PoE Ideal Diode Bridge PoE Data Pairs ±VIN1 EN + VOUT LT4321 – EN To PoE PD Controller PoE Spare ±VIN2 Pairs Active Diode Bridge Controller Minimizes Power Loss and Heat in Power over Ethernet Powered Device The LT®4321 ideal diode bridge controller 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). 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 below PoE classification power levels, or to add value-rich functionality while maintaining class. 1.8 Features Low Loss Replacement for Two PoE PD Diode Bridges 1.6 Reduces Heat to Ease Thermal Design 1.4 n Maximizes Available Power and Voltage PoE/PoE+/LTPoE++™ Compatible −− Works with 2- and 4-Pair PoE −− Does Not Corrupt Detection and Classification −− IEEE 802.3 Compliant When Paired with a PD Controller n n 100V Absolute Maximum n Less Than 5µA Quiescent Current During Detection n –40°C to 125°C Guaranteed Temperature Range POWER DISSIPATION (W) n LT4321 (50mΩ FETs) DIODES (S2B) IN12 = 55V IN36 = 0V IN45 = FLOAT IN78 = FLOAT 1.2 1.0 0.8 POWER SAVED 0.6 0.4 0.2 n 16-Pin 4mm × 4mm QFN Package n 0 0 200 400 600 CURRENT (mA) L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and LTPoE++ is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. 800 1000 LT4321 PoE Active Bridge Controller Power Savings PPD(MAX) VPD(MIN) IPD(MAX) PSAVED(MAX) Per Bridge Powered Bridges Power Savings Efficiency Gain PoE, IEEE 802.3af 13W 37V 350mA 0.46W 1 PoE+, IEEE 802.3at 25.5W 42.5V 600mA 0.83W 1 0.46W 3.6% 90W 41V 2.2A 1.6W at 1.1A 2 3.2W 3.6% PoE Type LTPoE++ 0.83W 3.3% High Efficiency 90W PD Solution Using LT4321 and LT4275 LTPoE++/PoE+/PoE PD Controller Top Views Bottom Views Thermograph Conditions: 4-Pair 90W LTPoE++, 2.2A at 41V without Forced Airflow www.linear.com/4321 n 1-800-4-LINEAR 1013