Datasheet

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4237
3020.514.510.12.7
D
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Detection Upper Limit
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Classification Upper Limit Mus
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Lower Limit - Operating Range
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Detect
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Shut-
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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
TPS23757
www.ti.com SLVS948D –JULY 2009–REVISED NOVEMBER 2013
DETAILED DESCRIPTION
PoE OVERVIEW
The following text is intended as an aid in understanding the operation of the TPS23757 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 will be referred to as a type 1 device (PD or PSE), and devices with high power and enhanced
classification will be referred to as type 2 devices (PD or PSE). Standards change and should always be
referenced when making design decisions. The TPS23757 supports type 1 PDs as a result of the limited-current
capability and lack of type 2 hardware class detection. Type 1 devices are encompassed within the new
standard, providing the same features and functions as devices in service since 2003.
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
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. A type 1 PD may
have passive classification ( class 0, < 5 mA) or active type 1 hardware class (1 through 3) per IEEE 802.3-2008.
DLL communication occurs after power-on and the ethernet data link has been established by the applications
circuits in the PD (not the power interface). It may be used by type 1 PDs and must be implemented by type 2
PDs.
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 18 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 18. Operational States for PD
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