The LTC4263 from Linear Technology is a single-chip Power over Ethernet (PoE) solution for a Power Sourcing Equipment (PSE) controller that is compliant with IEEE 802.3af. The device requires only input and output decoupling capacitors, a programming resistor, a transient voltage surge suppressor, and a fuse (see figure).
The device connects to the signal transformers that interface the PSE to the Ethernet cable. It performs detection and classification of the device at the opposite end of the cable, and if this device is a powered device (PD), then 48 V is supplied to its front-end power supply. The front-end supply of the PD then outputs an isolated voltage for use by the main load.
Stand-alone functionality is given special emphasis and enabled with an onboard control algorithm without the need of a microcontroller. By incorporating a MOSFET power switch and current-sense resistor in a 4-mm x 3-mm DFN package, the LTC4263 provides a compact stand-alone solution. The device can also operate independently of a microprocessor, a characteristic of other devices offered by Linear.
While the device supports a single channel, it facilitates the design of low-port-count PSE systems, because multiple devices can be driven from a single 48-V bus while remaining independently controllable. The controllers can also be connected together with a current share bus, whose voltage provides an indication of the total current consumption for all devices on the bus. These features enable advanced power management features for the system, such as limiting power to lower-priority ports in the event of a load spike or excessive total power consumption.
In addition to power management, the controller also provides protection to PD equipment. For example, the devices enable hot swapping, and either ac or dc disconnect sensing of the load is selected with a single pin on the controller.
As PoE has matured, endpoint systems, in which the PSE is an integral part of the Ethernet switch, are displacing midspan equipment, which is connected between the switch and PD. According to Bob Reay, Vice President and General Manager of Mixed Signal Products for Linear Technology, the next significant trend will be the increase of PoE power levels. The LTC4263 also supports this trend with the ability to deliver power using both the TX/RX twisted pairs together with the two spare twisted pairs available in Ethernet cable.
