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LTC4274

4274fa

APPLICATIONS INFORMATION

the OUT pin and measuring the resulting current; it then

reports the discovered class in the Port Status register. If

the LTC4274 is in AUTO pin mode, it will additionally use

the classification result to set the ICUT and ILIM thresholds.

See the Reset and the AUTO/MID Pin section for more

information.

The classification circuitry is disabled when the port is

initially powered up with the AUTO pin low, in shutdown

mode, or when the corresponding Class Enable bit is

cleared.

802.3at 2-Event Classification

The 802.3at spec defines two methods of classifying a

Type 2 PD.

One method adds extra fields to the Ethernet LLDP data

protocol; although the LTC4274 is compatible with this

classification method, it cannot perform classification

directly since it doesn’t have access to the data path.

LLDP classification requires the PSE to power the PD as

a standard 802.3af (Type 1) device. It then waits for the

host to perform LLDP communication with the PD and

update the PSE port data. The LTC4274 supports chang-

ing the ILIM and ICUT levels on the fly, allowing the host

to complete LLDP classification.

The second 802.3at classification method, known as

2-event classification or ping-pong, is fully supported by

the LTC4274. A Type 2 PD that is requesting more than

13W will indicate Class 4 during normal 802.3af classifi-

cation. If the LTC4274 sees Class 4, it forces the port to a

specified lower voltage (called the mark voltage, typically

9V), pauses briefly, and then re-runs classification to

verify the Class 4 reading (Figure 1). It also sets a bit in

the High Power Status register to indicate that it ran the

second classification cycle. The second cycle alerts the

PD that it is connected to a Type 2 PSE which can supply

Type 2 power levels.

2-event ping-pong classification is enabled by setting a bit

in the port’s High Power Mode register. Note that a ping-

pong enabled port only runs the second classification cycle

when it detects a Class 4 device; if the first cycle returns

Class 0 to 3, the port assumes it is connected to a Type

1 PD and does not run the second classification cycle.

Invalid Type 2 Class Combinations

The 802.3at spec defines a Type 2 PD class signature as

two consecutive Class 4 results; a Class 4 followed by a

Class 0-3 is not a valid signature. In AUTO pin mode, the

LTC4274 will power a detected PD regardless of the clas-

sification results, with one exception: if the PD presents

an invalid Type 2 signature (Class 4 followed by Class 0

to 3), the LTC4274 will not provide power and will restart

the detection process. To aid in diagnosis, the Port Status

so an invalid Class 4–Class 2 combination would report

a second class pulse was run in the High Power Status

and Class 2 in the Port Status register.

POWER CONTROL

External MOSFET, Sense R Summary

The primary function of the LTC4274 is to control the

delivery of power to the PSE port. It does this by control-

ling the gate drive voltage of an external power MOSFET

while monitoring the current via an external sense resis-

tor and the output voltage at the OUT pin. This circuitry

serves to couple the raw VEE input supply to the port in

a controlled manner that satisfies the PD’s power needs

while minimizing power dissipation in the MOSFET and

disturbances on the VEE backplane.

The LTC4274 is designed to use 0.25Ω sense resistors to

minimize power dissipation. It also supports 0.5Ω sense

resistors, which are the default when LTC4258/LTC4259A

compatibility is desired.

Inrush Control

Once the command has been given to turn on the port,

the LTC4274 ramps up the GATE pin of the port’s external

MOSFET in a controlled manner. Under normal power-up

circumstances, the MOSFET gate will rise until the port

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