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LM2678-ADJ Inductor selection
I am designing a regulator circuit using the LM2678-ADJ. The input voltage is 15V and the output is 12V. I need to be able to operate near the full 5A output that the LM2678 is rated for.
I am having a little trouble with the inductor selection when I use the design guide provided in the datasheet. According to the datasheet, the first step in the inductor selection is to calculate the ET. When I plug in all the correct values from my circuit into this equation I get 11.4. This value is used with Figure 6 then with Table 1 in the datasheet. The recommended inductor turns out to be 15uH 5.6A and there are a couple suggested manufacturers and part numbers. This is all fine and makes sense. I get confused when I get to the step of selecting the output capacitors.
The datasheet gives Table 6 to help select capacitors to match the output voltage and inductor. Since I will set the output to 12V I am looking at the 10 to 12.5V output section of this table. There are 4 inductor values listed in this part of the table: 33uH, 47uH, 68uH, 100uH. I saw a note in the datasheet (page 14) that says if the inductor selected by the ET equation is less than the lowest inductor in the capacitor table, it is best to choose the lowest value in the capacitor table to get stable operation, but this note says it is specific to low voltage output applications.
Taking all that into consideration I originally designed this regulator circuit to use a 33uH inductor with appropriate capacitors the table. I am starting to see some availability issues trying to find a 33uH greater than 5A inductor in a package that will fit in my design. Would I still get a stable output if I used a different inductor in the range of 15uH to 33uH? This would give me a lot more options so that I can solve the availability issues.
Any information would be appreciated.
I recommend using the Webench tool. From the product folder: http://www.ti.com/product/lm2678
you can enter your input and output conditions in the Webench window and click "open design". Once Webench opens, click on the BOM icon. You will see the optimum calculated component values and also have the option to select from a list of components that fit your design. It will also show the range of values that work for each component. This is the quickest and most reliable way to get a stable design with a known range of components.
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