LT4321 - PoE Ideal Diode Bridge Controller


  • Reduces Heat, Eliminates Thermal Design Problems
  • Maximizes Power Efficiency
  • Less than 800μA Quiescent Operating Current
  • Fully Compatible with IEEE 802.3 Detection and Classification
  • IEEE 802.3 Compliant When Paired with a Powered Device (PD) Controller
  • Works with 2-Pair and 4-Pair PoE Applications
  • Compatible with PoE, PoE+, and LTPoE++™
  • 100V Absolute Maximum Voltage
  • H-Grade Version Operates Up to 125°C
  • 16-Lead 4mm × 4mm QFN Package

Typical Application

LT4321 Typical Application


The LT®4321 is a dual ideal diode bridge controller that enables a Power over Ethernet (PoE) powered device (PD) to receive power in either voltage polarity from RJ-45 data pairs, spare pairs, or both. The LT4321 and eight N-channel MOSFETs replace the eight diodes in a passive PoE rectifier bridge. The LT4321 eases thermal design and increases delivered power.

An internal charge pump allows an all-NMOS bridge eliminating larger and more costly PMOS switches. The LT4321 works with 2-pair and 4-pair applications. High impedance input sense pins prevent reverse current on unused pairs. If the power source fails or is shorted, a fast turn-off minimizes reverse current transients. Unlike discrete ideal bridge solutions, the LT4321 will operate through transients without enabling the MOSFETs on unpowered pairs.


CAD Symbols and Footprints: The downloadable Zip file below contains the schematic symbol and PCB footprints.

For complete and up to date package information and drawings, please refer to our packaging page

Part Number Package Code Temp Package
LT4321HUF#PBF 4x4 QFN-16 UF H 05-08-1692 Yes
LT4321HUF#TRPBF 4x4 QFN-16 UF H 05-08-1692 Yes
LT4321IUF#PBF 4x4 QFN-16 UF I 05-08-1692 Yes
LT4321IUF#TRPBF 4x4 QFN-16 UF I 05-08-1692 Yes

LT4321 Package Drawing

Order Info

  • Part numbers ending in PBF are lead free. Solder plated terminal finish (SnPb) versions are non-standard and special terms and conditions and pricing applies if available. Please contact LTC marketing for information.
  • Part numbers containing TR or TRM are shipped in tape and reel or 500 unit mini tape and reel, respectively
  • Please refer to our general ordering information or the product datasheet for more details

Package Variations and Pricing

Part Number Package Temp Price
LT4321HUF#PBF 4x4 QFN-16 H $4.85 $3.39 Yes
LT4321HUF#TRPBF 4x4 QFN-16 H $3.45 Yes
LT4321IUF#PBF 4x4 QFN-16 I $4.21 $2.95 Yes
LT4321IUF#TRPBF 4x4 QFN-16 I $3.01 Yes
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* The USA list pricing shown is for BUDGETARY USE ONLY, shown in United States dollars (FOB USA per unit for the stated volume), and is subject to change. International prices may differ due to local duties, taxes, fees and exchange rates. For volume-specific price or delivery quotes, please contact your local Linear Technology sales office or authorized distributor.

Demo Boards

Linear Technology offers many demo boards free of charge to qualified customers. Contact your local sales office or distributor to inquire about a demo board. Certain demo boards are also available for sale via credit card on this website. Demo boards are for evaluation purposes only. It remains the customer’s responsibility to verify proper and reliable operation in the actual end application.

Part Number Description Price Documentation
DC2046A-A LT4276B/LT4321 Demo Board | PoE+ (25W, 3.3V/6.8A) PD with DC/DC and Ideal Diode Bridge $95.00
DC2046A-B LT4276B/LT4321 Demo Board | PoE+ (25W, 5V/4.7A) PD with DC/DC and Ideal Diode Bridge $95.00
DC2046A-C LT4276B/LT4321 Demo Board | PoE+ (25W, 12V/1.9A) PD with DC/DC and Ideal Diode Bridge $95.00
DC2046A-D LT4276B/LT4321 Demo Board | PoE+(25W, 24V/1A) PD with DC/DC and Ideal Diode Bridge $95.00
DC2046A-E LT4276A/LT4321 Demo Board | LTPoE++ (38.7W, 5V/7A) PD with DC/DC and Ideal Diode Bridge $95.00
DC2046A-F LT4276C/LT4321 Demo Board | PoE (13W, 5V/2.3A) PD with DC/DC and Ideal Diode Bridge $95.00
DC2047A-A LT4276B/LT4321 Demo Board | PoE+ (25W, 12V/1.9A) PD with DC/DC, Ideal Diode Bridge and 9V-57V AUX $95.00
DC2093A-A LT4275A/LT4321 LTPoE++/PoE+/PoE PD Demo Board $95.00
DC2093A-B LT4275B/LT4321 PoE+/PoE PD Demo Board $95.00
DC2093A-C LT4275C/LT4321 PoE PD Demo Board $95.00
DC2125A LT4275/LT4321/LTM8027 Demo Board | LTPoE++ PD Plus Diode Bridge & DC/DC Controller $125.00
DC2475A-A LT4295/LT4321 Demo Board | PoE++ (40W, 12V/3A) PD with Flyback DC/DC and Ideal Diode Bridge $95.00
DC2476A-A LT4295/LT4321 Demo Board | PoE++ (71W/24V/2.7A or 62W/24V/2.3A) PD with Flyback DC/DC and Ideal Diode Bridge $95.00
DC2539A-A LT4295/LT4321 Demo Board | PoE++ (51W, 12V/3.9A) PD with Flyback DC/DC and Ideal Diode Bridge $95.00
DC2583A LT4294/LT4321 Demo Board | PoE++ (71W/37V-57V) PD and Ideal Diode Bridge $95.00
DC2584A-A LT4295/LT4321 Demo Board | PoE++ (71W/12V/5.5A) PD with Forward DC/DC and Ideal Diode Bridge $95.00
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Companion Boards

Part Number Description Price Documentation
DC1814A-A LTC4274A-1 Demo Board | LTPoE++ (38.7W) PSE $100.00
DC1815A-A LTC4266A-1 Demo Board | 4-Port 38.7W LTPoE++ PSE (req. DC590) $200.00
DC1814A-B LTC4274A-2 Demo Board | LTPoE++ (52.7W) PSE $100.00
DC1814A-C LTC4274A-3 Demo Board | LTPoE++ (70W) PSE $100.00
DC1814A-D LTC4274A-4 Demo Board | LTPoE++ (90W) PSE $100.00
DC1815A-B LTC4266A-2 Demo Board | 4-Port 52.7W LTPoE++ PSE (req. DC590) $200.00
DC1815A-C LTC4266A-3 Demo Board | 4-Port 70W LTPoE++ PSE (req. DC590) $200.00
DC1815A-D LTC4266A-4 Demo Board | 4-Port 90W LTPoE++ PSE (req. DC590) $200.00
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Click here to view our complete list of demo boards


  • PoE/PoE+/ LTPoE++ Powered Devices
  • DC Polarity Correction and Ideal Diode-ORing of Telecom Supplies

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Design Tools

Linear Technology provides a variety of custom design simulation tools and device models to allow even novice designers to quickly and easily evaluate circuits using high performance switching regulators, amplifiers, data converters, filters and more. Please visit our Design Tools page to view and download:

  • LTspice - A powerful SPICE simulator, schematic capture and waveform viewer with enhancements and models for easing the simulation of switching regulators
  • LTpowerCAD - A complete power supply selection, synthesis and design tool
  • LTpowerPlay - A development environment supporting Linear Technology's Digital Power System Management (PSM) products
  • PScope & QuikEval - USB-based product demonstration and data acquisition system for use with our ADCs, DACs, and more
  • Linduino - Arduino compatible system for developing and distributing firmware libraries and example code
  • LinearLab Tools - A collection of Matlab and Python programs that provide direct access to Linear Technology’s data converter evaluation boards
  • And More!

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