Datasheet

57
4237
3020.514.510.12.7
D
etecti
on
Lo
wer L
imit
Detection Upper Limit
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L
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Classification Upper Limit Mus
t Turn
Off by
-
Volta
ge Fal
ling
Lower Limit - Operating Range
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ta
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Detect
Classify
Shut-
down
PIVoltage(V)
0
Lower Limit - 13W Op.
Mark
Class-Mark Transition
250ms
Transient
6.9
NormalOperation
IEE
E 802
.3-200
5
IEEE
802.3
at
NormalOperation
T2 Reset Range
42.5
TPS23754
TPS23754-1
TPS23756
www.ti.com SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013
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 are 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 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 (for example, 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.
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