元器件交易网

SGLS371–JANUARY2007

GENERALINFORMATION(continued)

ProgrammingtheSoft-StartTime

Anexternalcapacitor,CSS,connectedfromthesoft-start(SS)pintogroundcontrolstheTPS40200soft-startinterval.AninternalchargingresistorconnectedtoVDDproducesarisingreferencevoltage,whichisconnectedthougha700-mVoffsettothereferenceinputoftheTPS40200erroramplifier.Whenthesoft-startcapacitorvoltage(VCSS)isbelow150mV,thereisnoswitchingactivity.WhenVCSSrisesabovethe700-mVoffset,theerroramplifierstartstofollowVSST–700mV,andusesthisrisingvoltageasareference.WhenVCSSreaches1.4V,theinternalreferencetakesover,andfurtherincreaseshavenoeffect.Anadvantageofinitiatingaslowstartinthisfashionisthatthecontrollercannotovershootbecauseitsoutputfollowsascaledversionofthecontroller'sreferencevoltage.AconceptualdrawingofthecircuitthatproducestheseresultsisshowninFigure29Aconsequenceofthe700-mVoffsetisthatthecontrollerdoesnotstartswitchinguntiltheVCSShasto700mV.Theoutputremainsat0Vduringtheresultingdelay.WhenVCCSexceedsthe700-mVoffset,theTPS40200outputfollowsthesoft-starttimeconstant.Onceabove1.4V,the700-mVinternalreferencetakesover,andnormaloperationbegins.

Css

Figure29.Soft-StartCircuit

Theslow-starttimeshouldbemore(slower)thanthetimeconstantoftheoutputLCfilter.Thistimeconstraintmaybeexpressedas:

tS³2p´O´CO

(5)

Thecalculationofthesoft-startintervalissimplythetimeittakestheRCnetworktoexponentiallychargefrom0Vto1.4V.Aninternal105-k chargingresistorisconnectedfromtheSSpintoVSST.Forapplicationswherethevoltageisabove8V,aninternalregulatorclampsthemaximumchargingvoltageto8V.Theresultofthisisaformulaforthestart-uptime,asgivenby:

æVSSTö

÷tSS=Rc´CSS´lnççV÷-1.4èSSTøWhere:

tSS=Requiredsoft-starttime(inseconds)CSS=Soft-startcapacitorvalue(inF)

Rc=Internalsoft-startchargingresistor(105k nominal)VSST=Inputvoltageuptoamaximumof8V

(6)

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