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APPLICATION INFORMATION

OVERVIEW

Normal Operation

57

42

36

30

20.5

14.5

10.1

2.7

Detect

Classify

Shut -

down

PI Voltage (V)

0

INTERNAL THRESHOLDS

TPS2375-1

TPS2377-1

SLVS570 – MARCH 2005

The IEEE 802.3af specification defines a process for safely powering a powered device (PD) over a cable, and

then removing power if a PD is disconnected. The process proceeds through three operational states: detection,

classification, and operation. The intent behind the process is to leave an unterminated cable unpowered while

the PSE periodically checks for a plugged-in device; this is referred to as detection. The low power levels used

during detection are unlikely to cause damage to 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. Knowing the power

demand of each PD allows the PSE to intelligently allocate power between PDs, and also to protect itself against

overload. The PSE powers up a valid PD, and then monitors its output for overloads. The maintain power

signature (MPS) is presented by the powered PD to assure the PSE that it is there. The PSE monitors its output

for the MPS to see if the PD is removed, and turns the port off, if it loses the MPS. Loss of MPS returns the PSE

to the initial state of detection. Figure 16 shows the operational states as a function of PD input voltage range.

The PD input is typically an RJ-45 (8-pin) connector, referred to as the power interface (PI). PD input

requirements differ from PSE output requirements to account for voltage drops in the cable and 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, there is only

12.95 W 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, or between the two spare pairs as shown in

Figure 1. The applied voltage can be of either polarity. The PSE cannot apply voltage to both paths at the same

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 cause a difference between the IEEE 802.3af limits at the PI and the

TPS2375-1 specifications.

The PSE is required to current limit 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. The PSE may set lower output current limits

based on the PD advertised power requirements, as discussed in the Classification section.

The following discussion is intended as an aid in understanding the operation of the TPS2375-1, but not as a

substitute for the actual IEEE 802.3af standard. Standards change and should always be referenced when

making design decisions.

Figure 16. IEEE 802.3 PD Limits

In order to implement the PoE functionality as shown in Figure 16, the TPS2375-1 has a number of internal

comparators with hysteresis for stable switching between the various states. Figure 17 relates the parameters in

the Electrical Characteristics section to the PoE states. The mode labeled idle between classification and

detection implies that the DET, CLASS, PG, and RTN pins are all high impedance.

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