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ANALOG DESIGN Lowering the cost of ownership of PoE By Alison Steer and Heath Stewart Power over Ethernet (PoE), defined by the IEEE 802.3at specification, is a method to safely deliver application data and power over a single CAT-5 Ethernet cable. It has been increasing in popularity due to its flexibility for installing equipment almost anywhere, without the constraint of AC-power proximity or requiring installation by an electrician. The original IEEE 802.3af PoE specification limited the power delivered to the powered device (PD) to just 13W, which in turn limited the scope of applications to devices such as IP phones and basic security cameras. In 2009, the IEEE 802.3at specification increased this available power to 25.5W. However, this was still insufficient to satisfy the growing number of power-hungry PoE applications, such as picocells, wireless access points, LED signage and heated pan-tiltzoom Fig. 1: The LTC4290/LTC4271 chipset achieves isolation without any opto-isolators and eliminates the need for a dedicated isolated DC/DC converter. (PTZ) outdoor cameras. In 2011 Linear Technology released a new proprietary standard, LTPoE++TM, which extends the PoE and PoE+ specifications to 90W of delivered power, while maintaining 100% interoperability with the IEEE PoE standards. Four different power levels are available (38.7W, 52.7W, 70W, 90W), allowing the power supply to be sized according to the application’s requirements. LTPoE++ PSEs employ a clever PSE isolation architecture to minimize component count and enable the use of less expensive external components. Comprehensive cable discharge protection and 80V abs max pins ensure high reliability in the field. Use of external FETs allows thermal performance to be matched to the application requirements, improves the efficiency of the system, and increases long-term reliability. The LTPoE++ architecture requires only one PSE (power sourcing equipment) and PD controller to deliver up to 90W over 4-pair 100m CAT-5e cable. System isolation requirements Implementation of Power over Ethernet requires careful architecture and component selection to minimize system cost, while maximizing performance and reliability. A successful design must adhere to IEEE isolation requirements, protect the Hot Swap™ FET during short-circuit and overcurrent events, and otherwise comply with the IEEE specification. The PoE specification clearly lays out isolation requirements, guaranteeing ground loops are broken, maintaining Ethernet data integrity and minimizing noise in the PD application circuit. Traditional PSE isolation architectures isolate the digital interface and power at the host-to-PSE controller interface. Digital isolation elements such as opto-couplers are inherently expensive and unreliable. ICs capable of performing the isolation function are cost-prohibitive or do not support fast I2C transfer rates. In addition, isolated DC/DC converters needed to power the PSE logic increase board space and system cost. Isolation made easy Linear Technology’s 12-port (LTC4270/LTC4271) and 8-port PSE (LTC4290/LTC4271) chipsets take a different approach to PSE isolation by moving all digital functions to the host side of the isolation barrier – see figure 1. This significantly reduces the cost and complexity of required components. There is no longer the need for a separate, isolated DC/DC power supply; the LTC4271 digital controller can use the host’s logic supply. The LTC4271 controls the LTC4290 or LTC4270 using a transformerisolated communication scheme. An inexpensive and ubiquitous Ethernet transformer pair replaces six opto-couplers. I2C communication including port management, reset and fast port shutdown are encoded in a protocol designed to minimize radiated energy and provide 1500V of isolation. Robust cable discharge protection It is important to consider the robustness of your PoE design, especially when dealing with high cable counts, high voltages, high currents or high temperatures. Linear Technology has a lot of experience in this area and has designed a low cost, well thought-out, circuit protection scheme that is scalable to match IEC61000 cable discharge voltage requirements. Only a single TVS is required to protect the high voltage analog supply while a pair of inexpensive clamping diodes is used on each output port – see figure 2. The diodes at the ports steer harmful surges into the supply rails, where they are absorbed by the surge suppressor and the VEE bypass capacitance. The surge suppres- Alison Steer is Product Marketing Manager at Linear Technology Corporation – www.linear.com Heath Stewart is Senior Design Engineer for Mixed Signal Products at Linear Technology. Fig. 2: Robust cable discharge protection. 34 Electronic Engineering Times Europe January 2015 www.electronics-eetimes.com


EETE JAN 2015
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