WT7527
Rev. 1.20
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APPLICATION NOTE
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Whenꢀtheꢀloadꢀcurrentꢀincreased,ꢀtheꢀvoltageꢀ(VL)ꢀcrossꢀtheꢀinductorꢀisꢀincreased.ꢀ
Andꢀwhenꢀinductorꢀvoltageꢀexceedsꢀtheꢀresistorꢀvoltageꢀ(VR),ꢀtheꢀOCPꢀisꢀactive.ꢀ
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Sometimesꢀ powerꢁonꢀ orꢀ loadꢀ dynamicsꢀ willꢀ causeꢀ falseꢀ outputꢀ ofꢀ overꢁcurrentꢀ detection.ꢀ Itꢀ canꢀ beꢀ
solvedꢀbyꢀconnectingꢀaꢀcapacitorꢀbetweenꢀVSꢀpinꢀandꢀISꢀpin.ꢀInꢀtypicalꢀcase,ꢀCꢀ≥ꢀ0.47uFꢀisꢀsuggested.ꢀ
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OCPꢀpointꢀcanꢀbeꢀcalculatedꢀbyꢀtheꢀfollowingꢀequation:ꢀ
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LetꢀVRꢀ=ꢀVLꢀ ꢀ
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R × IR = RL × IL
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VREF
RI
Q IR = 8× IREF = 8×
RL× IL
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∴R =
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VREF
8×
RI
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Forꢀexample：ꢀ ꢀ
AssumeꢀRI=30Kꢂ,ꢀRL=5mꢂ,ꢀOCPꢀIL=20A.ꢀ
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Sol：Rꢀ=ꢀ(ꢀILꢀ*ꢀRLꢀ)ꢀ/ꢀ(ꢀ8ꢀ*ꢀIREFꢀ)ꢀ
=ꢀ(ꢀ20Aꢀ*ꢀ5mꢁ)ꢀ/ꢀ{ꢀ8ꢀ*ꢀ(ꢀ1.2Vꢀ/ꢀ30Kꢁ)}ꢀ
=ꢀ312.5ꢁ
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Weltrend Semiconductor, Inc.
Pageꢀ7
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