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LTC4270/LTC4271

42701f

APPLICATIONS INFORMATION

the classification signature current (in this case, Class 3)

in the VCLASS range. Table 4 shows the possible clas-

sification values.

Table 4. 802.3af and 802.3at Classification Values

CLASS

RESULT

Class 0

No Class Signature Present; Treat Like Class 3

Class 1

Class 2

Class 3

13W

Class 4

25.5W (Type 2)

If classification is enabled, the port will classify the PD

immediately after a successful detection cycle in semi-auto

or AUTO pin modes, or when commanded to in manual

mode. It measures the PD classification signature by ap-

plying 18V for 12ms (both values typical) to the port via

the OUTn pin and measuring the resulting current; it then

reports the discovered class in the Port Status register.

If the LTC4270/LTC4271 is in AUTO pin mode, it will ad-

ditionally 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,orwhenthecorrespondingClassEnablebitiscleared.

VOLTAGE (VCLASS)

510

42701 F13

TYPICAL

CLASS 3

PD LOAD

LINE

48mA

33mA

PSE LOAD LINE

23mA

14.5mA

6.5mA

CLASS 4

CLASS 2

CLASS 1

CLASS 0

CLASS 3

OVER

CURRENT

Figure 13. PD Classification

802.3at 2-Event Classification

The 802.3at specification defines two methods of classify-

ing a Type 2 PD. A-grade and B-grade LTC4270/LTC4271

parts support 802.3at 2-event classification.

One method adds extra fields to the Ethernet LLDP data

protocol; although the LTC4270/LTC4271 is compatible

with this classification method, it cannot perform clas-

sification 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 LTC4270/LTC4271 supports

changing 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 supported by

the LTC4270/LTC4271. A Type 2 PD that is requesting

more than 13W will indicate Class 4 during normal

802.3af classification. If the LTC4270/LTC4271 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 determines it is connected to a Type 1

PD and does not run the second classification cycle.

Invalid Type 2 Class Combinations

The 802.3at specification defines a Type 2 PD class signa-

ture 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 LTC4270/LTC4271 will power a detected PD regardless

of the classification results, with one exception: if the PD

presents an invalid Type 2 signature (Class 4 followed by

Class 0 to 3), the LTC4270/LTC4271 will not provide power

and will restart the detection process. To aid in diagnosis,

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