TPS23750
TPS23770
www.ti.com
SLVS590A–JULY 2005–REVISED AUGUST 2005
APPLICATION INFORMATION
PoE OVERVIEW
The following text is intended as an aid in understanding the operation of the TPS23750 but not as a substitute
for the actual IEEE 802.3af standard. Standards change and should always be referenced when making design
decisions.
The IEEE 802.3af specification defines a method of safely powering a PD over a cable, and then removing
power if a PD is disconnected. The process proceeds through the three operational states of detection,
classification, and operation. The PSE leaves the cable unpowered 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, then the PSE may optionally inquire
how much power the PD requires; this is referred to as classification. The PD may return a default full-power
signature, or one of four other choices. The PSE may then power the PD if it has adequate capacity. Once
started, the PD must present the maintain power signature (MPS) to assure the PSE that it is still there. 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 initial state of detection. Figure 29 shows the operational states as a function of PD input voltage.
The PD input is typically an RJ-45 eight-lead connector which is referred to as the power interface (PI). PD input
requirements differ from PSE output requirements to account for voltage drops in the cable and operating
margin. The specification uses a cable resistance of 20 Ω to derive the voltage limits at the PD from the PSE
output requirements. Although the standard specifies an output power of 15.4 W at the PSE output, only 12.95 W
is available at the input of the PD due to the worst-case power loss in the cable.
The PSE can apply voltage either between the RX and TX pairs (pins 1–2 and 3–6), or between the two spare
pairs (4–5 and 7–8). The applied voltage can be of either polarity and can only be applied to one set of pairs at a
time. The PD uses input diode bridges to accept power from any of the possible PSE configurations. The voltage
drops associated with the input bridges create a difference between the IEEE 802.3af limits at the PI and the
TPS23750 specifications.
The PSE is required to current limit at an average of between 350 mA and 400 mA during normal operation, and
it must disconnect the PD if it draws this current for more than 75 ms. Class 0 and 3 PDs may draw up to
400 mA peak currents. The PSE may set lower output current limits based on the PD’s declared power
requirements, as discussed below.
Shut-
Normal Operation
Classify
Detect
down
0
2.7
10.1 14.5
20.5
30
PI Voltage (V)
36
57
42
Figure 29. IEEE 802.3 PD Limits
15
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