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| DN86 Reduced component count and compact reference design for MR16 replacement lamps using multiple 1W LEDs Silvestro Russo, October 2007 Introduction MR16 lamps are one variety of Multifaceted Reflector (MR) lamps that usually employ a halogen filament capsule as the light source. They are used in many retail and consumer lighting applications where their size, configurability, spot-lighting capability and aesthetics provide utility and creativity. Low efficiency, heat generation and halogen capsule handling issues are among the disadvantages of the technology. They typically operate from 12V DC or 12V AC, using conventional electromagnetic transformers. LEDs offer a more energy efficient and no radiated heat solution to replace some halogen lamp applications. This reference design is intended to fit into the base connector space of an MR16 style LED lamp. The design has been optimized for part count and thermal performance. The design can be used with up to 3 1W LEDs in the Lens section. This can be arranged to suit the luminary designer's requirements. Figure 1 Data sheet MR16 application with ZXLD1350 It is recommended that this design note is used with the data sheet for the ZXLD1350 see http://www.zetex.com/3.0/pdf/ZXLD1350.pdf Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 1 www.zetex.com DN86 Description The system diagram of the MR16 lamp solution with ZXLD1350 and ZXSBMR16PT8 is shown in Figure 2, and Table 1 provides the bill of materials. Figure 2 System diagram of ZXLD1350 MR16 Lamp Solution The ZXLD1350 is designed for LED current drive applications of up to 350mA. The monolithic NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which with the hysteretic mode of operation (the inductor current waveform will ramp +/-15% about the nominal current set point) provides sufficient margin. The main features of the ZXLD1350 are: * * * * * Up to 380mA output current Wide input voltage range: 7V to 30V Internal 30V 400mA NDMOS switch High efficiency (>90% possible) Up to 1MHz switching frequency The ZXSBMR16PT8 is a new space saving and thermally efficient device specifically designed for the critical requirements of MR16 applications. The device encompasses a full bridge and a freewheeling diode realized using extremely low leakage 1A, 40V Schottky diodes to allow a nominal 12V AC input operations. The Schottky bridge together with the embedded freewheeling diode enhance the system efficiency compared to the standard silicon diodes in a compact format. The reference design has solder tag pins to bypass the bridge rectifier should the final lamp design be used for purely DC operation. As the ZXLD1350 has a hysteretic switching topology, the switching frequency is dependent on several factors - input voltage, target current and number of LEDs. An Excel based calculator is available for system initial evaluation and component choice. See http://www.zetex.com/3.0/otherdocs/zxld1350calc.xls System efficiency and LED current have been measured keeping the ADJ pin floating and the current in the device at its rated value. The input impedance of the ADJ pin is high (200K) and is susceptible to leakage currents from other sources. Anything that sinks current from this pin will reduce the output current. In order to avoid any kind of electromagnetic coupling a guard track around this pin is used. www.zetex.com 2 Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 DN86 Quantity 1 2 1 1 1 1 1 Part reference R1 C1, C2 C3 C4 L1 U1 U2 Value 0.33 150F/20V 0.1F/25V 1F/25V 100H ZXLD1350 ZXSBMR16PT8 Description Resistor, 1%, 0805 Type D SMD Tantalum Cap SMD 0805 X7R SMD 1210 X7R MSS6132-104 LED driver IC Schottky bridge rectifier and freewheeling diode Source Various Kemet NIC Componenents NIC Componenents Coilcraft ZETEX ZETEX Table 1 Bill of Material Referring to circuit schematic in Figure 2; the jumper connection could be used utilizing a zero ohm resistor, in order to enable the pure DC operations. Care has to be taken in this case, since the system is not reverse polarity protected. In Figure 3 the circuit layout is shown, highlighting its space saving features and compactness. Both bottom layer and top layer are shown to display effective devices arrangement. TOP COPPER AND SILKSCREEN BOTTOM COPPER AND SILK SCREEN Figure 3 Circuit layout The main layout design suggestions are: * * * * All thin devices on one side Employ a star connection for ground tracks Use a ground ring protecting ADJ pin Check that: * * * Tracks connecting R1 to ZXLD1350 are as short as possible (being sense tracks) The filter capacitor C3 is connected as close as possible to the Vin pin The freewheeling current path is as short as possible to ensure system precision and efficiency 3 www.zetex.com Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 DN86 Circuit board views Top Layer Bottom Layer MR16 Pins - Anode LED connections + Cathode Figure 4 Circuit board views Choice of Inductor and switching circuit layout A 100 H screened inductor was chosen to set the nominal frequency around 250kHz. A screened inductor is chosen to minimize radiated EMI. The layout with any switching regulator is crucial to minimize radiated EMI. This reference design keeps the critical track lengths to a minimum. Ground areas have been maximized around critical areas. Circuit performances Circuit performances have been evaluated taking into account two main parameters, the system efficiency and the current precision. The reference current is set to a nominal 300mA but can be adjusted to any value up to 350mA by changing the sense resistor Rsense according to the formula: Iref = 0.1/R1 For R1 = 0.33 [A] Iref = 300mA In Table 2 the data related to the system supplied with a DC voltage ranges from 12V to 15V. For these tests the Schottky bridge was included. The most important parameters are the system efficiency and the error between the rated LED current (300mA) and the actual LED current. In the DC case the frequency ranges between 150kHz and 300kHz, depending on the input voltage. Whatever the input voltage, the efficiency is higher than 87% and the error lower than 2%. Vin [V] 12.000 13.000 14.000 15.000 Iin[A] 0.275 0.252 0.232 0.220 Vout[V] 9.80 9.78 9.76 9.75 Iout[A] 0.296 0.294 0.294 0.294 Efficiency 87,9% 87.7% 87.6% 87.4% Current Accuracy 1.3% 2.0% 2.0% 2.0% Table 2 DC input voltage Table 3 shows the data related to the system supplied with an AC electromagnetic transformer. Using a SMD tantalum capacitor will save space and avoid using a larger aluminum electrolytic capacitor. This will improve the reliability of the system and stabilize performance during its lifetime. There is a trade off between physical size, reliability, cost and average LED current. Typical output voltages from a nominal 12V AC transformer can be 10%. With 3 LEDs the voltage across these will be around 10V. If the input capacitor value is lower then 200F, the AC input waveform is distorted (as can be seen in figure 8). When the rectified AC is not sufficiently www.zetex.com 4 Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 DN86 smoothed the ripple may drop below the combined LED forward voltage which stops the switching regulator and so reduces the average current in the LEDs. This will also reduce the average lumens output. C1 [F] 100 150 200 300 Vin [V] 12.70 12.60 12.53 12.50 Iin[A] 0.303 0.394 0.432 0.386 Vout[V] 9.28 9.50 9.55 9.70 Iout[A] 0.225 0.271 0.293 0.295 Efficiency 54% 52% 52% 60% Current Accuracy 25% 10% 2% 2% Table 3 AC input voltage Figures 5 to 7 show the input voltage ripple and LX voltage varying the input capacitance value Cin = C1 + C2 + C3. The higher the input capacitance the higher to output current precision and the average lumens outputs. The case with Cin=300F has the best performance both as efficiency and current precision. Reducing the input capacitance the output current precision will decrease up to 25% with system efficiency always above 50%. Figure 5 Cin=300F Figure 6 Cin=200F Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 5 www.zetex.com DN86 Figure 7 Cin=150F Figure 8 Cin=100F Figure 5 to 8: input ripple and LX voltage (Ch3 is the LX pin voltage and Ch4 is the input voltage) Gerber plots and further assistance are available from your local Zetex contact or Distributor. You can contact your local sales office by email. europe.sales@zetex.com usa.sales@zetex.com asia.sales@zetex.com Conclusion A compact, reliable, efficient and minimum part count solution can be realized using the ZXLD1350, ZXSBMR16PT8, and associated passive components. The compact design in the connector housing keeps the temperature sensitive semiconductors as far from the heat generating LEDs as possible. A compromise between LED current and size of capacitance is necessary for the final solution which accounts for efficiency, accuracy, size, and component count. This is the first design note in a series of reference designs MR16 variants solutions and options. www.zetex.com 6 Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 DN86 Intentionally left blank Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 7 www.zetex.com DN86 Definitions Product change Zetex Semiconductors reserves 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. No representation or warranty is given and no liability whatsoever is assumed by Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Zetex does not assume any legal responsibility or will not be held legally liable (whether in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business, contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances. Life support Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the Chief Executive Officer of Zetex Semiconductors plc. As used herein: A. 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. Terms and Conditions All products are sold subjects to Zetex' terms and conditions of sale, and this disclaimer (save in the event of a conflict between the two when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement. For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Zetex sales office . Quality of product Zetex is an ISO 9001 and TS16949 certified semiconductor manufacturer. To ensure quality of service and products we strongly advise the purchase of parts directly from Zetex Semiconductors or one of our regionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com/salesnetwork Zetex Semiconductors does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels. ESD (Electrostatic discharge) Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices. The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time. Devices suspected of being affected should be replaced. Green compliance Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or exceeding regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use of hazardous substances and/or emissions. All Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance with WEEE and ELV directives. Product status key: "Preview" Future device intended for production at some point. Samples may be available "Active" Product status recommended for new designs "Last time buy (LTB)" Device will be discontinued and last time buy period and delivery is in effect "Not recommended for new designs" Device is still in production to support existing designs and production "Obsolete" Production has been discontinued Datasheet status key: "Draft version" This term denotes a very early datasheet version and contains highly provisional information, which may change in any manner without notice. "Provisional version" This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance. However, changes to the test conditions and specifications may occur, at any time and without notice. "Issue" This term denotes an issued datasheet containing finalized specifications. However, changes to specifications may occur, at any time and without notice. Zetex sales offices Europe Zetex GmbH Kustermannpark Balanstrae 59 D-81541 Munchen Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Asia Pacific Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Corporate Headquarters Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com (c) 2007 Published by Zetex Semiconductors plc www.zetex.com 8 Issue 1 - October 2007 (c) Zetex Semiconductors plc 2007 |
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