News and New Products

POE devices cater to higher-powered applications

By Margery Conner, Technical Editor -- EDN, 10/16/2008

Manufacturers of PDs (powered devices) that rely on the IEEE 802.3af POE (power-over-Ethernet) standard rather than ac wall power can shrink board space and the BOM (bill-of-materials) costs. They achieve this reduction by using a PD-interface-control IC to handle PWM (pulse-width-modulation) control for the PD’s external-power-supply magnetics and switching devices, as well as network-interface chores. (For more on the emerging higher-powered POE standard and its nonstandard versions, see "Power-over-Ethernet chips: to the spec and beyond.")  

Both the current IEEE 802.3af standard, which sets the load to the PD at 12.95W, as well as the draft 802.3at version, which sets the load power at 25W, require PDs and PSE (power-sourcing-equipment) to perform network detection, classification, and UVLO (undervoltage lockout). Using these functions, the PSE and PD cooperate to determine the PD’s power requirement and ensure that the PSE applies power only to POE-enabled equipment. The system controller at the central location can determine the overall power budget and, if necessary, allocate power during power failure from its available UPS (uninterruptible-power-supply) budget.

After a user plugs an Ethernet cable from a PSE into a PD, the PSE interrogates the PD to determine whether it is POE-enabled. This detection phase prevents the PSE from applying power to a non-PE device. The PSE applies a voltage ramp to the PD and looks for a 25-kΩ impedance. If the PSE sees an incorrect impedance, it knows that the device is not POE-capable, assumes a standard Ethernet connection, and applies no power. If the PSE detects the signature impedance, the PSE moves on to the classification phase. The signature-identification ramp voltage is 2.5 to 10V. A 24.9-kΩ resistor provides the correct signature impedance for detection.

As the PSE ramps the voltage to the PD, the PD must draw a specified current to identify the device class when the ramp is 15 to 20V. The amount of current the PD draws indicates its classification, which communicates the amount of power the PD will require during normal operation. The PSE sends this power information to the Ethernet-system controller, and the controller uses the information to determine its power budget. After the classification phase, the PSE continues to ramp up the input voltage to the operating voltage of 30V, after which the PSE releases the UVLO pin, and the PD powers up.

All PD-interface-control ICs provide these network-interface functions. However, the ICs vary in the amount of power they can make available to the PD. In anticipation of the higher-power IEEE 802.3at POE version, On Semiconductor and Texas Instruments have introduced two POE-PD-interface controllers.

On Semi’s newest POE-family members, the NCP1080 and NCP1081 PD controllers, are both compatible with the 13W 802.3af standard, and the 1081 is compatible with the 25W 802.3at version. In addition, for designs requiring a proprietary higher-power scheme, the 1081 supports nonstandard extended, regulated power as high as 40W to the application load in a two-wire-pair configuration. This higher-power capability allows equipment manufacturers to offer higher-powered devices that comply with the POE standard in all respects except for power, making a straightforward route for designers to implement proprietary versions of POE. The chips also feature 3-kV cable-ESD (electrostatic-discharge) protection, an important feature for applications that rely on extended Ethernet cabling outside a building. This scenario is typical of many surveillance-camera installations, for example.

The two devices are pin-compatible, allowing designers to start with the lower-power version and swap in the higher-power version as power needs increase. The NCPP1080 and NCP1081 are available in 4.4×6.5-mm TSSOP-EP20 packages and sell for $1.45 and $1.80 (1000), respectively.

TI’s  TPS23754 PD-interface-controller IC, available in April 2009 in a 20-pin TSSOP package, is IEEE 802.3at-compliant and includes an active-clamp-rectification circuit. The currently available 14-pin 802.3at-compliant TPS23754 costs $1.65 (1000).