BL8516
toꢁpeakꢀinductorꢀrippleꢀcurrentꢀisꢀequalꢀtoꢀoneꢀ
thirdꢀofꢀswitchꢀcurrentꢀlimit.ꢀꢀHenceꢀtheꢀvalueꢀ
ofꢀ inductanceꢀ andꢀ theꢀ maximumꢀ outputꢀ
currentꢀcanꢀbeꢀexpressedꢀas:ꢀꢀ
largeꢀ outputꢀ capacitance.ꢀ ꢀ Theꢀ aboveꢀ
equationꢀalsoꢀoffersꢀaꢀgoodꢀstartingꢀpointꢀforꢀ
obtainꢀtheꢀproperꢀoutputꢀcapacitanceꢀvalueꢀtoꢀ
achieveꢀsatisfactoryꢀperformance.ꢀꢀ
Lꢀ=ꢀ3(1ꢀ–ꢀD)(VOUTꢀ+ꢀVD)/(ILIMITꢀ•ꢀFOSC)ꢀ
andꢀ
Applicationꢀ engineersꢀ canꢀ startꢀ withꢀ theꢀ
datasheetꢀrecommendedꢀvalue.ꢀ
IOUTꢀ(MAX)ꢀ=ꢀ(5/6)ꢀILIMITꢀ
Theꢀhighꢀperformanceꢀ(lowꢀESR),ꢀsmallꢀsizeꢀ
andꢀ robustnessꢀ ofꢀ ceramicꢀ capacitorsꢀ makeꢀ
themꢀ theꢀ preferredꢀ typeꢀ forꢀ BL8516ꢀ
applications.ꢀ Oneꢀ needsꢀ keepꢀ inꢀ mindꢀ thatꢀ
notꢀ allꢀ ceramicꢀ capacitorsꢀ areꢀ theꢀ same.ꢀ
Manyꢀofꢀtheꢀhigherꢀvalueꢀcapacitorsꢀuseꢀpoorꢀ
dielectricsꢀwithꢀhighꢀtemperatureꢀandꢀvoltageꢀ
coefficients.ꢀ
Fromꢀ aboveꢀ discussionꢀitꢀ isꢀ derivedꢀ thatꢀtheꢀ
switchꢀcurrentꢀlimitꢀisꢀdependantꢀonꢀtheꢀdutyꢀ
cycle,ꢀwhichꢀalsoꢀisꢀaꢀfunctionꢀofꢀinputꢀvoltage.ꢀ
Forꢀ aꢀ specificꢀ application,ꢀ oneꢀ canꢀ calculateꢀ
theꢀ valueꢀ ofꢀ inductanceꢀ andꢀ theꢀ rangeꢀ ofꢀ
availableꢀ outputꢀ currentꢀ accordingꢀ toꢀ theꢀ
maximumꢀandꢀminimumꢀallowedꢀinputꢀvoltageꢀ
values.ꢀꢀ
ꢀ
ꢀ
Catch Diode
Capacitor Selection
Schottkyꢀ diodeꢀ withꢀ 3Aꢀ currentꢀ ratingꢀ isꢀ
recommendedꢀforꢀtheꢀcatchꢀdiodeꢀD1.ꢀꢀꢀ
ꢀ
Theꢀ inputꢀ capacitorꢀ (C1)ꢀ suppliesꢀ theꢀ ACꢀ
componentꢀ ofꢀ theꢀ currentꢀ drawnꢀ byꢀ theꢀ
converter.ꢀ Aꢀ 10uFꢀ ceramicꢀ capacitorꢀ isꢀ
sufficientꢀinꢀmostꢀcases.ꢀ
Inꢀcaseꢀtheꢀapplicationꢀcircuitryꢀhasꢀrelativelyꢀ
largeꢀ parasiticꢀ parametersꢀ (mostꢀ oftenꢀ
causedꢀ byꢀ longꢀ wiresꢀ orꢀ filterꢀ components),ꢀ
additionalꢀ bulkꢀ inputꢀ capacitanceꢀ mayꢀ beꢀ
required.ꢀ Forꢀ highꢁpowerꢀ applicationsꢀ itꢀ isꢀ
suggestedꢀtoꢀ increaseꢀ theꢀ inputꢀ capacitanceꢀ
toꢀ100uF.ꢀꢀ
BOOST Pin Considerations
Inꢀ theꢀ applicationꢀ circuitꢀ shownꢀ inꢀ theꢀ coverꢀ
page,ꢀcapacitorꢀC3ꢀandꢀdiodeꢀD2ꢀareꢀusedꢀtoꢀ
boostꢀinputꢀvoltageꢀtoꢀaꢀhigherꢀlevel.ꢀInꢀmostꢀ
casesꢀaꢀ10nFꢀcapacitorꢀandꢀaꢀfastꢀswitchingꢀ
diodeꢀ areꢀ suitableꢀ forꢀ thisꢀ purpose.ꢀ Theꢀ
voltageꢀ atꢀ theꢀ BOOSTꢀ pinꢀ mustꢀ beꢀ atꢀ leastꢀ
2.5Vꢀ higherꢀ thanꢀ theꢀ SWꢀ pinꢀ forꢀ bestꢀ
efficiency.ꢀꢀ
Theꢀ outputꢀ capacitorꢀfiltersꢀ theꢀ squareꢀ waveꢀ
generatedꢀ byꢀ theꢀ chipꢀ toꢀ produceꢀ theꢀ DCꢀ
outputꢀandꢀstoresꢀenergyꢀtoꢀsupplyꢀtransientꢀ
loadsꢀ asꢀ wellꢀ asꢀ stabilizesꢀ theꢀ controlꢀ loop.ꢀ
Whileꢀ forꢀ mostꢀ switchingꢀ modeꢀ converters,ꢀ
theꢀoutputꢀrippleꢀisꢀmainlyꢀdeterminedꢀbyꢀtheꢀ
equivalentꢀ seriesꢀ resistanceꢀ (ESR)ꢀ ofꢀ theꢀ
outputꢀcapacitor,ꢀtheꢀBL8516’sꢀcontrolꢀloopꢀisꢀ
independentꢀ onꢀ theꢀ outputꢀ capacitor’sꢀ ESRꢀ
forꢀstableꢀoperation.ꢀIfꢀaꢀlowꢀESRꢀcapacitorꢀisꢀ
selected,ꢀ forꢀ exampleꢀ ceramicꢀ capacitor,ꢀ
Outputꢀ rippleꢀ canꢀ beꢀ estimatedꢀ byꢀ theꢀ
followingꢀequation:ꢀ
TheꢀminimumꢀoperatingꢀvoltageꢀofꢀanꢀBL8516ꢀ
applicationꢀ isꢀ limitedꢀ byꢀ theꢀ underꢀ voltageꢀ
lockoutꢀ (<ꢀ 3.6V)ꢀ andꢀ byꢀ theꢀ Maximumꢀ dutyꢀ
cycleꢀasꢀoutlinedꢀabove.ꢀꢀ
Forꢀproperꢀstartup,ꢀtheꢀboostꢀcircuitꢀalsoꢀlimitsꢀ
theꢀ minimumꢀ inputꢀ voltage.ꢀ Ifꢀ theꢀ inputꢀ
voltageꢀ isꢀ rampedꢀ slowly,ꢀ orꢀ theꢀ BL8516ꢀ isꢀ
turnedꢀonꢀwithꢀitsꢀCEꢀpinꢀwhenꢀtheꢀoutputꢀisꢀ
alreadyꢀinꢀregulation,ꢀthenꢀtheꢀboostꢀcapacitorꢀ
mayꢀnotꢀbeꢀfullyꢀcharged.ꢀBecauseꢀtheꢀboostꢀ
capacitorꢀisꢀchargedꢀwithꢀtheꢀenergyꢀstoredꢀinꢀ
theꢀ inductor,ꢀ theꢀ circuitꢀ willꢀ relyꢀ onꢀ someꢀ
minimumꢀloadꢀcurrentꢀtoꢀgetꢀtheꢀboostꢀcircuitꢀ
runningꢀ properly.ꢀ Thisꢀ minimumꢀ loadꢀ willꢀ
dependꢀonꢀinputꢀandꢀoutputꢀvoltages,ꢀandꢀonꢀ
theꢀ arrangementꢀ ofꢀ theꢀ boostꢀ circuit.ꢀ Theꢀ
minimumꢀ loadꢀ generallyꢀ goesꢀ toꢀ zeroꢀ onceꢀ
theꢀ circuitꢀ hasꢀ started.ꢀ Inꢀ manyꢀ casesꢀ theꢀ
dischargedꢀ outputꢀ capacitorꢀ willꢀ presentꢀ aꢀ
loadꢀtoꢀtheꢀswitcher,ꢀwhichꢀwillꢀallowꢀitꢀtoꢀstart.ꢀꢀ
ꢀ
VRIPPLEꢀ=ꢀꢃIL•ꢀ[ESR+1/(8•ꢀFOSC•C2)]ꢀ
Theꢀ outputꢀ capacitorꢀ mustꢀ haveꢀ anꢀ energyꢀ
capacityꢀ largerꢀ thanꢀ theꢀ inductorꢁotherwiseꢀ
theꢀtransferꢀofꢀenergyꢀfromꢀtheꢀinductorꢀtoꢀtheꢀ
outputꢀcanꢀcauseꢀlargeꢀovershoot.ꢀForꢀaꢀ5%ꢀ
overshoot,ꢀ theꢀ minimumꢀ valueꢀ ofꢀ theꢀ outputꢀ
capacitorꢀcanꢀbeꢀpresentedꢀas:ꢀ
C2 ≥ 10 • L • (ILIMIT /VOUT )2
ꢀ
Goodꢀ transientꢀ performanceꢀ alsoꢀ requiresꢀ
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