Design Idea DI-131

LinkSwitch-TN

LED Driver for Halogen Light Bulb Retrofit

Figure 1. 3 W LED, Halogen Bulb Replacement using LinkSwitch-TN in Non-Isolated Buck Converter Technology.

DI-131

March 2007

Application

Device

Power Output Input Voltage

Output Voltage

Topology

LED Lighting LNK306DN

3 W

85-265 VAC

10 V

Non-isolated Buck

Design Highlights

• FitsinsidestandardGU10lampbase

• Constantcurrent(CC)output

• DrivecurrentindependentofLEDV

• Tightlineregulation(±5%)forconstantlightoutput

• Noannoyinglinefrequencyflicker

• Universalinputvoltagerange

• Asingledesigncanbesoldworldwide

• Consistentlyhighefficiencyacrossentirerange

• MeetsEN55022BconductedEMIlimits

• Completeovervoltageandovertemperatureprotection

• Operatesunloaded:canbetestedwithouttheLEDs

• Auto-restart:withstandsshortedoutputindefinitely

• Thermalshutdown:protectsentirelampassembly

Operation

The non-isolated buck converter shown in Figure 1 was

designedaroundamemberoftheLinkSwitch-TNICfamily,

the LNK306DN (U1).A LNK306 was selected since its

minimumcurrentlimit(450mA)ensuresthatitwillbeable

todelivertherequired330mAofoutputcurrent.

DiodebridgeBR1rectifiestheACinputwhilecapacitorsC1

andC2filterit.InductorL1andRF1attenuateconductedEMI.

Incaseofacomponentfailure,RF1willactasafuse.

TheMOSFETwithinU1,diodeD3,inductorL2andcapacitor

C3formthebuckconverterstage.Thecontrollerwithin

U1regulatestheoutputcurrentbyenablinganddisabling

MOSFETswitchingcycles.

During normal operation, the output current develops a

voltagedropacrossR8andR10,whichisimpressedacross

C6.WhenthevoltageacrossC6exceedstheV

Q1andQ2bothturnon.WhenQ2turnson,currentis

injectedintotheFEEDBACK(FB)pinofU1fromC5,via

R3.WheneverthecurrentintotheFBpinexceeds49mA,

MOSFETswitchingisdisabled.Thecontrolleradjuststhe

ratioofenabledanddisabledcyclestoregulatetheoutput

currentat330mA.

Duringno-loadoperation,theoutputvoltageissensedby

D1,D2andC2.ThevoltageacrossC5feedscurrentinto

theFBpinofU1throughdividerR7andR1.TheFBpin

®

J4

9-15 V,

300 mA

J3

L

N

PI-4575-030707

FB

BP

S

D

LinkSwitch-TN

C1

4.7

µF

380 V

C4

100 nF

25 V

D1

BAV21WS-7-F

Q1

MMST3904

Q2

MMST3906

D2

BAV21WS-7-F

BR1

MB6S

600 V

D3

MURS160T3

85-265

VAC

U1

LNK306DN

C3

2.2

µF

25 V

C5

100 nF

25 V

C6

100 nF

25 V

RF1

10

Ω

2.5 W

R7

30 k

Ω

R3

10 k

Ω

R1

4.7 k

Ω

R9

4.7 k

Ω

R6

10 k

Ω

R4

10 k

Ω

R8

2

Ω

L2

1 mH

L1

1 mH

R10

12

Ω

C2

4.7

µF

380 V

R11

1 k

Ω