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Power Management I Product Focus Feeling the

Roberto Massolini and Michele Sclocchi consider how it’s possible to squeeze isolated power from an Ethernet cable by using a single chip solution

Power over Ethernet (PoE) is based on an old and pretty simple idea: allowing the electrical current, necessary for the operation of each device connected to a Local Area Network (LAN), to be carried by the data cables rather than by power cords. In other words: transmit power and data over the same cable. The result is lower cost, easier installation and maintenance and a much wider range of possible applications. PoE is becoming more popular and it is used in a wide class of devices such as IP based telephone systems, IP based control camera devices, remote controlled power domes, WiFi access points, environmental sensors, and Uninterruptible Power Supplies (USP). In standard installations all the cited equipment tends to be placed in close proximity to an AC power outlet, which results in suboptimum installation or significant additional costs if new power outlets need to be installed. While installations can also be affected by different safety codes, isolations requirements and AC voltages, for example, PoE avoids these problems by supplying low-voltage DC power to the PDs through the worldwide available and standardised RJ-45 power connector using the same data cable.

PoE brief overview

The US based IEEE (Institute of Electrical and Electronic Engineers) approved the standard for PoE products back in June 2003. The IEEE802.3af standard ensures

that the power is delivered safely to both legacy devices and power-enabled devices, the cabling infrastructure is preserved, and the delivery of power does not cause data degradation.

The IEEE has standardised the use of 48VDC voltage transferred through a standard Cat 5e Ethernet cable via the two spare pairs present in the cable or the two data pairs. The spec does not allow both sets of wires to be used – a choice must be made. The Power Sourcing Equipment (PSE) applies power to either set of wires. The Powered Device (PD) must be able to accept power from both options. The amount of power available is limited to four classes from 4 watts to a maximum of 15.4 watts delivered per port. The new standards introduced after 2005 called IEEE802.3at - also known as PoE Plus - increase the maximum amount of deliverable power up to 25.5W, increasing the range of available hardware options. As cited above, the system should be

backward compatible with legacy devices and this is ensured by a “discovery process” run from the Power Sourcing Equipment (PSE) looking for devices that comply with the specification. It does this by applying a small current-limited voltage to the cable and checking for the presence of specified signature impedance in the remote device. Only if the detected impedance is above 23.75k and below 26.25k with a parallel capacitance of 0.1uF does the PSE consider a PD to be present and start applying the full 48V. The PD must also continue to draw a minimum

current. If it does not (for example, when the device is unplugged) then the PSE removes the power and the discovery process begins again.

Design considerations A Power Sourcing Equipment (PSE) and a Powered Device (PD) are the essential blocks building the PoE system (Figure 1). In these kinds of applications a big chunk of the complexity is given by the Power Device Interface (PDI) that is responsible for the power on sequence, complying with the minimum current drawing and in-rush current protection.

Clearly one of the PoE enabled device’s focal points is the presence of a reliable and sure interface circuit that fulfills all the above specifications and guarantees compatibility with many PSE. To minimise the possibility of damage to equipment in the event of a malfunction, the PoE system’s interface should employ fault protection. Even if such an interface could be implemented with discrete parts the obvious shortcoming of this approach is that of complexity and component count coupled with greater demands on the design skills of the power supply designer. The limitation on the DC current, which should not exceed 350mA (transient current limit is 400 mA), is another big issue. This limits the maximum input power to 12.95 Watts (37V x 350 mA) and makes the DC-DC conversion efficiency a key factor for using as much as possible of that scarce resource. Add to this the fact that many of the designers that use the PoE solution are not necessarily power supply design experts and may prefer existing power supply

solutions, and an integrated solution able to provide IEEE802.3af specs compatibility, cost efficiency, small area footprint and good efficiency is the solution of choice.

The LM5072

National Semiconductor’s LM5072 integrates a fully IEEE 802.3af compliant PD interface and PWM controller in a single integrated circuit, providing a complete power solution for devices that connect to PoE systems. The implementation requires a minimal number of external components and its Hot Swap PD interface provides four major advantages:

1. An input voltage rating up to 100V for improved flexibility in surge suppressor 2. Fully integrated PD signature resistor and programmable inrush current limit 3. Input voltage under-voltage lock-out (UVLO)

4. DC current limit that includes an easy to use PWM controller based on the peak current mode control technique.

Current mode control provides inherent advantages such as line voltage feed- forward, cycle-by-cycle current limit, and simplified closed-loop compensation. The controller’s PWM gate driver is capable of sourcing and sinking peak currents of 800 mA to directly drive the power MOSFET switch of the DC-DC converter. The PWM controller also contains a high gain, high bandwidth error amplifier, a high voltage startup bias regulator, a programmable oscillator for a switching frequency between 50 kHz to 500 kHz, a bias supply (VCC) under-voltage lock-out circuit, and a programmable soft-start circuit.

Figure 1:PoE block diagram including PSE and PD devices 40 July/August 2011 Components in Electronics

Figure 2:PoE block diagram with LM5072 controller

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