57

42

37

30

20.5

14.5

10.1

2.7

Detect

Classify

Shut-

down

PIVoltage(V)

0

Mark

6.9

NormalOperation

NormalOperation

42.5

TPS23754

TPS23754-1

TPS23756

The IEEE 802.3-2005 (802.3at) standard defines a method of safely powering a PD (powered device) over a

cable by power sourcing equipment (PSE), and then removing power if a PD is disconnected. The process

proceeds through an idle state and three operational states of detection, classification, and operation. The PSE

leaves the cable unpowered (idle state) while it periodically looks to see if something has been plugged in; this is

referred to as detection. The low power levels used during detection are unlikely to damage devices not designed

for PoE. If a valid PD signature is present, the PSE my inquire how much power the PD requires; this is referred

to as classification. The PSE may then power the PD if it has adequate capacity.

Type 2 PSEs are required to do type 1 hardware classification plus a (new) data-layer classification, or an

enhanced type 2 hardware classification. Type 1 PSEs are not required to do hardware or data link layer (DLL)

classification. A type 2 PD must do type 2 hardware classification as well as DLL classification. The PD may

return the default 12.95W (often referred to as 13W) current-encoded class, or one of four other choices. DLL

classification occurs after power-on and the ethernet data link has been established.

Once started, the PD must present the maintain power signature (MPS) to assure the PSE that it is still present.

The PSE monitors its output for a valid MPS, and turns the port off if it loses the MPS. Loss of the MPS returns

the PSE to the idle state. Figure 21 shows the operational states as a function of PD input voltage. The upper

half is for IEEE 802.3-2005, and the lower half shows specific differences for IEEE 802.3at. The dashed lines in

the lower half indicate these are the same (e.g., Detect and Class) for both.

Figure 21. Operational States for PD

The PD input, typically an RJ-45 eight-lead connector, is referred to as the power interface (PI). PD input

requirements differ from PSE output requirements to account for voltage drops and operating margin. The

standard allots the maximum loss to the cable regardless of the actual installation to simplify implementation.

IEEE 802.3-2005 was designed to run over infrastructure including ISO/IEC 11801 class C (CAT3 per

TIA/EIA-568) that may have had AWG 26 conductors. IEEE 802.3at cabling power loss allotments and voltage

drops have been adjusted for 12.5

Ω power loops per ISO/IEC11801 class D (CAT5 or higher per TIA/EIA-568,

typically AWG #24 conductors). Table 2 shows key operational limits broken out for the two revisions of the

standard.

Table 2. Comparison of Operational Limits

POWER LOOP

PSE

PSE STATIC

PD INPUT

STATIC PD INPUT VOLTAGE

RESISTANCE

OUTPUT POWER

OUTPUT VOLTAGE

POWER

STANDARD

POWER

≤

POWER >

(max)

(min)

(min)

(max)

12.95 W

12.95 W

'2005

20

Ω

15.4 W

44 V

12.95 W

37 V–57 V

N/A

802.3at

12.5

Ω

36 W

50 V

25.5 W

37 V–57 V

42.5 V–57 V

16

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