damaging legacy data equipment that does not

expect to see DC voltage, the PoE spec defines a protocol

that determines when the PSE may apply and remove

power. Valid PDs are required to have a specific 25k com-

mon mode resistance at their input. When such a PD is

connected to the cable, the PSE detects this signature

resistance and turns on the power. When the PD is later

disconnected, the PSE senses the open circuit and turns

power off. The PSE also turns off power in the event of a

current fault or short circuit.

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

classification signature that tells the PSE the maximum

power the PD will draw. The PSE can use this information

toallocatepoweramongseveralports,topolicethecurrent

consumptionofthePD,ortorejectaPDthatwilldrawmore

power than the PSE has available. The classification step

is optional; if a PSE chooses not to classify a PD, it must

assume that the PD is a 13W (full 802.3af power) device.

New in 802.3at

Thenewer802.3atstandardsupersedes802.3afandbrings

several new features:

• A PD may draw as much as 25.5W. Such PDs (and the

PSEs that support them) are known as Type 2. Older

13W 802.3af equipment is classified as Type 1. Type 1

PDs will work with all PSEs; Type 2 PDs may require

Type 2 PSEs to work properly. The LTC4290/LTC4271

APPLICATIONS INFORMATION

is designed to work in both Type 1 and Type 2 PSE de-

signs, and also supports non-standard configurations

at higher power levels.

• The Classification protocol is expanded to allow Type 2

PSEs to detect Type 2 PDs, and to allow Type 2 PDs to

determine if they are connected to a Type 2 PSE. Two

versions of the new Classification protocol are avail-

able: an expanded version of the 802.3af Class Pulse

protocol, and an alternate method integrated with the

existing LLDP protocol (using the Ethernet data path).

The LTC4290/LTC4271 fully supports the new Class

Pulse protocol and is also compatible with the LLDP

protocol(whichisimplementedinthedatacommunica-

tions layer, not in the PoE circuitry).

• Fault protection current levels and timing are adjusted

to reduce peak power in the MOSFET during a fault; this

allows the new 25.5W power levels to be reached using

the same MOSFETs as older 13W designs.

BACKWARD COMPATIBILITY

The LTC4290/LTC4271 chipset is designed to be backward

compatible with the LTC4266, operating in Type 2 mode,

without software changes; only minor layout changes

are required to implement a fully compliant IEEE 802.3at

design.

Some LTC4266 registers have been obsoleted in the

LTC4290/LTC4271chipset.Theobsoletedregistersarenot

required for 802.3at compliant PSE operation. For more

details about software differences between the LTC4266

and LTC4290/LTC4271, refer to the LTC4271 Software

Programming document.

Operation with high power mode disabled is obsoleted in

the LTC4290/LTC4271 chipset. All operations previously

available in low power mode are fully implemented as a

subset of the high power mode capabilities.

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