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1、IT'SEASIERTHANYOUTHINKTODE-signLED'displaydriverswithNationalSemiconductor'sLM391XandLM2917seriesIC's.111thisarticle,we'llshowyouhowtousethoseIC'stobuildmoving-dotandbar-graphvoltmetersaswellasfrequency-to-voltageconverters.We'llalsogiveyouanintroducti
2、ontobin-arycodeddecimal(BCD)to7-segmentdecoderldrivercircuitsthatarecommonlyusedinelec-tronicsdesign.LM391X-seriesbasicsNationalSemiconductor'sLM391Xdothardisplaydriversareversatile18-pinDIPIC'sthatcanbeusedtodriveupto10Let'stakeanin-depthlookatLED'sin
3、eitherdotorbarmode.ThethreemembersoftheLEDdisplaydrivers.LM391XdisplaydriverseriesaretheLM3914,LM3915.andRAYMARSTONLM3916.Allthreeversionsusethesamebasicinternalcircuit-ry,asshowninFig.1,buthavedifferentoutputscalingmodes,asshowninTable1.TheLM3914isali
4、nearly-scaleddevicethat'sintendedforuseinLEDvoltmeters,withthenumberoflitLED'sbeingdi-rectlyproportionaltotheinputvoltage(pin5).TheLM3915isalogarithmically-scaleddevicethat'sintendedforuseinpowermeters,andspansarangeof0-30dBinten3-dBsteps.Fi-nally,theL
5、M3916isasemi-log-arithmically-scaleddevicethat'sintendedforuseinvolume-unit(W)meters.Figure1showsanLM3914usedinasimple10-LEDvolt-meter,whichrangesfrom0to1.25voltsDC.TheLM3914has10internalcomparators,eachwithitsnon-invertingterminalconnectedtoaspecifict
6、aponafloating,precision,10-stage,in-ternalresistivevoltage-divider.Theinvertingterminalsonalltenofthecomparatorsarefedbyaunity-gainbufferonpin5.Eachcomparatorisexternallyaccessible,andcansinkupto30milliamps.Thesinkcur-rentsareinternallylimited,andareex
7、ternallypre-setviaR1.TheLM3914alsohasafloat-ing1.25-voltDCreferencebe-tweenpins7and8,externallyconnectedtothe10-stageinter-FIG.1-INTERNALCIRCUITOFTHELM3914,withconnectionsformakingaIO-LED,0nalvoltagedivideronpins4andto1.25voltsDC,linearmeterusingeither
8、dotorbarmode.656.Pins4and8aregrounded,soTABLE1-COMPARISONOFTYPICALINPUTTHRESHOLDSFORTHEthatthebottomofthe10-stageLM391XICFAMILYinternalvoltagedividerisatground,andthetopisat1.25LEDLM3914LM3915LM3916voltsDC.TheLM3914alsohasVVIdBVIdBIVUan
