LTC4292 datasheet(22/44 Pages) AD

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LTC9101-2/LTC9102

Rev. 0

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APPLICATIONS INFORMATION

Status or Channel Status register, as appropriate. If the

LTC9101-2/LTC9102 is in auto mode, it will additionally

use the classification result to set 2P Police, 4P Police,

4P PCUT Enable and DC Disconnect thresholds.

Classification is disabled for a port when the LTC9101-2/

LTC9102 is initially powered up with Auto mode disabled,

when the port is in shutdown mode, or when the corre-

sponding Class Enable bit is cleared.

Figure 14.

VOLTAGE (VCLASS)

9101-2 F14

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

≈

PD Classification

Figure 15.

9101-2 F15

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

CLASS

POWER ON

Type 1 or 2 PSE, 1-Event Class Sequence

Table 9. Type 1 and Type 2 PD Classification Values

CLASS

RESULT

Class 0

No Class Signature Present; Treat Like Class 3

Class 1

3.84W

Class 2

6.49W

Class 3

13W

Class 4

25.5W (Type 2)

auto mode, 2P Police, 4P Police, and DC Disconnect are

automatically set; see the Reset and the PWRMD section

for more information.

Detection is disabled for a port when the LTC9101-2/

LTC9102 is initially powered up in Auto mode, when the

port is 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 stan-

dard are commonly referred to today as legacy PDs. One

type of legacy PD uses a large common-mode capacitance

(>10μF) as the detection signature.

When large capacitance devices are detected, connec-

tion check will be skipped, and detection/classification will

only be performed on the channel where a large capaci-

tance device was detected. Legacy PDs may be inferred

by detection and classification result, per the Palpatine

specification. Legacy PDs are not automatically powered

in Auto mode.

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 sig-

nature as a constant current draw when the PSE port

voltage is in the VCLASS range (between 15.5V and 20.5V)

as shown in Figure 15, with the current level indicating

one of five possible PD signatures. Figure 14 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 9 shows the possible clas-

sification values.

If classification is enabled, the PSE will classify the PD

immediately after a successful detection cycle. The PSE

measures the PD classification signature by applying

VCLASS to the port via OUTn and measuring the resulting

current; it then reports the discovered class in the Port

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