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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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HT7939A датащи(PDF) 4 Page - Holtek Semiconductor Inc |
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HT7939A датащи(HTML) 4 Page - Holtek Semiconductor Inc |
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4 / 13 page  Rev. 1.00 4 April 19, 2012 HT7939A Functional Description VIN Under-Voltage Lockout -- UVLO The HT7939A contains an Input UnderVoltage Lockout,UVLO,circuit.ThepurposeoftheUVLO circuitistoensurethattheinputvoltageishigh enoughforreliableoperation.WhentheInputVoltage isbelowtheUVLOthreshold,theinternalpower MOSFET will remain switched off .The UVLO thresholdissetbelowtheminimuminputvoltage of2.6VtoavoidanytransientVINdropsunderthe UVLOthresholdandcausingtheconvertertoturnoff. Current Limit Protection TheHT7939Ahasacycle-by-cyclecurrentlimitto protecttheinternalpowerMOSFET.Iftheinductor current reaches the current limit threshold, the MOSFETwillbeturnedoff.Itisimportanttonote thatthiscurrentlimitwillnotprotecttheoutputfrom excessivecurrentduringanoutputshortcircuit.If anoutputshortcircuitoccurs,excessivecurrentcan damageboththeinductorandthediode. Over-Voltage Protection -- OVP TheHT7939Aprovidesanover-voltagefunction. IftheFBpinisshortedtogroundoranLEDis disconnectedfromthecircuit,theFBpinvoltage willbezeroandtheinternalpowerMOSFETwill switchatitsfullydutycycle.Thismaycausethe output voltage to exceed its maximum voltage rating,possiblydamagingthedeviceandtheexternal components.Internalovervoltageprotectioncircuitry turnsoffthepowerMOSFETandshutsdownthe deviceassoonastheoutputvoltageexceedsthe threshold.Asaresult,theoutputvoltagefallstothe leveloftheinputsupplyvoltage.Thedeviceremains inthisshutdownmodeuntilitisenabledonceagain toaresetconditionbytheENpinorafterthepower isrestarted. Over-Temperature protection -- OTP Athermalshutdownfunctionisimplementedto preventdevicedamageduetoexcessiveheatand powerdissipation.Typicallythethermalshutdown thresholdis150°C.Whenthethermalshutdown istriggeredthedevicestopsswitchinguntilthe temperaturefallsbelowatypicalvalueof125°C.After thisthedevicewillonceagainresumeswitching. Application Information Inductor Selection Therearethreeimportantelectricalparametersthat needtobeconsideredwhenchoosinganinductor. Thesearetheinductorvalue,theDCR(parasiticserial DCresistance)andthesaturationcurrent. Theinductor’svaluedeterminestheinputripple current.Lowerinductorvaluesdecreasethephysical sizeoftheinductor,butincreasetheinputripple current.However,largerinductorvaluesdecreasethe inputripplecurrent,buthavehigherseriesresistances andlowersaturationcurrents.Agoodruletochoose asutiableinductorvalueistoallowthepeak-to- peakripplecurrenttobeapproximately30~50%of themaximuminputcurrent.Calculatetherequired inductancevalueusingthefollowingequations: L MA X IN PEAK L MA X IN L IN MA X OU T OU T MA X IN L SW OU T IN OU T IN I I I I I V I V I I F V V V V L = D D ´ ´ D ´ ´ - ´ ! # h ´ = = = + ~ Intheequationsabove,IOUT(MAX)isthemaximum loadcurrent,∆IListhepeak-to-peakinductorripple current,ηistheconverterefficiency,FSWisthe switchingfrequencyandIL(PEAK)isthepeakinductor current.Topreventinductorcoresaturation,ensure thattheinductor-saturationcurrentisratedhigher thanthepeakinductorcurrent.A10µHinductorvalue isrecommendedformostHT7939Aapplications. |
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