RF1

8.2

Ω, 1 W

Fusible

D1

1N4007

D2

1N4007

D3

UF4005

1 A, 600 V

Ultrafast

12.5 V,

100 mA

D4

UF4005

VR1

BZX79C18

18 V, 500 mW

C1

4.7 uF

400 V

C2

4.7 uF

400 V

C5

1 uF, 50 V

low ESR

LED 1-20

25 mA per String

C3

0.1 uF

50 V

RF2

47

Ω, 0.5 W

Flame Proof

R1

2 k

Ω

1%

R2

300

Ω

1%

R3

20

Ω

0.5 W

1%

C4

47 uF

10 V

U1

LNK304P

85-265

VAC

PI-3846-062106

FB

BP

S

D

LinkSwitch-TN

L1

1.2 mH

220 mA

Application

LED Driver

LNK304P

12.5 V, 100 mA

Power Output

1.25 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Buck-Boost

Figure 1. Schematic of a 1.25 W, 12.5 V, 100 mA constant current LED Driver, Using a Non-Isolated Buck-Boost Topology.

Design Highlights

• Universal AC input range – single design worldwide

• Accurate and stable constant current output

• Small size and weight

• Low cost, low component count solution replaces passive

capacitor or resistor droppers

• Fail safe topology – protects load

• High efficiency (>60% at 85 VAC)

• Scalable output power using larger LinkSwitch-TN devices

• Meets EN55022 B EMI limits

Operation

Using the Buck-Boost topology, the circuit shown in Figure 1

generates a non-isolated constant current (CC) output for

driving LEDs. Typical uses include night-lights, neon sign

replacements, emergency exit signs or any application utilizing

LEDs for lighting.

The AC input is rectified and filtered by D1, D2, C1, C2, RF1,

and RF2. Two diodes improve both line surge withstand (2 kV)

andconductedEMI. ResistorRF1shouldbeafusibleflameproof

type, whereas RF2 can be flameproof only.

www.powerint.com

DI-74

LinkSwitch-TN

uses ON/OFF control to regulate the output

current. When the current into the FEEDBACK (FB) pin

exceeds 49 µA, MOSFET switching is disabled for the

next switching cycle. The 49 µA threshold is specified at

a pin voltage of 1.65 V (±7%), allowing it to be used as a

reference.

The voltage developed across R3, which is averaged by C4,

represents the output current. When this voltage exceeds

2 V, the voltage on the FB pin, via divider of R1 and R2,

exceeds 1.65 V and >49 µA current is fed into the pin. The

2 V across R3 sets an output current of 100 mA, or 25 mA

for each string of LEDs. If the load is disconnected or the

outputisshorted,nofeedbackisprovidedandLinkSwitch-TN

enters auto-restart (5% on time). To prevent a high output

voltage under no load the optional voltage feedback circuit

can be included by adding VR1 and D4. The voltage of VR1

is selected to be above the normal output voltage.

Select the value of L1 following AN-37, LinkSwitch-TN

Design Guide or using the PIXls design spreadsheet. Enter

the output voltage as the voltage of the LED string and the

output current as the total combined LED current.

Design Idea DI-74

Constant Current LED Driver

June 2006

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