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LM3686 Datasheet(PDF) 17 Page - Texas Instruments |
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LM3686 Datasheet(HTML) 17 Page - Texas Instruments |
17 / 27 page ¸ ¹ · ¨ © § x x x f 1 L 2 ! ISAT IOUTMAX + IRIPPLE where IRIPPLE = VOUT VBATT ¸ ¹ · ¨ © § ¸ ¹ · ¨ © §V BATT - V OUT LM3686 www.ti.com SNVS520E – AUGUST 2008 – REVISED MAY 2013 APPLICATION INFORMATION Application Selection It is strongly recommended to select the required components of LM3686 as described within the datasheet. If other components are selected, the device will not perform up to standard and electrical characteristics cannot be ensured. Inductor Selection There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. The minimum value of inductance to ensure good performance is 0.7 µH at ILIM (typ) dc current over the ambient temperature range. Shielded inductors radiate less noise and should be preferred. There are two methods to choose the inductor saturation current rating. METHOD 1 The saturation current should be greater than the sum of the maximum load current and the worst case average to peak inductor current. This can be written as: ISAT > IOUT_DCDC_MAX + IRIPPLE (1) where • IRIPPLE: average to peak inductor current • IOUT_DCDCMAX: maximum load current (600 mA) • VBATT: maximum input voltage in application • L: minimum inductor value including worst case tolerances (30% drop can be considered for method 1) • f: minimum switching frequency (2.55 MHz) (2) METHOD 2 A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum current limit of 1375 mA. A 1.0 µH inductor with a saturation current rating of at least 1375 mA is recommended for most applications. The inductor’s resistance should be less than 0.3 Ω for good efficiency. Table 1 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. For noise critical applications, a toroidal or shielded- bobbin inductor should be used. A good practice is to lay out the board with overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor, in the event that noise from low-cost bobbin models is unacceptable. Table 1. Suggested Inductors and their Suppliers Model Vendor Dimensions LxWxH (mm) DCR (max) BRL2518T1R0M TAIYO YUDEN 2.5 X 1.8 X 1.2 80 MDT2520CR1R0M TOKO 2.5 X 2.0 X 1.0 80 KSLI252010AG1R0 HITACHI METALS 2.5 X 2.0 X 1.0 75 External Capacitors As common with most regulators, the LM3686 requires external capacitors to ensure stable operation. The LM3686 is specifically designed for portable applications requiring minimum board space and the smallest size components. These capacitors must be correctly selected for good performance. Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 17 Product Folder Links: LM3686 |
Similar Part No. - LM3686_15 |
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Similar Description - LM3686_15 |
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