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Figure 10. PD Classification

APPLICATIONS INFORMATION

LTC9101-3/LTC9103

Rev. 0

For more information www.analog.com

VOLTAGE (VCLASS)

91013 F10

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

≈

Detection of Legacy PDs

Proprietary PDs that predate the original IEEE 802.3af

standard are commonly referred to today as legacy

devices. 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 powers legacy PDs is technically noncom-

pliant with the IEEE spec. The LTC9101-3/LTC9103 can

be configured to detect this type of legacy PD using the

LEGACY pin. When enabled, the port will accept the detec-

tion result as valid when it sees either a valid IEEE PD or a

high-capacitance legacy PD. With legacy mode disabled,

only valid IEEE PDs will be powered.

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

current level indicating one of 5 possible PD class sig-

natures. Figure 10 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 3 shows

the possible classification values.

Table 3. 802.3af and 802.3at Classification Values

CLASS

RESULT

No Class Signature Present; Treat Like Class 3

13W

25.5W (Type 2)

If classification is enabled, the PSE will classify the PD

immediately after a successful detection cycle. It mea-

sures the PD classification signature by applying 18V for

12ms (both values typical) via the OUTn pin and measures

the resulting current. The LTC9101-3/LTC9103 uses the

classification result to set the ICUT, ILIM, and PCUT thresh-

olds as shown in Table 4.

Table 4. ICUT, ILIM and PCUT Values

PSE ASSIGNED

CLASS

ICUT

ILIM

PCUT

94mA

425mA

4.86W

150mA

425mA

7.56W

338mA

425mA

16.2W

638mA

850mA

32.4W

802.3at 2-Event Classification

In 802.3at, 802.3af classification is named Type 1 classi-

fication. The 802.3at standard introduces an extension of

Type 1 classification: Type 2 (2-event) classification. Type

2 PSEs are required to perform classification.

A Type 2 PD requesting 25.5W presents class signature 4

during all class events. If a Type 2 PSE with 25.5W of

available power measures class signature 4 during the

first class event, it forces the PD to VMARK (9V typical),

pauses briefly, and issues a second class event as shown

in Figure 12. The second class event informs the PD that

the PSE has allocated 25.5W.

Note that the second classification event only runs if

required by the IEEE classification procedure. For exam-

ple, a single-signature Class 0 to 3 PD will only be issued

a single class event as shown in Figure 11.

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