Design Idea DI-74
®
LinkSwitch-TN 1.25 W Non-Isolated
Constant Current LED Driver
Topology
Device
Power Output
Output Voltage
Application
Input Voltage
Buck-Boost
LED Driver
1.25 W
85-265 VAC
LNK304P
12.5 V, 100 mA
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.
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
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.
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.
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.
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.
VR1
R2
300 Ω
1%
BZX79C18
D4
18 V, 500 mW UF4005
R3
20 Ω
0.5 W
1%
R1
RF2
RF1
C5
2 kΩ
D1
47 Ω, 0.5 W
LED 1-20
25 mA per String
8.2 Ω, 1 W
Fusible
1 uF, 50 V
low ESR
1%
FB
BP
S
1N4007 Flame Proof
D
C3
D3
0.1 uF
50 V
UF4005
1 A, 600 V
Ultrafast
LinkSwitch-TN
U1
C4
47 uF
10 V
LNK304P
C1
4.7 uF
400 V
C2
4.7 uF
400 V
L1
12.5 V,
100 mA
85-265
VAC
1.2 mH
220 mA
D2
1N4007
PI-3846-062106
Figure 1. Schematic of a 1.25 W, 12.5 V, 100 mA constant current LED Driver, Using a Non-Isolated Buck-Boost Topology.
June 2006
DI-74
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