LTC4271 datasheet(20/30 Pages) LINER

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LTC4266A/LTC4266C

4266acfc

APPLICATIONS INFORMATION

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 ICUT and ILIM thresholds are automatically set;

see the Reset and the AUTO/MID 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 LTC4266A/LTC4266C can be configured to detect

this type of legacy PD. Legacy detection is disabled by

default, but can be manually enabled on a per-port basis.

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 13

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 5 shows the possible clas-

sification values.

Table 5. 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 LTC4266A/LTC4266C 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.

Figure 13. PD Classification

VOLTAGE (VCLASS)

510

4266AC 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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