LTC4263 datasheet(18/28 Pages) LINER

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LTC4274

4274fa

APPLICATIONS INFORMATION

The port will then wait for at least 100ms (or 2 seconds if

midspan mode is enabled), and will repeat the detection

cycle to ensure that the data in the Port Status register

is up-to-date.

If the port is in semi-auto mode and high power opera-

tion is enabled, the port will not turn on in response to

a power-on command unless the current detect result is

detect good. Any other detect result will generate a tSTART

fault if a power-on command is received. If the port is not

in high power mode, it will ignore the detection result and

apply power when commanded, maintaining backwards

compatibility with the LTC4259A.

Behavior in AUTO pin mode is similar to semi-auto; however,

after Detect Good is reported and the port is classified (if

classification is enabled), it is automatically powered on

without further intervention. In standalone (AUTO pin)

mode, the ICUTandILIMthresholdsareautomaticallyset;see

the Reset and the AUTO Pin section for more information.

The signature detection circuitry is disabled when the

port is initially powered up with the AUTO pin low, in

shutdown mode, or when the corresponding Detect

Enable bit is cleared.

Detection of Legacy PDs

Proprietary PDs that predate the original IEEE 802.3af

standard are commonly referred to today as legacy de-

vices. One type of legacy PD uses a large common mode

capacitance (>10μF) as the detection signature. Note that

PDs in this range of capacitance are defined as invalid, so

a PSE that detects legacy PDs is technically noncompliant

with the IEEE spec.

The LTC4274 can be configured to detect this type of

legacy PD. Legacy detection is disabled by default, but

can be manually enabled. When enabled, the port will

report Detect Good when it sees either a valid IEEE PD or

a high-capacitance legacy PD. With legacy mode disabled,

only valid IEEE PDs will be recognized.

CLASSIFICATION

802.3af Classification

A PD can optionally present a classification signature to

the PSE to indicate the maximum power it will draw while

operating. The IEEE specification defines this signature as

a constant current draw when the PSE port voltage is in the

VCLASS range (between 15.5V and 20.5V), with the current

level indicating one of 5 possible PD classes. Figure 14

shows a typical PD load line, starting with the slope of the

25kΩ signature resistor below 10V, then transitioning to

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

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

sification values.

Table 4. 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

Figure 13. PD Classification

VOLTAGE (VCLASS)

510

4274 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

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