元器件交易网

SGLS371–JANUARY2007

EXAMPLEAPPLICATIONS(continued)

ComponentSelection

Table1.DesignParameters

SYMBOLVINVOUT

PARAMETER

InputvoltageOutputvoltageLineregulationLoadregulation

VOUT

OutputvoltageLineregulationLoadregulation

VRIPPLEVOVERVUNDERIOUTISCP

OutputripplevoltageOutputovershootOutputundershootOutputcurrent

Short-circuitcurrenttrippointEfficiency

FS(1)

Switchingfrequency

AtnominalinputvoltageandmaximumoutputcurrentIOUTat2.5A±0.2%VOUT±0.2%VOUTIOUTat2.5A±0.2%VOUT±0.2%VOUT

Atmaximumoutputcurrent

For2.5-Aloadtransientfrom2.5Ato0.25AFor2.5-Aloadtransientfrom0.25Ato2.5A

0.1253.75

90300

TESTCONDITION

MIN83.2003.2933.2934.854.9904.990

NOM123.33.33.35556010060

2.55.00MAX163.400(1)3.3073.3075.150(1)5.0105.010

UNITVVVVVVVmVmVmVAA%kHz

Set-pointaccuracyisdependentonexternalresistortoleranceandtheICreferencevoltage.Lineandloadregulationvaluesarereferencedtothenominaldesignoutputvoltage.

FETSelectionCriteria

Themaximuminputvoltageforthisapplicationis16V.Switchingtheinductorcausesovershootvoltagesthatcanequaltheinputvoltage.SincetheRDSONoftheFETriseswithbreakdownvoltage,selectaFETwithaslowabreakdownvoltageaspossible.Inthiscase,a30-VFETwasselected.

TheselectionofapowerFET’ssizerequiresknowingboththeswitchinglossesanddclossesintheapplication.AClossesareallfrequencydependentanddirectlyrelatedtodevicecapacitancesanddevicesize.Conversely,dclossesareinverselyrelatedtodevicesize.Theresultisanoptimumwherethetwotypesoflossesareequal.SincedevicesizeisproportionaltoRDSON,astartingpointistoselectadevicewithanRDSONthatresultsinasmalllossofpowerrelativetopackagethermalcapabilityandoverallefficiencyobjectives.

Inthisapplication,theefficiencytargetis90%andtheoutputpower8.25W.Thisgivesatotalpower-lossbudgetof0.916W.TotalFETlossesmustbesmall,relativetothisnumber.

2

ThedcconductionlossintheFETisgivenby:

PDC=Irms´RDSON

(8)

12öù

Thermscurrentisgivenby:

éæDIpp÷2úIrms=êD´çIOUT+

ç÷ê12ú

øûëè

2

(9)

18

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