Need help choosing inductor!

Hi, i am building a car bluetooth module and i am almost finished... :smiley:
Only one thing left and thats the inductor for the 12v-5v buck converter, i know it should be 100μH
but i dont know anything more about its ratings :slightly_frowning_face:
can anyone help me? :roll_eyes:
(The inductor is L1)
Thanks.

Links:
Design Site:

Schematic:

Check the datasheet for the switcher you are using. They typically have a table with suggested manufacturer part numbers.

I did that yesterday
All the datasheet said was that it was a 100μH inductor :confused:

The inductor should be rated with a rated saturation current higher than the intended maximum load (with some margin), as well as meeting any other constraints imposed by the buck controller IC.

Be aware that dc-dc converters are layout sensitive - they give recommendations on layout in the datasheet, typically, but basically, the traces around the inductor and diode and back to the IC need to be as short as possible, otherwise they'll destroy efficiency and generate electromagnetic noise.

How many mA higher do you recommend?
If i have 260ma max draw is 300ma rating good?
And what about the series resistance? :confused:

I'd want more headroom than that... how about 500mA?

Series resistance? Lower is better. All the current that goes into the load will be passing through that series resistance, so you want low series resistance.

(Also - you know that the diode needs to be a fast schottky diode, not a floor-sweepings diode, right? Since it seems that the datasheet you're looking had has provided precious little in the way of supporting component selection guidance)

Eirikki211299:
All the datasheet said was that it was a 100μH inductor

Find a better datasheet. The LM2576 "Simple Switcher" regulator was originally developed by National Semiconductor, which was bought by Texas Instruments a few years back. The TI datasheet has a couple paragraphs on inductor selection, as well as a bunch of additional "application note" type documents that go into even more detail.
Here's one: http://www.ti.com/lit/pdf/snva038

I think I want to correct one thing that DrAzzy said: "a rated saturation current higher than the intended maximum load" - I think you need the rated current to be higher than the "peak current through the inductor" - since switchmode power supplies pulse the inductor, the peak current is going to be higher than the output current (by a predictable ratio, probably.)

Ideally you also check the magnetic losses at the frequency the switcher chip is using to
see if inductor self-heating is going to be a problem. Choosing a generously rated inductor
will help (this means larger in size). Larger also tends to mean lower series resistance too.

So do these look alright?
Inductor:
http://www.digikey.com/product-detail/en/tdk-corporation/SLF7045T-101MR50-PF/445-2015-1-ND/755422
Fast recovery Diode:
http://www.digikey.com/product-detail/en/diodes-incorporated/HER302-T/HER302DICT-ND/190631

So i think my project is finished now :slight_smile: