A Product Line of Diodes Incorporated DN93 Wide input voltage range SEPIC LED driver using ZXLD1321 with external power switch Ray Liu, Applications Engineer, Diodes Incorporated Introduction Some LED lighting applications like the solar and automotive system, require an LED driver with a wide input voltage range where the LED voltage falls between minimum and maximum input voltage. In this case, neither boost nor buck topology would be a suitable solutions. SEPIC (Single Ended Primary Inductance Converter) resolves this issue and provide reasonable efficiency with relatively simple topology. The following is a design example of a SEPIC LED driver which is capable of driving multi-chip LEDs with forward voltage up to 17V and LED current of 1A . The input voltage is wide with range from 8V to 30V. 1A Multi-chips LED driver with external SEPIC MOSFET switch Figure 1 and Table 1 show the schematic and bill of materials of the design respectively. R1* 1 10 M_LOAD 11 2 9 8 L ED1 C9 L ED2 C8 Q2 EMITTER1 EMITTER2 ISENSE Q3 5 GND L1 R6 C7 R7 C6 D1 L ED3 6 7 Exp'd Pad COLLECTOR1 COLLECTOR2 CFB N/C TADJ 3 4 13 ADJ D2 VREF BIAS 14 M_VIN VIN 12 R1 U1 ZXLD1321 R8 Q1 VI N C5 C3 C4 R4 C15 Z1 C14 L 1' L ED4 R5 Q4 C10 C11 C12 C13 R9 R3 C1 C2 R2 R2* Z2 Figure 1 - Schematic Issue 1 - November 2008 © Diodes Incorporated 2008 1 www.zetex.com www.diodes.com DN93 Ref No. Value Part No. Manufacturer Note U1 LED Driver IC ZXLD1321 Diodes Zetex Q1 60V 10A Mosfet ZXMN6A25K Diodes Zetex Q2 Gate Driver Pairs ZXTC2045 Diodes Zetex Q3 50V NPN ZXTN2031 Diodes Zetex D1 PDS5100H Diodes Zetex D2 100V 5A Schottky Rectifier 100V 1A Rectifier S1B Diodes Zetex Z1 12V 350mW Zener BZX84C12 Diodes Zetex Z2 18V 350mW Zener BZX84C18 Diodes Zetex L1 22uH 5.2A SEPIC Inductor NPIS25DW220MTRF 744870220 MSD1278-223ML NIC Components Wurth Electronik Coilcraft www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.diodes.com www.zetex.com www.niccomp.com www.we-online.com www.coilcraft.com C1 10nF 50V SMD 0603 47nF 50V SMD 0603 470uF 50V NMC0603X7R103K50TRPF GRM188R71E103KA01 NMC0603X7R473K50TRPF GRM188R71E473KA01 Generic Electrolytic NIC Components Murata NIC Components Murata www.niccomp.com www.murata.com www.murata.com 4.7uF 50V X7R SMD 1206 GRM31CR71H475KA12 Murata www.murata.com 200mΩ NIC Components www.niccomp.com C2 C15 C5 C3 C4 C6 C7 C8 C9 C10 C11 C12 C13 C14 R1 R1* R2 R2* 20mΩ NCST12GR200GTRF SMD 1206 SMD 1206 R3 0Ω SMD 0805 / 0603 R4 1.2KΩ SMD 0805 / 0603 R5 33Ω SMD 0805 / 0603 R6 18Ω SMD 0805 / 0603 R7 10Ω SMD 0805 / 0603 R8 4.7KΩ SMD 0805 / 0603 R9 2.7KΩ SMD 0805 / 0603 Table 1 - Bill of materials www.zetex.com www.diodes.com 2 Issue 1 - November 2008 © Diodes Incorporated 2008 DN93 Circuit description In this design, the ZXLD1321 is used as an LED driver controller to drive the MOSFET power switch through a ZXTC2045 gate drive transistor pair. Although the input voltage rating of ZXLD1321 is 12V, by using an external 60V Mosfet power switch (ZXMN6A25K) and a simple 12V regulator formed by Q3 (ZXTN2031) and Z1, the system could operate up to 30V supply voltage with absolute maximum rating of 40V. The SEPIC power stage is formed by Q1, D1, L1, L1’ and C6 to C13. The same inductance is used for both L1 and L1’ such that a coupled inductor can also be used. Instead of using two inductors for the power stage, a single standard coupled inductor is used to minimize the component count. The output of the SEPIC stage is connected to the LEDs through the current sensing resistor R1. The voltage drop across R1 provides feedback to the LED controller IC and the sensing threshold is 100mV. Since the rated voltage of ZXLD1321’s MVIN and MLOAD pin are limited to 18V, the system is capable of driving LEDs with total LED voltage below 18V. To prevent MVIN and MLOAD pins from over-voltage, Z2 and Q4 formed a voltage detection circuit which will pull down the ADJ pin. This will shutdown the driver IC whenever the LED voltage is above 18V. Design Considerations Since the voltage spike caused by the high switching current and stray inductance of PCB’s copper track would be high compared to the low ISENSE voltage threshold of ZXLD1321 which is below 55mV, RC filter formed by C2 and R5 is used to prevent false triggering of ISENSE pin. i.e. C2 should be placed as close as possible between the ISENSE pin and the exposed GND pad of the IC. In SEPIC design, high ripple current appears at the output filtering capacitor (C10 – C13) and the SEPIC coupling capacitor (C6 - C9). Hence, a low ESR capacitor with an appropriate ripple current rating should be used. In this design, we recommend use of a standard X7R ceramic SMD capacitor working in parallel rather than the specialized pulse capacitor. Performance The typical efficiency throughout the input voltage range is shown in Figure 2 with average efficiency of 85%. The line regulation of LED current is shown in Figure 3 with line regulation better than 3%. Issue 1 - November 2008 © Diodes Incorporated 2008 3 www.zetex.com www.diodes.com DN93 Typical performance graphs 100 90 80 E ffic ienc y (% ) 70 60 50 40 30 20 V F = 16V or 4 LEDs V F = 13V or 3 LEDs 10 0 8 10 12 14 16 18 20 22 24 26 28 30 LE D V oltage (V ) E ffic ie n c y v s L E D V o lta g e Figure 2 1200 LE D Current (m A ) 1000 800 600 400 V F =1 6 V or 4 LEDs V F =1 3 V or 3 LEDs 200 0 8 10 12 14 16 18 20 22 24 26 28 30 Input V oltage (V ) L E D C u r r e n t v s In p u t V o lta g e Figure 3 www.zetex.com www.diodes.com 4 Issue 1 - November 2008 © Diodes Incorporated 2008 DN93 Conclusion The ZXLD1321 can be used in SEPIC mode to produce a high buck-boost LED driver with scalable output current capability at an efficiency of around 85%. The input voltage can be between 8 and 30V but the number of LEDs must be limited to ensure the output voltage is less than 17V. Issue 1 - November 2008 © Diodes Incorporated 2008 5 www.zetex.com www.diodes.com DN93 www.zetex.com www.diodes.com 6 Issue 1 - November 2008 © Diodes Incorporated 2008 DN93 Definitions Product change Diodes Incorporated the right to alter, without notice, specifications, design, price or conditions of supply of any product or service. Customers are solely responsible for obtaining the latest relevant information before placing orders. Applications disclaimer The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for the user’s application and meets with the user’s requirements. 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Life support devices or systems are devices or systems which: 1. are intended to implant into the body or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labelling can be reasonably expected to result in significant injury to the user. B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. Reproduction The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. 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