VOUT
±VIN1
±VIN2
To PoE PD
Controller
PoE
Data
Pairs
PoE
Spare
Pairs
–
+
LT4321
EN
EN
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.
Features
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Low Loss Replacement for Two PoE PD Diode Bridges
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Reduces Heat to Ease Thermal Design
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Maximizes Available Power and Voltage
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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
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100V Absolute Maximum
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Less Than 5µA Quiescent Current During Detection
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–40°C to 125°C Guaranteed Temperature Range
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16-Pin 4mm × 4mm QFN Package
Active Diode Bridge Controller Minimizes Power Loss and Heat
in Power over Ethernet Powered Device
PoE Ideal Diode Bridge
CURRENT (mA)
0
POWER DISSIPATION (W)
0
0.8
1.0
1.2
1.8
1.4
600
0.6
0.4
0.2
1.6
200 400 800 1000
IN12 = 55V
IN36 = 0V
IN45 = FLOAT
IN78 = FLOAT
LT4321 (50mΩ FETs)
DIODES (S2B)
POWER
SAVED
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.
Thermograph Conditions: 4-Pair 90W LTPoE++, 2.2A at 41V without Forced Airflow
LT4321 PoE Active Bridge Controller Power Savings
High Efficiency 90W PD Solution
Using LT4321 and LT4275 LTPoE++/PoE+/PoE PD Controller
Top Views Bottom Views
PoE Type 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 0.46W 3.6%
PoE+, IEEE 802.3at 25.5W 42.5V 600mA 0.83W 1 0.83W 3.3%
LTPoE++ 90W 41V 2.2A 1.6W at 1.1A 23.2W 3.6%
1013
www.linear.com/4321 n 1-800-4-LINEAR
