9.1 Application Information

The IEEE 802.3af specification defines a process for safely providing power over an ethernet cable when a capable device is connected, and then removing power if it is disconnected. The process proceeds through three operational states: detection, classification, and operation. An unterminated cable is not powered. The PSE periodically probes the cable with low voltage, looking for a 25 kΩ signature; 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 powered device (PD) signature is present during detection, then the PSE may optionally inquire about the amount of power the PD requires; this is referred to as classification. The PD may return a default full-power signature, or one of four other defined choices. In a high-power system, class may not be required, or the levels may be redefined to suit that particular system. The PSE may use the class power to determine if it has adequate power to operate this device, and later to determine if a device is using more power than it requested. At this point in the process, the PSE may choose to power the PD. The PSE output is protected against shorts and overloads when the PD is powered. The maintain power signature (MPS) is presented by the powered PD to assure the PSE that it is present. The MPS is either a minimum dc current, a maximum ac impedance, or both. When the MPS disappears, the PSE removes power and returns to its initial state. Figure 11 shows the operational states as a function of PD input voltage range as defined in IEEE 802.3af.

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. The IEEE 802.3af specification uses a cable resistance of 20 Ω to derive the voltage limits at the PD from the PSE output requirements. While the 20 Ω specification covers telecom type wiring, CAT-5 infrastructure will meet a 12.5 Ω limit. Specifying the high-power system to operate over CAT-5 cable allows significantly more power to be delivered.

A high-power nonstandard system need not support all combinations of voltage delivery polarities and pair sets. The IEEE 802.3af PSE allows voltage of either polarity between the RX and TX pairs, or between the two spare pairs. An input diode or bridge is recommended to provide reverse input polarity protection. The bridge maintains compatibility with auto-MDIX systems that have reverse RX-TX pair assignments. The voltage drops associated with the input diode(s) cause a difference between the limits at the PI and the TPS2376-H specifications.

Two-pair power delivery is the simplest to implement, and is preferred if adequate power can be achieved. Application report SLVA225 presents a number of considerations for a high power PoE end-to-end system. Power delivery on all four pairs is significantly more complex, and is only recommended when two pair systems do not suffice. Considerations for high power systems are presented in Application Report SLVA225.

The following discussion is intended as an aid in understanding the operation of the TPS2376-H, but not as a substitute for the IEEE 802.3af standard. Standards change and should always be referenced when making design decisions.

Figure 11. IEEE 802.3 PD Voltage Limits