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

SLVS885D – OCTOBER 2008 – REVISED DECEMBER 2009

www.ti.com

DETAILED DESCRIPTION

PoE OVERVIEW

The following text is intended as an aid in understanding the operation of the TPS23754 but not as a substitute

for the IEEE 802.3at standard. The IEEE 802.3at standard is an update to IEEE 802.3-2008 clause 33 (PoE),

adding high-power options and enhanced classification. Generally speaking, a device compliant to IEEE

802.3-2008 is referred to as a type 1 device, and devices with high power and enhanced classification will be

referred to as type 2 devices. Standards change and should always be referenced when making design

decisions.

The IEEE 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, 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 802.3-2008, 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 802.3-2008 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 type 2 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.

16

Submit Documentation Feedback

Copyright © 2008–2009, Texas Instruments Incorporated