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1、附录AMAX232-5V-Powered,MultichannelRS-232Drivers/ReceiversTheMAX232isadualdriver/receiverthatincludesacapacitivevoltagegeneratortosupplyTIA/EIA-232-Fvoltagelevelsfromasingle5-Vsupply.EachreceiverconvertsTIA/EIA-232-Finputsto5-VTTL/CMOSlevels.Thesereceivershaveatypicalthresholdof1.3V,atypi
2、calhysteresisof0.5V,andcanaccept±30-Vinputs.EachdriverconvertsTTL/CMOSinputlevelsintoTIA/EIA-232-Flevels.Thedriver,receiver,andvoltage-generatorfunctionsareavailableascellsintheTexasInstrumentsLinASIClibrary.TheMAX220–MAX249containfoursections:dualcharge-pumpDC-DCvoltageconverters,RS-23
3、2drivers,RS-232receivers,andreceiverandtransmitterenablecontrolinputs.DualCharge-PumpVoltageConverterTheMAX220–MAX249havetwointernalcharge-pumpsthatconvert+5Vto±10V(unloaded)forRS-232driveroperation.ThefirstconverterusescapacitorC1todoublethe+5Vinputto+10VonC3attheV+output.Thesecondconv
4、erterusescapacitorC2toinvert+10Vto-10VonC4attheV-output.Asmallamountofpowermaybedrawnfromthe+10V(V+)and-10V(V-)outputstopowerexternalcircuitry(seetheTypicalOperatingCharacteristicssection),exceptontheMAX225andMAX245–MAX247,wherethesepinsarenotavailable.V+andV-arenotregulated,sotheoutput
5、voltagedropswithincreasingloadcurrent.DonotloadV+andV-toapointthatviolatestheminimum±5VEIA/TIA-232EdriveroutputvoltagewhensourcingcurrentfromV+andV-toexternalcircuitry.WhenusingtheshutdownfeatureintheMAX222,MAX225,MAX230,MAX235,MAX236,MAX240,MAX241,andMAX245–MAX249,avoidusingV+andV-topo
6、werexternalcircuitry.Whenthesepartsareshutdown,V-fallsto0V,andV+fallsto+5V.Forapplicationswherea+10VexternalsupplyisappliedtotheV+pin(insteadofusingtheinternalchargepumptogenerate+10V),theC1capacitormustnotbeinstalledandtheSHDNpinmustbetiedtoVCC.ThisisbecauseV+isinternallyconnectedtoVCC
7、inshutdownmode.RS-232DriversThetypicaldriveroutputvoltageswingis±8Vwhenloadedwithanominal5kΩRS-232receiverandVCC=+5V.OutputswingisguaranteedtomeettheEIA/TIA-232EandV.28specification,whichcallsfor±5Vminimumdriveroutputlevelsunderworst-caseconditions.Theseincludeaminimum3kΩload,V
