Four pair, high current
Four pair, high current
Combining high current circuitry with four-pair hookup allows more power down the cable than any other technique. Four-pair high current allows as much as 50W to be delivered to the PD over a 100m CAT-5 cable, more if the cable length is kept short. Although this scheme includes the drawbacks of all of the previous schemes, it is the highest power option available.
Beyond 50W, long cables rapidly approach an "impedance matched" situation where the cable dissipates more power than it delivers to the PD. If the length of the cable is kept short, the current level can be further increased, ultimately limited by the RJ45 connector, offset current in the magnetics and the temperature rise in the CAT-5 cable. Extremely high power (>50W) should only be used in systems where the entire solution is specified by the same supplier.
Classification: When to apply high power
Notably absent from the above circuits is a method to determine when it is appropriate to apply high power to the line. All of the techniques will successfully power standard 802.3af PDs under normal conditions. The dual-threshold circuits need to have some information from the PD to know when to switch thresholds, and the four-pair schemes need to know when it is appropriate to switch on the second set of conductors. The IEEE 802.3at committee is working to address these issues, but no schemes have yet been finalized. In the interim, ad-hoc solutions are needed to identify high power PDs.
802.3af defines an unused class (Class 4) that looks tailor-made for high power, and both the LTC4258/59 PSE chips and the LTC4257/67 PD chips support Class 4. Unfortunately, a Class 4 PD will be powered with Class 3 current limits if it is plugged into a standard 802.3af PSE, which may cause it to cycle on and off repeatedly if it attempts to draw higher power. Class 4 can be used as a "warning" that a high power PD is connected, but it is advisable to have an additional handshake before higher power is delivered. Ideally, a high power PD should receive some sort of signal from a high-power PSE, acknowledging it is acceptable to operate in a high power mode. If no handshake is received, the PD should give some sort of indication to the user that it is plugged into the wrong kind of PSE.
Recommendations
The best technique to use for pre-standard high power depends on the application. At power levels up to 30W, the two-pair high current techniques provide the lowest cost and complexity, and full 802.3af compliance by using the dual-threshold circuit. If maximum power is required (50W or more), the four-pair high current circuit is the best choice
About the authors:
Todd Nelson has been the Marketing Manager for Linear Technology's Mixed Signal Business Unit since 1998. He is responsible for all data converter products, interface transceivers, system monitoring products, Hot Swap controllers and powered Ethernet devices. He has held various marketing and applications positions at Linear Technology and at National Semiconductor prior to joining Linear Technology in 1995. Todd earned his Bachelor's degree in Engineering from Kettering University, and his Masters in Engineering Management from Santa Clara University. He has published several technical articles and papers and participated on various standards committees. Todd enjoys bicycling with his wife and son in his leisure time.
