SILICON SUPPORT MAGNETICS Compatibility Guide SELECTION OF INDUCTORS FOR SWITCHING REGULATORS The choice of Inductors for switching products is a critical path in any new design. Inductance values, current ratings, and DC Resistance values will alter for different applications. The mode of operation, load current, and input voltage will all impact inductor selection in some way. C&D Technologies (NCL)’s compatibility guide provides an easy-to-use reference in establishing a valid design. Providing simple cross-references between the regulator part number, the suggested C&D Technologies (NCL) family of inductors, a part number suitable for a typical application and including a worked example. Inductor selection may vary between individual application, inductors are available from C&D Technologies (NCL) in SMD, radial and axial format with current ratings from 350mA-13A and inductance ranges from 2.2µH-47mH which are suitable for most applications. Analog silicon vendors produce a wide range of switching circuits. Design notes published by these vendors provide detailed guidance on the selection of inductors for optimum performance and efficiency, and should be consulted to finalise designs. as low as possible to reduce losses and minimise the self-heating effect of the inductor. Current ratings must be selected with care, and the core material chosen must be capable of storing the required energy without saturating. Switching regulators have two types of operation: continuous and discontinuous modes. The difference between the two modes is directly related to the current flowing through the inductor chosen for the regulator. In discontinuous mode the cur rent through the inductor will drop to 0A for a period of time during the switching cycle, the inductor used in continuous mode of operation will be exposed to current through out the entir e switching period. If the maximum cur rent rating of the inductor is exceeded the component will begin to heat up and drive the core material towards it Curie temperature causing the inductance to fall rapidly. Under specifying the cur rent can lead to inductor saturation and the component becomes a mainly resistive element, the effect this has on the switching circuit is to increase the switch current considerably and reduce the load current. The mode of operation is dependant on individual application discontinuous mode is preferred for low load cur rents, and will utilise small inductor values. Continuous operation will provide greater output power however the continual flow of current will require larger values of inductance for high input voltages. The inductance value for a switching regulator must be high enough to prevent excessive current and low enough to store sufficient energy in the core. DC Resistance should be Inductors are available in a wide range of ferrite and iron powder materials and produced in formats ranging from open wound ferrite rods to fully shielded components. Practical inductor ranges are typically 2.2µH to 47mH however the majority of switching products are covered by the inductance ranges of 22µH to 100µH. C&D Technologies (NCL) offer a wide range of products based upon bobbin wound components, toroids and shielded products for optimum magnetic paths and low EMI radiation. INDUCTOR SELECTION The following equations provide a guide for a typical application, and are based on the Maxim 1636 Step Down controller for portable power. The three key inductor parameters must be specified: Inductance value (L), peak cur rent (IPEAK), and DC resistance (R DC). The following equation includes a constant, LIR, which is the ratio of inductor peak-to-peak AC current to DC load current. A good compromise between size and losses is a 30% ripple-current to load-current ratio (LIR = 0.3), which corresponds to a peak inductor current 1.15 times higher than the DC load current. The following equation can be applied: L= VOUT (VIN(MAX) -VOUT) VIN(MIN) x f x I OUT x LIR Where: f = switching frequency, normally 200kHz or 300kHz IOUT = maximum DC load cur rent. The peak current can be calculated by: IPeak = I LOAD + VOUT (V IN(MAX) -VOUT) 2 x f x L x VIN(MAX) Using the following table of parameters the inductors performance can be calculated. I IN (A) VIN Min (V) VO (V) LIR Operating Frequency (kHz) 1 12 5 0.3 200 The inductor calculation for this particular application shows an inductance of 69.4µH with a peak current of 1.12 Amps. www.dc-dc.com The C&D Technologies (NCL) 26S680 SMD Shielded inductor would provide a suitable solution for this example. Higher efficiency and output current are achieved with lower inductor DC resistance. SILICON SUPPORT MAGNETICS Compatibility Guide NATIONAL SEMICONDUCTOR LINEAR TECHNOLOGY C&D Technologies (NCL) MAXIM C&D Technologies (NCL) C&D Technologies (NCL) Family Series Part Number Family Series Part Number Family Series Part Number LM1575 LM1577 LM2574 LM2575 LM2576 LM2577 LM2594 LM2595 LM2596 LM2597 LM2598 LM2599 LM2651 LM2653 LM2670 LM2671 LM2672 LM2673 LM2674 LM2675 LM2676 LM2677 LM2678 LM2679 26S 1400 24S 26S 1400 1400 24S 26S 1800R 24S 1800R 1800R 26S 26S 1800R 24S 26S 1400 26S 26S 1800R 1400 1400 1400 26S330 1410454 24S330 26S330 1410454 1410454 24S100 26S680 18R153 24S330 18R683 18R153 26S220 26S220 18R153 26S101 26S470 1433606 26S101 26S470 18R153 1422606 1422606 1422606 LT149 LT1010 LT1013 LT1020 LT1109 LT1143 LT1300 LT1301 LT1302 LT1303 LT1317 LT1370 LT1371 LT1372 LT1374 LT1375 LT1377 LT1500 LT1504 LT1576 LT1578 LT1610 LT1611 LT1613 LT1617 LT1624 LT1777 LT1930 LT1504A LT1576-5 LT1578-2.5 26S 1400 1400 1400 22R 1800R 26S 1800R 1800R 1800R 26S 1800R 24S 22R 1800R 24S 22R 26S 1800R 26S 24S 1400 26S 24S 26S 24S 26S 24S 26S 26S 1800R 26S470 1422514 1422514 1447448 22R473 18R223 26S220 18R333 18R220 18R473 26S100 18R472 24S100 22R473 18R472 24S100 22R683 26S330 18R333 26S150 24S150 1447506 26S220 24S220 26S220 24S100 26S150 24S100 26S470 26S680 18R153 MAX608 MAX629 MAX649 MAX651 MAX652 MAX668 MAX742 MAX749 MAX756 MAX764 MAX774 MAX796 MAX797 MAX798 MAX799 MAX856 MAX866 MAX867 MAX1626 MAX1627 MAX1636 MAX1652 MAX1653 MAX1654 MAX1655 MAX1664 MAX1672 MAX1705 MAX1706 MAX1714 MAX1715 MAX1771 1800R 2400 26S 26S 26S 1800R 1800R 24S 26S 24S 26S 1400 1400 1400 1400 24S 2200R 2200R 26S 26S 1800R 26S 26S 26S 26S 2400 26S 1800R 26S 1800R 1800R 1800R 18R153 24220 26S220 26S220 26S220 18R472 18R104 24S470 26S220 24S470 26S220 1447423 1447423 1447423 1447423 24S470 22R683 22R683 26S220 26S220 18R472 26S150 26S150 26S150 26S150 24S100 26S100 18R223 26S220 18R472 18R472 18R223 MOTOROLA C&D Technologies (NCL) Family Series Part Number C551411 C551412 C551413 C551414 C55171 C55172 C55173 LM2574 LM2575 LM2576 MC33163 MC33166 MC33167 MC33463 MC33465 MC33467 MC33680 MC34167 MA78540 MC33167 26S 26S 26S 26S 26S 26S 26S 24S 26S 1400 1400 1400 1400 2600 2600 2600 2400 2600 26S 2600 26S150 26S150 26S150 26S150 26S220 26S220 26S220 24S331 26S331 1415449 1415449 1415449 1415449 26S470 26S470 26S470 24330 26S221 26S221 26S331 TEXAS INSTRUMENTS C&D Technologies (NCL) Family Series Part Number TPS5102 TPS56302 TPS61000 TPS61001 TPS61002 TPS61003 TPS61004 TPS6734 TPS6735 UC257 UCC2941 UCC2942 UCC3941 UCC39411 UCC39412 UCC39413 1400 1400 2400 2400 2400 2400 2400 2400 2400 1400 26S 1400 26S 26S 26S 26S 1410313 1410313 24150 24150 24150 24150 24150 24220 24220 144738 26S220 1410313 26S220 26S220 26S220 26S220 C&D Technologies (NCL) Limited reserve the right to alter or improve the specification, internal design or manufacturing process at any time, without notice. Please check with your supplier or visit our web site to ensure that you have the current and complete specification for your product before use. © C&D Technologies (NCL) Limited 2000 AN003.2 No part of this publication may be copied, transmitted or stored in a retrieval system or reproduced in any way including, but not limited to, photography, photocopy, magnetic or other recording means, without prior written permission from C&D Technologies (NCL) Limited. Instructions for use are available from www.dc-dc.com C&D Technologies (NCL) Ltd Tanners Drive, Blakelands North Milton Keynes MK14 5BU, England Tel: +44 (0)1908 615232 Fax:+44 (0)1908 617545 email: [email protected] www: http://www.dc-dc.com The information provided in this application note is believed to be accurate and reliable, but is for guidance only and should not be relied upon. 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