Author:
Reported by Cliff Keys, Editorial Director, Editor-in-Chief, Power Systems Design
Date
05/01/2011
PDF
.jpg)
Ethernet standards require the data cable to be isolated from the system chassis due mainly to ground loops, which can corrupt data, and possible ground potential differences in the Ethernet cable plant. By extension, PoE (Power over Ethernet) subsystems which leverage Ethernet architectures need to be isolated for the same reasons. This normally calls for at least one isolated supply to power the PSE (power sourcing equipment) controller, as well as multiple opto-couplers to isolate the PSE-to-host digital communications, which is almost always an I2C serial bus interface.
Linear Technology Corporation introduces the LTC4270/ LTC4271 isolated 12-port Power Source Equipment (PSE) controller chipset designed for use in IEEE 802.3at (PoE+) Type 1 and Type 2 compliant Power over Ethernet (PoE) systems. The LTC4270/71 provides 12 independent PSE channels for simpler, lower component count designs, reduced board space and lower solution costs. The LTC4271 provides a digital interface to the PSE host, while the LTC4270 provides the high voltage Ethernet interface; the two ICs are then bridged by inexpensive Ethernet transformers. The transformer-isolated communication protocol replaces up to six expensive opto-couplers and a complex isolated 3.3V supply used in traditional designs, resulting in significant cost savings and a more robust and manufacturable design.
.jpg)
The LTC4270/LTC4271 is suitable for a wide variety of PSE applications, including next generation switches, routers, hubs and midspans. Users will appreciate the robustness provided by 80V port pins and lowest-in-industry power dissipation, making thermal design significantly easier than designing with PSEs that integrate more fragile normally higher RDS(ON) MOSFETs. Powered Device (PD) discovery is accomplished using a proprietary dual-mode, four point detection mechanism that ensures the best immunity from false PD detection. Advanced power management includes prioritized fast shutdown, 14-bit per-port voltage and current readback, 8-bit programmable current limits and 7-bit programmable overload current limits and field-upgradeable firmware. A 1MHz I2C interface allows a host controller to digitally configure the IC or query port readings. "C" libraries are available to reduce engineering costs and improve time to market. More Features - More Ports, More Power When the latest PoE standard, IEEE 802.3at (PoE+), finalized in 2009, it increased the power available to powered devices (PDs) from 13W to 25.5W so that end applications could advance in feature set (for example, pan-tilt-zoom cameras). Today, PoE+ PSEs and PDs are continuing to proliferate quickly in the field with many new applications emerging. The LTC4270/LTC4271 provides a cost-effective 12-port PSE solution via reduced BOM cost, component count and board space, as well as an increase in reliability.
The LTC4270/LTC4271 chipset is the first PSE solution to offer LTPoE++, Linear Technology's implementation of a higher power PoE standard which provides up to 90W of delivered power to an LTPoE++ PD. This proprietary scheme uses LTPoE++ signaling methods to identify LTPoE++ PDs requiring more than the 25.5W limit of the PoE+ specification. LTPoE++ is interoperable and backward compatible with IEEE PoE/PoE+ compliant devices, so LTPoE++, PoE+ and PoE devices can work together seamlessly. The LTC4270/LTC4271 chipset is Linear Technology's first 12-port PSE controller, and the first of a family of 4th generation PoE products targeted for high power applications. The integrated isolation and high port count provides the lowest BOM cost and component count solution in the market. The use of external, low RDS(ON) MOSFETs and 0.25? sense resistors increase port reliability and provide the lowest power dissipation through each port. Very high reliability 4-point PD detection uses two test voltages and two test currents to guard against false PD detections. A host of new advanced power management hardware and software features are now available to PSE designers, such as temperature monitoring and "C" libraries that address multiple implementation levels: from basic PSE operation to fully managed, over allocated and prioritized switch systems - simplifying PSE designs. The LTC4270 is available in three power grades: A-grade uses Linear's LTPoE++™ signaling to support up to 90W PDs, B-Grade uses PoE+ signaling to support up to 25.5W PDs, and C-grade uses PoE signaling to support up to 13W PDs. All chipsets are offered in industrial temperature grades. The LTC4270 is in a RoHS-compliant 52-pin 7mm x 8mm QFN package, while the LTC4271 is in a 24-pin 4mm x 4mm QFN package. The LTC4270/71 chipset is priced starting at $12.00 each in 1,000 piece quantities and is now available in production quantities. The LTC4270/LTC4271 is backward compatible with the LTC4266 4-port PoE+ PSE controller and LTC4274 single-port PoE+ PSE controller, supported by Linear's extensive technical experience in PoE circuit design, ensuring a smooth transition to the new PoE+ standard or higher power levels. Visit www.linear.com/product/LTC4270 for more product information.
