Single-Port PSEs Put Power in More Places

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

Single-Port PSE Controller

In many applications, a single port or just a few ports with PoE capability are required. For these applications, one of the multichannel controllers is typically used, and unused channels are left floating. However, Linear Technology’s LTC4263, an autonomous single-channel PSE controller, permits an alternative approach in such applications (Fig. 2).
The LTC4263 provides fully compliant IEEE 802.3af PD detection, classification, port turn on, fault monitoring, and ac or dc disconnect sensing. In addition, an efficient LED driver provides the user with port status, indicating port on or faults through a single LED. Selectable options include a midspan back-off timer, legacy detection and power class enforcement set by tying their respective pin to VSS or VDD5. The LTC4263 features simple power management, programmed with an RC that works across multiple LTC4263s in a multiple-port application. An analog approach is used through a common PWRMGT pin with multiple LTC4263s to perform the power management described previously. The key advantage of the LTC4263 is its independent operation without the need for a microcontroller.

In addition to eliminating the microcontroller and its peripheral components, the LTC4263 also has an onboard power MOSFET that properly switches the negative rail, and a built-in ac disconnect signal and digital supply. Fig. 2 shows a minimum component circuit for operation with the LTC4263. An isolated 48-V supply comes in and provides power to the LTC4263. Bypass capacitors are placed at the input supply pin VDD48 as well as the internally generated 5-V supply brought out at VDD5. The output lines, the 48-V positive rail and the switched negative rail, go out to the RJ-45 port connection. If going through data pairs, these lines are connected to the center taps of the Ethernet transformer. Otherwise, they are connected directly to the spare pairs. A 58-V transient voltage suppressor (TVS) and a 0.1-F capacitor help protect against transients, overvoltage and reverse voltage at the port. An optional 1-A fuse can also be used to meet safety requirements.

The low-component-count solution allows for a layout area smaller than a single RJ-45 connector. Another layout advantage of the single-port PSE over multichannel PSEs is the parts placement. The individual controller circuits can be placed behind or close to the Ethernet port as opposed to long traces to meet at the shared controller. A 58-V transient voltage suppressor can also be placed close to the controller it is designed to protect. Furthermore, a simplified layout helps meet isolation requirements mentioned in the IEEE 802.3af standard, an area sometimes overlooked in PSE design.

Separating the controller and MOSFET to the individual ports also benefits the PSE thermally. Space between components provides better thermal management than multichannel controller solutions, where the power paths are close to each other. The payoffs are most apparent in areas where the PSE is enclosed in a tight space such as an integrated RJ-45 connector.

Other options (not shown in Fig. 2) are the LED connection and ac disconnect. An LED with a resistor to set the current goes to the LED pin. The LED driver can also act as a switching current source to increase power efficiency, only needing an additional inductor and diode. In applications where ac disconnect is needed in place of dc disconnect, the LTC4263 configuration is changed simply with a capacitor at the OSC pin to set the internal oscillator, an RC at ACOUT to apply the ac signal to the port, and an ac blocking diode in the negative power path.

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© 2007 Penton Media, Inc.