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LT4321 - PoE Ideal Diode Bridge Controller

LT4321 - PoE Ideal Diode Bridge Controller
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LT4321 - PoE Ideal Diode Bridge Controller - 1

LT4321 PoE Ideal Diode Bridge Controller Features 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. Reduces Heat, Eliminates Thermal Design Problems Maximizes Power Efficiency Less than 800µA Quiescent Operating Current Fully Compatible with IEEE 802.3 Detection and Classification n IEEE 802.3 Compliant When Paired with a Powered Device (PD) Controller n Works with 2-Pair and 4-Pair PoE Applications n Compatible with PoE, PoE+, and LTPoE++™ n 100V Absolute Maximum Voltage n H-Grade Version Operates Up to 125°C n 16-Lead 4mm × 4mm QFN Package n Applications PoE/PoE+/ LTPoE++ Powered Devices DC Polarity Correction and Ideal Diode-ORing of Telecom Supplies 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. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and PowerPath and LTPoE++ are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application Powered Device for PoE (to 13W), PoE+ (to 25.5W), or Linear LTPoE++ (to 90W) Systems SPARE PAIRS ISOLATED POWER SUPPLY

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LT4321 - PoE Ideal Diode Bridge Controller - 2

LT4321 Absolute Maximum Ratings UF PACKAGE 16-LEAD (4mm × 4mm) PLASTIC QFN TJMAX = 125°C, θJC = 4.5°C/W EXPOSED PAD (PIN 17) IS OUTN, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on nonstandard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape...

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LT4321 - PoE Ideal Diode Bridge Controller - 3

The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, OUTP = 20V to 80V unless otherwise noted. SYMBOL Operating Supply Range Undervoltage Lockout Total Supply Current in Detect Region Total Supply Current in Shutdown Total Operating Supply Current INn = OUTP + ∆VSD(MAX) + 5mV, 10µA Out of TGn (Note 3) Bottom Gate Drive Top Gate Pull-Up Current Top Gate Pull-Down Current Bottom Gate Pull-Up Current Bottom Gate Pull-Down Current EN Pull-Up Resistance (Active Low) EN Pull-Down Resistance (Active High) Digital Input...

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LT4321 - PoE Ideal Diode Bridge Controller - 4

LT4321 Typical Performance Characteristics Total Supply Current in Ideal Bridge Mode, 2-Pair Total Supply Current in Shutdown 700 EN Open Circuit Voltage Total Supply Current in Ideal Bridge Mode, 4-Pair IN45 = IN78 = FLOAT EN = OUTP IN12 = OUTP –250mV IN36 = OUTN –50mV 0

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LT4321 - PoE Ideal Diode Bridge Controller - 5

LT4321 Pin Functions IN12: Data Pair Input 1. In a PoE system, IN12 connects to the center tap of the transformer connected to pins 1 and 2 on an RJ45 connector. BG36: Bottom-Side Gate Driver Output. BG36 pin pulls high with respect to OUTN when IN12 is greater than OUTP and IN36 is less than OUTN. IN36: Data Pair Input 2. In a PoE system, IN36 connects to the center tap of the transformer connected to pins 3 and 6 on an RJ45 connector. BG45: Bottom-Side Gate Driver Output. BG45 pin pulls high with respect to OUTN when IN78 is greater than OUTP and IN45 is less than OUTN. IN45: Spare Pair...

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LT4321 - PoE Ideal Diode Bridge Controller - 6

LT4321 Applications Information OVERVIEW A very common application is an IEEE 802.3 powered device which is required to accept voltage in either polarity at its RJ-45 input. Polarity correction devices allow the PD to work equally well with standard or cross-over cables and endspan or midspan PSEs. They also prevent the PD from back feeding current into the Ethernet cable. PD polarity correction is commonly done with a traditional diode bridge, but this results in an efficiency loss due to the forward drop generated across two conducting diodes. This voltage drop reduces the available...

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LT4321 - PoE Ideal Diode Bridge Controller - 7

LT4321 Applications Information Table 1. Conditions for Ideal Bridge Mode on IN12/IN36 PoE MODE OUTP – OUTN Table 2. Conditions for Ideal Bridge Mode on IN45/IN78 PoE MODE OUTP – OUTN Shutting down the LT4321 does not disconnect the load. The external MOSFETs are shorted gate to source and bridge current is carried by the MOSFETs’ body diodes. The eight body diodes will act like two traditional diode bridges. EXTERNAL INTERFACE AND COMPONENT SELECTION At light load, the power dissipated in the forward drop of the body diodes will be less than the power dissipated by the LT4321 quiescent...

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LT4321 - PoE Ideal Diode Bridge Controller - 8

LT4321 Applications Information Transient Voltage Suppressor The LT4321 specifies an absolute maximum voltage of 100V and is designed to tolerate brief overvoltage events. However, pins that interface to Ethernet cables or remote telecom supplies can routinely see excessive peak voltages. To protect the LT4321, install a unidirectional transient voltage suppressor (TVS) such as an SMAJ60A between OUTP and OUTN. This TVS must be mounted as close as possible to the LT4321. For extremely high cable discharge and surge protection contact Linear Technology Applications. MOSFET Selection Select...

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LT4321 - PoE Ideal Diode Bridge Controller - 9

ISOLATED POWER SUPPLY Figure 3. PD Interface Using the EN Pin ISOLATED POWER SUPPLY RUN Figure 4. PD Interface Using the EN Pin

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LT4321 - PoE Ideal Diode Bridge Controller - 10

LT4321 Typical Applications High Efficiency 25W PD Solution with 12VDC and 24VAC Auxiliary Input BSZ110N06NS3 ×4 TG2 TG1 ISOLATED POWER SUPPLY SPARE PAIRS

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