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ILC6391 Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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ILC6391 Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 10 page ILC6390/91 6 ©2001 Fairchild Semiconductor Corporation External Components and Layout Consideration The ILC6390 is designed to provide a complete DC-DC con- verter solution with a minimum of external components. Ide- ally, only three externals are required: the inductor, a pass diode, and an output capacitor. The inductor needs to be of low DC Resistance type, typi- cally 1 Ω value. Toroidal wound inductors have better field containment (less high frequency noise radiated out) but tend to be more expensive. Some manufacturers like Coilcraft have new bobbin-wound inductors with shielding included, which may be an ideal fit for these applications. Contact the manufacturer for more information. The inductor size needs to be in the range of 47mH to 1mH. In general, larger inductor sizes deliver less current, so the load current will determine the inductor size used. For load currents higher than 10mA, use an inductor from 47mH to 100mH. [The 100mH inductor shown in the datasheet is the most typical used for this application.] For load currents of around 5mA, such as pagers, use an inductor in the range of 100mH to 330mH. 220mH is the most typical value used here. For lighter loads, an inductor of up to 1mH can be used. The use of a larger inductor will increase overall conversion effi- ciency, due to the reduction in switching currents through the device. For the ILC6391, using an external transistor, the use of a 47mH inductor is recommended based on our experience with the part. The capacitor should, in general, always be tantalum type, as tantalum has much better ESR and temperature stability than other capacitor types. NEVER use electrolytics or chemical caps, as the C-value changes below 0×C so much as to make the overall design unstable. Different C-values will directly impact the ripple seen on the output at a given load current, due to the direct charge-to- voltage relationship of this element. Different C-values will also indirectly affect system reliability, as the lifetime of the capacitor can be degraded by constant high current influx and outflux. Running a capacitor near its maximum rated voltage can deteriorate lifetime as well; this is especially true for tantalum caps which are particularly sensitive to over- voltage conditions. In general, then, this capacitor should always be 47mF, Tan- talum, 16V rating. The diode must be of shottkey type for fast recovery and minimal loss. A diode rated at greater than 200mA and max- imum voltage greater than 30V is recommended for the fast- est switching time and best reliability over time. Different diodes may introduce different levels of high frequency switching noise into the output waveform, so trying out sev- eral sources may make the most sense for your system. For the IL6391, much of the component selection is as described above, with the addition of the external NPN tran- sistor and the base drive network. The transistor needs to be of NPN type, and should be rated for currents of 2A or more. [This translates to lower effective on resistance and, there- fore, higher overall efficiencies.] The base components should remain at 1k Ω and 3300pF; any changes need to be verified prior to implementation. As for actual physical component layout, in general, the more compact the layout is, the better the overall perfor- mance will be. It is important to remember that everything in the circuit depends on a common and solid ground reference. Ground bounce can directly affect the output regulation and presents difficult behavior to predict. Keeping all ground traces wide will eliminate ground bounce problems. It is also critical that the ground pin of CL and the VSS pin of the device be the same point on the board, as this capacitor serves two functions: that of the output load capacitor, and that of the input supply bypass capacitor. Layouts for DC-DC converter designs are critical for overall performance, but following these simple guidelines can sim- plify the task by avoiding some of the more common mis- takes made in these cases. Once actual performance is completed, though, be sure to double-check the design on actual manufacturing prototype product to verify that noth- ing has changed which can affect the performance. |
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