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Rev. B 03/07

Power Integrations

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voltageisspecifiedas1.63Vat49mA,whichallowsittobe

usedasareference.Theoutputvoltageisregulatedto<18V

bydisablingMOSFETswitchingcyclesasthecurrentintothe

FBpinexceeds49mA.

Key Design Points

• TopreventEMIcouplingbetweentheswitchingnodeson

themainboardandtheinputfilterboard,ashieldboard

(connectedtoC2negative)wasplacedbetweenthetwo

PCboards(seeFigure3).Thisgreatlyimprovedthe

margintotheconductedEMIlimits.

• DiodesD1andD2canbereplacedbyasingle600V

diode.Two250Vdiodeswereusedbecausetheytookup

lessspacethanasingle600Vdiode.

• IncreasingthevalueofC3willreducetheripplecurrent

throughtheLEDloadattheexpenseofincreased

componentsizeandcost.

• Pre-loadresistorR11ensurescorrectno-loadoperation,

whichenablesthepowersupplytobetestedwithoutthe

loadconnectedtoit.

• VerifythatL2doesnotsaturateheavilyatthemaximum

currentlimitofU1(647mA).RefertotheInductor

Manufacturer’sL–20%currentrating.

• Inlessthermallychallengedenclosures,theFBpincan

beusedtodirectlysensethevoltageacrossresistors

R8andR10.ThiscaneliminateQ1,Q2andassociated

components,butrequiresthecurrentlimitsensevoltage

toincreasefrom0.65Vto1.65V.Thiswillincreasethe

senseresistordissipationbyabout300mWandcausethe

internalambienttemperatureofthesupplytoincrease.

• MaximizethePCBareathattheU1SOURCEpins

connectto,foroptimalheatsinking.

• Sincethisdesigndoesnotprovideinputtooutput

isolation,theloadLEDsmustbeisolatedfromtheuservia

thelightbulbenclosure.

Figure 3. Assembled Unit, Dimensions: 0.78 × 1 inch

(19.8 × 25.4 mm).

Figure 2. Worst Case Conducted EMI (230 VAC).

1.080

”

TOL ± 0.050

”

0.742

”

0.614

”

PI-4577-030707

1.024

”

0.780

”

1.0

0.15

10.0

100.0

-20

-10

0

10

20

30

40

50

60

80

70

MHz

QP

AV

QP

AV