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Original question:

LMR33610: Inductor Selection

LMR33610: LMR33610 Datasheet clarification

E. Jongerius
Prodigy 250 points
Part Number: LMR33610
Other Parts Discussed in Thread: LMR33620

Hi,

I have a very similar question like this one:

e2e.ti.com/.../905577

The datasheet refers to Isc, while it is not specified.

(page 19, 9.2.2.4 Inductor selection): "Ideally, the saturation current rating of the inductor must be at least as large as the high-side switch current limit,
Isc (see the Electrical Characteristics section). ", butt the datasheet does not specify this current. It does state a maximum output current, but I think this is to the total solution, which implies the Isc current must be quite a bit larger. Which value should I use to spec the saturation current of my inductor?

Also, in the inductor selection I see the same problems as the similar thread. Clearly, Iripple = 0.3 .. 0.4 * Ioutmax is not a hard limit. I get into very high values (decades higher than advised), and I figured this is because the device goes into low power mode (non-continous switching). With the larger, calculated inductances (47uH to 68uH) I have very annoying audible sound and very large Vpp at low load conditions. Changing to a lot smaller inductor solves both. I am now at a 2.2uH inductor at 400kHz, for a 10..28V input to 7V output and it performs well in the first tests. A bit unsure if it is correct as I have even lower values than in your datasheet. (Iout max 250mA, but tested OK to 500mA)

I might suggest to elaborate on this procedure in your application information a bit more; as the calculations do not match with the advised value.

  • 0
    TI__Guru 64365 points

    Hello

    I have attached the current limit values for this device.

    When selecting the inductor you would use a range of 0.3 to 0.4 times the maximum output current of the converter.

    Values of 47uH to 68uH are too large.  The 2.2uH is somewhat small for 400kHz.  I think that 10uH may be closer to 

    what you need.  You can test this value of you wish.

    The current rating of the inductor can be based on the typical Isc shown below.

    Also, there is a calculator tool on the product web page for this device that you can use to help with your design.

    When using the spreadsheet for this device, use the LMR33620 pull down menu.

    Thanks