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Data Sheet

LTC4296-1

THEORY OF OPERATION

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Classification for Discoverable Systems

The SPoE class defines the PSE output voltage range and maxi-

mum power sourced or consumed in the system. PoDL comprises

Class 0 through Class 15. Class 10 through Class 15 are defined

by IEEE 802.3cg for discoverable systems as shown in Table 5 and

Table 6. Class 10 through Class 12 provide a 24 V nominal PSE

output voltage, as required by many industrial, factory automation,

legacy building automation, and wet locations. Each of the three

24 V classes represents a different cabling definition and accompa-

nying maximum power transfer. Class 13 through Class 15 provide

a 54 V nominal PSE output voltage, to maximize power transfer

without exceeding the safety extra low voltage (SELV). Compatible

SPoE, PSE and PD class pairs are shown in Table 7.

PoDL defines the SCCP for classifying a PD. The SCCP uses three

basic symbols: initialization, read-slot, and write-slot. Classification

is performed in the detection state by the host microcontroller using

the SCCP. Refer to the EVAL-SPoE-KIT-AZ Evaluation Kit User

Guide (the LTC4296-1 demonstration board) for additional informa-

tion on how to implement the SCCP with the host microcontroller.

IEEE 802.3cg also supports an optional CRM feature as part of the

SCCP. CRM allows a PSE to allocate additional power otherwise

allocated for cable loss to a PD when connected through less than

the maximum allowed cable resistance.

Table 5. IEEE 802.3cg Class Power Requirements Matrix for PSE and PDs

Class Symbol and Unit

Class Description

Class 10

Class 11

Class 12

Class 13

Class 14

Class 15

VPSE (V)

PSE output voltage

20 to 30

50 to 58

IPI(MAX) (mA)

Cable current

240

632

231

600

1579

PCLASS(MIN) (W)

PSE output power

1.85

4.8

12.63

11.54

VPD(MIN) (V)

PD input voltage

PPD(MAX) (W)

PD power

1.23

3.2

8.4

7.7

RLINK_SEG_LOOP (Ω)

Cable resistance

9.5

Table 6. IEEE 802.3cg Class Power Requirements Matrix Example Link Segment Maximum Distances

Example Cable

Maximum Lengths (m)

Class 10

Class 11

Class 12

Class 13

Class 14

Class 15

14AWG, 14 Gauge Cable

10001

1000

400

10001

1000

400

18AWG, 18 Gauge Cable

1000

400

158

1000

400

158

24AWG, 24 Gauge Cable

300

100

300

100

1 IEEE 802.3cg limits cable length to 1000 m.

Table 7. PSE and PD Class Compatibility Matrix

PD Class

PSE Class

Yes

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