DI-74 Datasheet, PDF (1/2 Pages) Power Integrations, Inc. – 1.25 W Non-Isolated Constant Current LED Driver

Design Idea DI-74
LinkSwitch^®-TN 1.25 W Non-Isolated
Constant Current LED Driver
Application
LED Driver
Device
LNK304P
Power Output
1.25 W
Input Voltage
85-265 VAC
Output Voltage
12.5 V, 100 mA
Topology
Buck-Boost
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)
and conducted EMI. Resistor RF1 should be a fusible flameproof
type, whereas RF2 can be flameproof only.
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
output is shorted, no feedback is provided and LinkSwitch-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.
R2
300 Ω
1%
VR1
BZX79C18
18 V, 500 mW
D4
UF4005
RF1
8.2 Ω, 1 W D1
Fusible ^1N4007
85-265
VAC
C1
4.7 uF
400 V
RF2
47 Ω, 0.5 W
Flame Proof
FB BP
R1
2 kΩ
1%
C2
4.7 uF
400 V
D
S _C3
LinkSwitch-TN
U1
0.1
50
uF
V
LNK304P
L1
1.2 mH
220 mA
R3
20 Ω
0.5 W
1%
C4
47 uF
10 V
C5
1 uF, 50 V
low ESR
D3
UF4005
1 A, 600 V
Ultrafast
12.5 V,
100 mA
LED 1-20
25 mA per String
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.
DI-74
www.powerint.com
June 2006