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LTC4270/ Datasheet(PDF) 27 Page - Linear Technology |
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LTC4270/ Datasheet(HTML) 27 Page - Linear Technology |
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27 / 32 page  LTC4290/LTC4271 27 429071f APPLICATIONS INFORMATION External MOSFET CarefulselectionofthepowerMOSFETiscriticaltosystem reliability. LTC recommends either Fairchild IRFM120A, FDT3612, FDMC3612 or Philips PHT6NQ10T for their proven reliability in Type 1 and Type 2 PSE applications. SOA curves are not a reliable specification for MOSFET selection.ContactLTCApplicationsbeforeusingaMOSFET other than one of these recommended parts. Sense Resistor The LTC4290/LTC4271 is designed to use 0.25Ω current sense resistors to reduce power dissipation. Four com- monly available 1Ω resistors (sized according to power dissipation) can be used in parallel in place of a single Figure 15. LTC4290/LTC4271 Proprietary Isolation 0.25Ω resistor. In order to meet the ICUT and ILIM accuracy required by the IEEE specification, the sense resistors should have ±1% tolerance or better, and no more than ±200ppm/°Ctemperaturecoefficient.Inaddition,thesense resistors must meet strict layout guidelines. Port Output Cap Each port requires a 0.22μF cap across its outputs to keep the LTC4290 stable while in current limit during startup or overload. Common ceramic capacitors often have signifi- cant voltage coefficients; this means the capacitance is reduced as the applied voltage increases. To minimize this problem, X7R ceramic capacitors rated for at least 100V are recommended and must be located close to the PSE. • • 3.3V 3.3V 0.1µF –54V 100 100 100 100 T1 • • 3.3V 1µF 1µF 0.1µF –54V 100 100 100 100 T2 GP0 GP1 MID RESET MSD INT AUTO SCL AD0 AD1 AD2 AD3 AD6 DGND CAP1 CAP2 DND DPD CND CPD DNA SENSE1 GATE1 OUT1 LTC4290 DPA CNA CPA XIO0 XIO1 0.22µF 100V 0.22µF 100V S1B S1B –54V –54V 429071 F15 –54V –54V S1B S1B PORTn PORT1 SDAIN SDAOUT LTC4271 NO ISOLATION REQUIRED ON I2C INTERFACE VDD33 2nF, 2kV VEE SENSEn GATEn OUTn AGND VSSK 0.25 0.25 >47µF SYSTEM BULK CAP + |
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