LTC4271 datasheet(17/30 Pages) LINER

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LTC4274A/LTC4274C

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OPERATION

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

to allocate power among several ports, police the current

consumption of the PD, or to reject a PD that will draw

more power that the PSE has available. For a 802.3af PSE,

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

The newer 802.3at standard supersedes 802.3af and brings

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 LTC4274A/LTC4274C

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 LTC4274A/LTC4274C fully supports the new Class

Pulse protocol and is also compatible with the LLDP

protocol (which is implemented in the data communica-

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.

Extended Power LTPoE++

The LTC4274A adds the capability to autonomously deliver

up to 90W of power to the PD. LTPoE++ PDs may forego

802.3 LLDP support and rely solely on the LTPoE++ Physi-

cal Classification to negotiate power with LTPoE++ PSEs;

this greatly simplifies high-power PD implementations.

LTPoE++ classification may be optionally enabled for the

LTC4274A by setting both the High Power Enable and

LTPoE++ Enable bits.

The higher levels of LTPoE++ delivery impose additional

layout and component selection constraints. The LTC4274A

is offered in four power levels (-1, -2, -3, and -4) which

allows the AUTO pin mode LTC4274A to autonomously

power up to supported power levels. If the AUTO pin is

high, internal circuitry determines the maximum deliver-

able power. PDs requesting more than the available power

limits are not powered.

Table 1. LTPoE++ Auto Pin Mode Maximum Delivered

Power Capabilities

PART

PAIRS

PD POWER

LTC4274A-1

38.7W

LTC4274A-2

52.7W

LTC4274A-3

70W

LTC4274A-4

90W

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