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SP7661 数据表(PDF) 7 Page - Sipex Corporation |
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SP7661 数据表(HTML) 7 Page - Sipex Corporation |
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7 / 20 page  7 Nov3-06 Rev I SP766 Wide Input Voltage Range 3A, 600kHz Buck Regulator © 2006 Sipex Corporation THEORY OF OPERATION ThereforewithoutexternalbiasingtheViN startthresholdis9.5V.Asmallcapacitormay berequiredbetweenUVINandGNDtofilter outnoise.ForapplicationswithViNof5V or3.3V,connectUVINdirectlytoViN.To programtheViN startthreshold,useapair ofexternalresistorsasshown.Ifexternal resistorsareanorderofmagnitudesmaller than internal resistors,then the ViN start thresholdisgivenby: ViN(start)=2.5•(R4+R5)/R5 Forexample,ifitisrequiredtohaveaViN startthresholdof7V,thenletR5=5KΩand usingtheViN startthresholdequationweget R4=9.09KΩ. Thermal and Short-Circuit Protection BecausetheSP7661isdesignedtodrive largeoutputcurrent,thereisachancethat the power converter will become too hot. Therefore, an internal thermal shutdown (145°C)hasbeenincludedtopreventthe IC from malfunctioning at extreme tem- peratures. A short-circuit detection comparator has alsobeenincludedintheSP7661toprotect againstanaccidentalshortattheoutputof thepowerconverter.Thiscomparatorcon- stantlymonitorsthepositiveandnegative terminalsoftheerroramplifier,andiftheVfB pinfallsmorethan250mV(typical)below thepositivereference,ashort-circuitfault is set. Because the SS pin overrides the internal0.8Vreferenceduringsoftstart,the SP7661iscapableofdetectingshort-circuit faultsthroughoutthedurationofsoftstartas wellasinregularoperation. Over-Current Protection The Over-current protection feature can onlybeusedonoutputvoltages≤3.3volts. It is limited by the common mode rating of the op-amp used to sense the voltage across the inductor. Over-current is de- tectedbymonitoringadifferentialvoltage acrosstheoutputinductorasshowninthe nextfigure. ISN SP7661 SWN ISP VOUT L=2.7uH,DCR=4.1mΩ RS 5.11KΩ RS2 5.11KΩ CS 0.1uF CSP 6.8nF Over-current detection circuit Inputs to an over-current detection com- parator,settotriggerat60mVnominal,are connectedtotheinductorasshown.Since the average voltage sensed by the com- paratorisequaltotheproductofinductor currentandinductorDCresistance(DCR), thenIMAx=60mV/DCR.Solvingthisequa- tionforthespecificinductorincircuit1,IMAx =14.6A.WhenIMAxisreached,a220ms time-outisinitiated,duringwhichtopand bottomdriversareturnedoff.Followingthe time-out,arestartisattempted.Ifthefault conditionpersists,thenthetime-outisre- peated(referredtoashiccup). Increasing the Current Limit IfitisdesiredtosetIMAx>{60mV/DCR}(in thiscaselargerthan14.6A),thenaresistor RS3 shouldbeaddedasshowninthenext figure.RS3formsaresistordividerandre- ducesthevoltageseenbythecomparator. Since:60mV (IMAx •DCR) RS3 = {RS+RS2+RS3} |
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