Posted on 01 April 2019

Single Port PoE Controller Simplifies PSE Design

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Power over Ethernet (PoE) has been enjoying rapid growth in popularity. PoE provides the user with numerous benefits including fast and convenient installation, reduced overall power consumption, fewer cable runs, lower installation costs and a unified, worldwide power standard based on the IEEE 802.3af specification.

By Alison Smith, Product Marketing Manager, Mixed Signal Products, Linear Technology Corporation


PoE is used in a multitude of applications from Voice over Internet Protocol (VoIP) telephony to wireless access points and security cameras. Delivering power over CAT-5 cable is no longer limited to enterprise networks, but is gaining popularity for small business and home office networks. With the extension of PoE to a wider consumer base, system designers are facing faster time-tomarket requirements while having to maintain product performance and standards compliance.

The LTC4263 provides the smallest footprint and simplest design for implementing IEEE 802.3af compliant Power Sourcing Equipment (PSE). Housed in a tiny 4mm x 3mm DFN package and integrating a highvoltage MOSFET, the LTC4263 requires only a few external components to provide a complete single-port PSE solution that will detect, classify and provide 48 Volts to a Powered Devices (PDs) such as a VoIP phone. Advanced detection and power monitoring techniques prevent damage to legacy data-only equipment, while still supplying power to newer, Ethernet-powered devices. Offering standard-compliant AC or DC methods to sense the removal of a PD and featuring sophisticated onboard control algorithms, the LTC4263 provides autonomous operation through all modes of operation without processor intervention. The most unique feature of the LTC4263, however, may be its ability to manage power allocation across a multi-port system.

Seamless Execution of Complex Tasks

The LTC4263 performs multiple functions autonomously, as required by the IEEE 802.3af standard. These include detection of a compliant PD, classification, power supply load management, powering on the port with current limit and circuit breaker functions, and removal of power after the PD has been disconnected. The LTC4263 uses a forcecurrent detection method in order to reduce noise sensitivity and provide a more robust detection algorithm. The IC checks for the signature resistance by forcing two test currents on the port in sequence and measuring the resulting voltages. It then subtracts and divides the two V-I points to determine the resistive slope while removing voltage offset caused by any series diodes or current offset caused by leakage at the port.

If detection is successful, the LTC4263 performs classification, identifying which of the three IEEE power levels is present and stores the detected class internally for use by the power management circuitry. The LTC4263 proceeds to power the port, while providing current limit and current monitoring functions. Inrush current limiting ensures that the PD is powered-up in a controlled manner without causing transients on the input supply. If at any time the port is shorted or an excessive load is applied, the LTC4263 limits port current to avoid hazardous conditions. If the port current limit is exceeded continuously for more the 62ms, the port is turned off and the LTC4263 waits four seconds before restarting detection.

Single – Port Fully Autonomous PSE

The LTC4263 can be pin configured for either Endpoint or Midspan operation to ensure proper powering of a PD in the case of two PSEs connected to a single PD. Midspan operation extends the detection backoff timing so as not to continually corrupt the signature as seen by the endpoint PSE.

The LTC4263 also offers standard-compliant AC or DC-disconnect methods to sense the removal of a PD and turn off 48V power.

Management in Multi-Port Systems

Easy Power Management in Multi-Port Systems

Multi-port systems rely on a microcontroller to arbitrate total power supply load. The LTC4263 presents a novel way to achieve proper allocation of power by using programmable onboard power management circuitry. The PWRMGT pins of the LTC4263 are tied together, along with an RC network. When a new PD is added, the LTC4263 immediately classifies it and checks the voltage at the shared PWRMGT node. If the voltage at the node is less than 1V, the power requirements of the new PD can be met and the LTC4263 proceeds to power the port. If over 1V, the current is removed from the node, port powering is aborted, and the LTC4263 goes back into detection mode. Thus no microcontroller is needed to manage total power supply loading, yet the LTC4263 successfully manages proper power allocation and provides maximum utilization of the 48V power supply.

New Frontiers for PoE

Power over Ethernet has fundamentally changed the way voice-over-IP telephones and wireless access points are powered, and it is spawning new breeds of Powered Devices in the home, office, and industrial settings. What the future holds for PoE is unknown and it remains to be seen how much of traditional electrical power will be replaced by the efficient and clean advantages of PoE. But next time your office phone rings, check to see where that power is coming from and start thinking of novel and exciting applications for this new worldwide power standard.



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