Datasheet

57
4237
3020.514.510.12.7
D
etection
L
ower Limit
Detection Upper Limit C
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Classification Upper Limit M
ust Turn Of
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oltage Falli
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Lower Limit - Operating Range
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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
IEEE 802.
3at
Normal Operation
T2 Reset Range
42.5
IEEE 802-2008
TPS2379
SLVSB98 –MARCH 2012 www.ti.com
DETAILED DESCRIPTION
PoE OVERVIEW
The following text is intended as an aid in understanding the operation of the TPS2379 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 may 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 a 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.
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Figure 18. Threshold Voltages
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