Satisfying the requirements set forth in the IEEE 802.3af standard for Power over Ethernet (PoE) isn't a trivial task, particularly with respect to the power-management operations required of power sourcing equipment (PSE). According to IEEE 802.3af, PSE must perform several precision measurements on the powered device (PD) to ensure that the PD complies with the standard. If the PD complies, the PSE may then apply 48-V power to it.
To detect a PD, the PSE must check for a 25-kΩ impedance (within a defined tolerance) by performing a two-point measurement of the current drawn between 2.7 and 10 V. A two-point measurement is needed to offset the effects of varying cable lengths, the mandated diode bridge, and other leakage currents. Such system-level variations make the impedance measurement challenging.
After PD detection, another challenge for PSE is to classify the PD according to its power consumption. (The IEEE standard defines different current ranges for each class.) This optional step requires the PSE to take another impedance measurement at a slightly higher voltage. Still another hurdle for the PSE to overcome occurs during normal operation. During this time, the PSE must monitor the current drawn by the PD to detect disconnection and different stages of overcurrent.
The LTC4258/4259 powered Ethernet controllers incorporate circuits specifically designed for each of these tasks. Moreover, these chips are specified and tested for all of the above conditions to help ensure that the PSE in which they're used meets the IEEE standard. In addition to the features previously mentioned, these ICs provide inrush current control, short-circuit protection, power-good indication, and on/off control.
Combining the LTC4258/4259 controllers on the PSE side with an LTC4257 controller on the PD side creates a complete PoE power-management design (see the figure). The LTC4257 features a 25-kΩ signature resistance, programmable classification, power switching, and fault protection such as inrush current limiting and thermal limiting.
Another feature of the LTC4257 is an integrated 100-V power MOSFET that keeps the PD circuitry disconnected from the Ethernet cable until the input voltage reaches a valid level. Housed in a SO-8 package, the LTC4257 starts at $1.65 each in quantities of 1000.
Linear Technology Corp.