DI-74 Design Idea LinkSwitch-TN 1.25 W Non-Isolated Constant Current LED Driver Application Device Power Output Input Voltage Output Voltage Topology LED Driver LNK304PN 1.25 W 85 265 VAC 12.5 V, 100 mA Buck-Boost Design Highlights LinkSwitch-TN uses ON/OFF control to regulate the output Universal AC input range single design worldwide current. When the current into the FEEDBACK (FB) pin exceeds Accurate and stable constant current output 49 A, MOSFET switching is disabled for the next switching cycle. Small size and weight The 49 A threshold is speci ed at a pin voltage of 1.65 V (7%), Low cost, low component count solution replaces passive allowing it to be used as a reference. capacitor or resistor droppers Fail safe topology protects load The voltage developed across R3, which is averaged by C4, High ef ciency (>60% at 85 VAC) represents the output current. When this voltage exceeds 2 V, the Scalable output power using larger LinkSwitch-TN devices voltage on the FB pin, via divider of R1 and R2, exceeds 1.65 V Meets EN55022 B EMI limits 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 Operation load is disconnected or the output is shorted, no feedback is Using the Buck-Boost topology, the circuit shown in Figure 1 provided and LinkSwitch-TN enters auto-restart (5% on time). To generates a non-isolated constant current (CC) output for driving prevent a high output voltage under no load the optional voltage LEDs. Typical uses include night-lights, neon sign replacements, feedback circuit can be included by adding VR1 and D4. The emergency exit signs or any application utilizing LEDs for lighting. voltage of VR1 is selected to be above the normal output voltage. The AC input is recti ed and ltered by D1, D2, C1, C2, RF1, and Select the value of L1 following AN-37, LinkSwitch-TN Design RF2. Two diodes improve both line surge withstand (2 kV) and Guide or using the PIXls design spreadsheet. Enter the output conducted EMI. Resistor RF1 should be a fusible ameproof type, voltage as the voltage of the LED string and the output current as whereas RF2 can be ameproof only. the total combined LED current. VR1 R2 BZX79C18 D4 300 18 V, 500 mW UF4005 1% R3 R1 RF2 RF1 20 C5 2 k D1 47 , 0.5 W 8.2 , 1 W LED 1-20 0.5 W 1 uF, 50 V 1% FB BP 1N4007 Flame Proof Fusible 25 mA per String 1% low ESR S D C3 D3 0.1 uF UF4005 LinkSwitch-TN 50 V C4 1 A, 600 V U1 47 uF Ultrafast LNK304PN 10 V C1 C2 L1 12.5 V, 85-265 4.7 uF 4.7 uF 1.2 mH 100 mA VAC 400 V 400 V 220 mA D2 1N4007 PI-3846-010208 Figure 1. Schematic of a 1.25 W, 12.5 V, 100 mA Constant Current LED Driver, Using a Non-Isolated Buck-Boost Topology. www.powerint.com January 2008Key Design Points 80 The output is not safety isolated from the input. 70 The circuit shown in Figure 1 has a total output current QP tolerance of 12% (including t of 50 C). 60 To prevent noise coupling and to decrease EMI, place the input AV 50 lter components physically away from the source node of the Quasi-Peak LinkSwitch-TN and L1 inductor. The DC input lter capacitors 40 C1 and C2 can be placed, as a barrier, between the AC input 30 and U1/L1. Select C4 so that C4 20 (15 ms / R3), to provide adequate 20 ltering of the current sense voltage. Values above 50 (15 ms / Average 10 R3) will yield little improvement in CC linearity. Select C5 based on the acceptable peak current through the 0 LEDs. Larger values of capacitance reduce peak LED current. -10 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. -20 With the values of R1 and R2 shown, the value of R3 is 0.15 1.0 10.0 100.0 calculated by R3 = 2 V / I . MHz O The supply s total required output current is determined by the Figure 2. Conducted EMI Plot of the Schematic Shown in Figure 1 for 3 Strings of number of LED strings in the load, and is limited by the value of 4 LED s (10 V, 66.6 mA). L1 and the current limit of U1. For this circuit, the load should be 100 mA and 1.25 W total. 25 AC Input = 85 V 20 AC Input = 265 V 15 10 5 0 04200 60 80 100 120 Output Current (mA) Figure 3. VI Characteristic of the Schematic Shown in Figure 1. Power Integrations 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. 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