LTC4270SLASH datasheet(20/32 Pages) LINER

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LTC4290/LTC4271

429071f

APPLICATIONS INFORMATION

More on Operating Modes

The port’s operating mode determines when the LTC4290/

LTC4271 runs a detection cycle. In manual mode, the port

will idle until the host orders a detect cycle. It will then

run detection, report the results, and return to idle to wait

for another command.

In semi-auto mode, the LTC4290/LTC4271 autonomously

polls a port for PDs, but it will not apply power until com-

manded to do so by the host. The Port Status register is

updated at the end of each detection cycle.

If a valid signature resistance is detected and classification

is enabled, the port will classify the PD and report that

result as well. 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

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. In high power

mode the port must be placed in manual mode to force a

port on regardless of detect outcome.

BehaviorinAUTOpinmodeissimilartosemi-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 Pins 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

ProprietaryPDsthatpredatetheoriginalIEEE802.3afstan-

dardarecommonlyreferredtotodayaslegacydevices.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 LTC4290/LTC4271 can be configured to

detect this type of legacy PD. Legacy detection is disabled

bydefault,butcanbemanuallyenabledonaper-portbasis.

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 may 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

currentlevelindicatingoneof5possiblePDclasses.Figure

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

the 25k 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. 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)

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