Single-Port PSEs Put Power in More Places

Mar 23, 2007 4:06 PM
By Dilian Reyes, Applications Engineer, Linear Technology, Milpitas, Calif.

Wall-Adapter PSE

Portable devices have different voltage and power requirements. Devices that are PoE capable (PDs) have their own dc-dc conversion built in and thus do not rely on connecting to the correct-voltage wall adapter. When traveling with such devices, a wall adapter is no longer needed; only an Ethernet cable is required. However, in situations where PoE is unavailable, but power is needed for a device such as a battery charger, a simple wall-adapter PSE could be handy to plug into a wall outlet to output PoE power to a PD. Such a device plugs directly into a wall socket. Power goes through an ac-dc converter to provide the PoE supply. This simple device does not require any smarts and only needs to provide power. The LTC4263 provides the PSE control function, enabling safe detection and power on of the PD. Users can also read out an LED driven by the LTC4263 for basic faults should a non-PoE device connect to this wall adapter.

If you need more than one port on a wall adapter, you can use multiple LTC4263s to implement a PoE wall-adapter expander. Power management can be provided by the LTC4263s and set according to the power limits from the supply. This would limit the number of ports of particular power classes that are powered on. Fig. 3 shows a three-port wall adapter configured for 30 W of available power for PD power on. Various combinations of PD power classes can be powered on as long as the set available power is not exceeded. For example, if a Class 3 PD is first powered on, the 15.4-W allocated power for the device leaves enough power for Class 1 or 2 PDs.

In the multiport solution, each port acts independently of the other ports, with the exception of power management. This is also useful for cases where the ports need to be isolated from each other. Here, a port would have its own isolated supply and the power management would not be used.

The wall-adapter PSE does not have to be limited to power. This device can be permanently installed in the wall and connect to an Ethernet data line from a legacy (non-PoE) switch and run through the wall. A PD plugged to the wall port can then receive both data and power. The wall adapter can have an assigned address, which is helpful information in an emergency when a call is made from a VoIP phone connected to that location. A response team would immediately know the location of that call.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.