DI-89
DI-131
80
70
60
50
40
30
20
10
0
voltageꢀisꢀspecifiedꢀasꢀ1.63ꢀVꢀatꢀ49ꢀmA,ꢀwhichꢀallowsꢀitꢀtoꢀbeꢀ
usedꢀasꢀaꢀreference.ꢀꢀTheꢀoutputꢀvoltageꢀisꢀregulatedꢀtoꢀ<18ꢀVꢀ
byꢀdisablingꢀMOSFETꢀswitchingꢀcyclesꢀasꢀtheꢀcurrentꢀintoꢀtheꢀ
FBꢀpinꢀexceedsꢀ49ꢀmA.
QP
AV
QP
Key Design Points
AV
•ꢀ ToꢀpreventꢀEMIꢀcouplingꢀbetweenꢀtheꢀswitchingꢀnodesꢀonꢀ
theꢀmainꢀboardꢀandꢀtheꢀinputꢀfilterꢀboard,ꢀaꢀshieldꢀboardꢀ
(connectedꢀtoꢀC2ꢀnegative)ꢀwasꢀplacedꢀbetweenꢀtheꢀtwoꢀ
PCꢀboardsꢀ(seeꢀFigureꢀ3).ꢀꢀThisꢀgreatlyꢀimprovedꢀtheꢀ
marginꢀtoꢀtheꢀconductedꢀEMIꢀlimits.
•ꢀ DiodesꢀD1ꢀandꢀD2ꢀcanꢀbeꢀreplacedꢀbyꢀaꢀsingleꢀ600ꢀVꢀ
diode.ꢀꢀTwoꢀ250ꢀVꢀdiodesꢀwereꢀusedꢀbecauseꢀtheyꢀtookꢀupꢀ
lessꢀspaceꢀthanꢀaꢀsingleꢀ600ꢀVꢀdiode.
-10
-20
0.15
1.0
10.0
100.0
MHz
•ꢀ IncreasingꢀtheꢀvalueꢀofꢀC3ꢀwillꢀreduceꢀtheꢀrippleꢀcurrentꢀ
throughꢀtheꢀLEDꢀloadꢀatꢀtheꢀexpenseꢀofꢀincreasedꢀ
componentꢀsizeꢀandꢀcost.
Figure 2. Worst Case Conducted EMI (230 VAC).
•ꢀ Pre-loadꢀresistorꢀR11ꢀensuresꢀcorrectꢀno-loadꢀoperation,ꢀ
whichꢀenablesꢀtheꢀpowerꢀsupplyꢀtoꢀbeꢀtestedꢀwithoutꢀtheꢀ
loadꢀconnectedꢀtoꢀit.
•ꢀ VerifyꢀthatꢀL2ꢀdoesꢀnotꢀsaturateꢀheavilyꢀatꢀtheꢀmaximumꢀ
currentꢀlimitꢀofꢀU1ꢀ(647ꢀmA).ꢀꢀReferꢀtoꢀtheꢀInductorꢀ
Manufacturer’sꢀLꢀ–ꢀ20%ꢀcurrentꢀrating.
0.780”
TOL 0.0ꢀ0”
•ꢀ Inꢀlessꢀthermallyꢀchallengedꢀenclosures,ꢀtheꢀFBꢀpinꢀcanꢀ
beꢀusedꢀtoꢀdirectlyꢀsenseꢀtheꢀvoltageꢀacrossꢀresistorsꢀ
R8ꢀandꢀR10.ꢀꢀThisꢀcanꢀeliminateꢀQ1,ꢀQ2ꢀandꢀassociatedꢀ
components,ꢀbutꢀrequiresꢀtheꢀcurrentꢀlimitꢀsenseꢀvoltageꢀ
toꢀincreaseꢀfromꢀ0.65ꢀVꢀtoꢀ1.65ꢀV.ꢀꢀThisꢀwillꢀincreaseꢀtheꢀ
senseꢀresistorꢀdissipationꢀbyꢀaboutꢀ300ꢀmWꢀandꢀcauseꢀtheꢀ
internalꢀambientꢀtemperatureꢀofꢀtheꢀsupplyꢀtoꢀincrease.
•ꢀ MaximizeꢀtheꢀPCBꢀareaꢀthatꢀtheꢀU1ꢀSOURCEꢀpinsꢀ
connectꢀto,ꢀforꢀoptimalꢀheatsinking.
1.080”
1.024”
0.742”
0.614”
•ꢀ Sinceꢀthisꢀdesignꢀdoesꢀnotꢀprovideꢀinputꢀtoꢀoutputꢀ
isolation,ꢀtheꢀloadꢀLEDsꢀmustꢀbeꢀisolatedꢀfromꢀtheꢀuserꢀviaꢀ
theꢀlightꢀbulbꢀenclosure.
PI-4ꢀ77-030707
Figure 3. Assembled Unit, Dimensions: 0.78 × 1 inch
(19.8 × 25.4 mm).
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