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Design Idea DI-74 (R) 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
LinkSwitch-TN uses ON/OFF control to regulate the output current. When the current into the FEEDBACK (FB) pin exceeds 49 A, MOSFET switching is disabled for the next switching cycle. The 49 A 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 A 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.
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.
VR1 R2 BZX79C18 D4 300 18 V, 500 mW UF4005 1% RF2 47 , 0.5 W Flame Proof R1 2 k 1% R3 20 0.5 W 1% D3 UF4005 1 A, 600 V Ultrafast 12.5 V, 100 mA
RF1 D1 8.2 , 1 W 1N4007 Fusible
FB
BP S
C5 1 uF, 50 V low ESR
LED 1-20 25 mA per String
85-265 VAC
C1 4.7 uF 400 V
C2 4.7 uF 400 V
D C3 0.1 uF LinkSwitch-TN 50 V U1 LNK304P L1 1.2 mH 220 mA
C4 47 uF 10 V
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
DI-74
PI-3847-020904
Key Design Points
* The output is not safety isolated from the input. * The circuit shown in Figure 1 has a total output current tolerance of 12% (including t of 50 C). * To prevent noise coupling and to decrease EMI, place the input filter components physically away from the source node of the LinkSwitch-TN and L1 inductor. The DC input filter capacitors C1 and C2 can be placed, as a barrier, between the AC input and U1/L1. * Select C4 so that C4 20 * (15 ms / R3), to provide adequate filtering of the current sense voltage. Values above 50 * (15 ms / R3) will yield little improvement in CC linearity. * Select C5 based on the acceptable peak current through the LEDs. Larger values of capacitance reduce peak LED current. Typical values are 100 nF to 100 F, low ESR. With no capacitor the peak output current is equal to the internal current limit of U1. * With the values of R1 and R2 shown, the value of R3 is calculated by R3 = 2 V / IO. * The supply's total required output current is determined by the number of LED strings in the load, and is limited by the value of L1 and the current limit of U1. For this circuit, the load should be 100 mA and 1.25 W total.
80 70 60 50 40
QP AV
Quasi-Peak
dBV
30 20 10 0 -10 -20 0.15 1.0 10.0 100.0
Average
MHz
Figure 2. Conducted EMI Plot of the Schematic Shown in Figure 1 for 3 Strings of 4 LED's (10 V, 66.6 mA).
Output Voltage (V)
20 15 10 5 0 0 20 40 60
AC Input = 85 V AC Input = 265 V
80
100
120
Output Current (mA)
Figure 3. VI Characteristic of the Schematic Shown in Figure 1.
For the latest updates, visit www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm.
The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, PeakSwitch, EcoSmart, Clampless, E-Shield, Filterfuse, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. (c)Copyright 2006, Power Integrations, Inc.
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PI-3848-021304
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