LT4295 datasheet(26/50 Pages) AD

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LTC9101-1/

LTC9102/LTC9103

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

In semi-auto mode, if a valid signature resistance is

detected and classification is enabled, the port will clas-

sify the PD and report that result as well. The port will then

wait for at least 100ms, and will repeat the detection cycle

to refresh the data in the Port Status registers.

The port will not turn on in response to a power-on com-

mand unless the current detect result is Detect Valid. Any

other detect result will generate a tSTART fault if a power-on

command is received.

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

after Detect Valid is reported and the port is classified, it

is automatically powered on without host intervention. In

auto mode the ICUT-2P, ILIM-2P, and PCUT-4P thresholds are

automatically set; see the Reset and the AUTO Pin section

for more information.

Detection is disabled for a port when the LTC9101-1/

LTC9102/LTC9103 is initially powered up with AUTO low,

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

sponding 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. Note that PDs in this range

of capacitance are defined as invalid, so a PSE that pow-

ers legacy PDs is noncompliant with the IEEE standard. The

LTC9101-1/LTC9102/LTC9103 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 detects either

a valid IEEE PD or a high-capacitance legacy PD. With legacy

mode disabled, only valid IEEE PDs will be recognized.

If a nonstandard PD presents an invalid detection signa-

ture not included by legacy detection, the LTC9101-1/

LTC9102/LTC9103 may be configured to perform classi-

fication and/or apply power regardless of detection result.

To accomplish this, the LTC9101-1/LTC9102/LTC9103

introduces per-port Force Power and Class Event over-

rides. These overrides intentionally defeat compliance

checks. See the LTC9101-1 Software Programming docu-

mentation for details.

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 16, with the current level indicating

one of five possible PD signatures. Figure 15 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 10 shows the possible clas-

sification values.

Figure 15.

VOLTAGE (VCLASS)

9101123 F15

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 16.

9101123 F16

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

CLASS

POWER ON

Type 1 or 2 PSE, 1-Event Class Sequence

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