LTC4271 datasheet(16/30 Pages) LINER

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LTC4266A/LTC4266C

4266acfc

OPERATION

Figure 10. Power over Ethernet System Diagram

4266AC F10

S1B

SMAJ58A

0.22μF

100V

X7R

1μF

100V

X7R

SMAJ58A

58V

DATA PAIR

VEE

SENSE GATE OUT

VDD

INT

SCL

SDAIN

SDAOUT

0.25Ω

SPARE PAIR

1/4

LTC4266

DGND

AGND

I2C

3.3V

INTERRUPT

–48V

CAT 5

20Ω MAX

ROUNDTRIP

0.05μF MAX

RJ45

1N4002

PSE

RCLASS

–48VIN

PWRGD

–48VOUT

LTC4265

GND

DC/DC

CONVERTER

5μF ≤ CIN

≤ 300μF

–

VOUT

GND

0.1μF

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

common-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

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 LTC4266A/LTC4266C

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 LTC4266A/LTC4266C 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.

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