Ideal Diode Bridge Controller Minimizes Power Loss & Heat in PoE Powered Devices

Oct 22 2013 - MILPITAS, CA
  • Linear Technology Corporation introduces the LT4321, an ideal diode bridge controller that replaces two diode bridge rectifiers with low-loss N-channel MOSFET bridges to increase the available power and reduce heat dissipation in a Power over Ethernet Powered Device (PoE PD). IEEE 802.3 PoE specifications require PDs to accept DC supply voltages of any polarity over their Ethernet inputs. The space- and power-efficient LT4321 dual active bridge rectifies and smoothly combines power from both the data and spare pairs into a single, polarity-correct supply output. Circuit size and cost are reduced as the enhanced power efficiency eliminates heat sinking requirements. Power savings of 10× or more enables PDs to stay within PoE classification levels or to add value-rich functionality while maintaining class.

    Carefully designed to be compliant with IEEE 802.3, the LT4321’s bias current does not corrupt detection and classification. Working with 2- or 4-pair Ethernet, the controller is compatible with PoE, PoE+ and LTPoE++ standards. An integrated charge pump provides the gate drive for the eight low on-resistance N-channel MOSFETs without requiring external capacitors. Although designed for PoE PDs, the LT4321’s 20V to 80V operating range with a 100V absolute maximum makes it suitable and robust for telecom applications employing batteries or reversible supplies. Dual-polarity enable pins provide shutdown capability to the LT4321, reducing its bias current to 32µA from the 0.5mA operating current.

    Specified over the –40°C to 125°C ambient temperature range, the LT4321 is offered in a compact 16-pin 4mm × 4mm QFN package. Pricing begins at $2.95 each for 1,000-piece quantities. Samples and evaluation circuit boards are available online or from your local Linear Technology sales office. For more information, visit

    Summary of Features: LT4321

    • Low Loss Replacement for Dual ORed Diode Bridges
    • Controls Eight N-Channel MOSFETs
    • Reduces Heat to Ease Thermal Design
    • Maximizes Available Power & Voltage
    • 20V to 80V DC Operation, 100V Absolute Maximum
    • PoE/PoE+/LTPoE++ Compatible
      • Works with 2-Pair & 4-Pair PoE Applications
      • Does Not Corrupt IEEE 802.3 Detection & Classification
      • IEEE 802.3 Compliant When Paired with a Powered Device (PD) Controller
    • DC Polarity Correction & ORing of Telecom Supplies
    • 0.8mA (max) Quiescent Current, 60µA (max) in Shutdown
    • –40°C to +125°C Guaranteed Ambient Temperature Range
    • 16-Pin 4mm × 4mm QFN Package

    Power over Ethernet (PoE) Active Bridge Rectifier Maximizes Available Power & Voltage


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