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LT8392 Inductor Selection - Formula doesn't seem to result values in typical application examples


I am currently trying to implement LT8392 into a design. The typical applications are actually already very close to my desired specifications, however I wanted to derive my own values to better understand the design process. However, I am struggling to calculate results similar to the inductors used in the examples:

One example: 98% Efficient 48W (12V 4A) Buck-Boost Voltage Regulator (Vin range of 3-56V), uses an inductor value of 6 uH.

The inductor selection section gives a couple of formulas to find the minimum inductance:

The ripple current percentage is a bit confusing but is stated to be ripple current delta  / average inductor current. This i take to be something like 30% or 0.3.

Plugging in the numbers for the example:

Vout = 12

Vinmax = 56

Ioutmax = 4

f = 400e3

ripple current % = 0.3 or  30% 

Gives a minimum inductor size of 19 uH (with 0.3) or or 0.19uH (with 30 %).

the 6 uH specified in the example is far less than 19 uH (which is meant to be a minimum) and far more than 0.19 uH.

I'd be very grateful if this could be explained to get me going in the right direction, as I'd like to understand it better than just copying the example! Many thanks in advance.

  • Hi t12,

    Hope you are doing well.

    I think you are having discrepancy with the minimum inductor size since you have set a ripple current % and adjusted your computation accordingly. the ripple current % you should choose is the 0.3 (19uH). Be reminded that this is a buck-boost converter and it can operate as either a buck or a boost converter. Since we don't have a nominal voltage set in the input, we can sweep the max and min as the Vin for the buck or the boost, respectively. Note that with the computation you made for the inductor value is for a buck setup. This will vary the resulting current ripple % you will set for the boost. The application circuit components are tested within the specified VIN range so the 6uH value should be okay. Do note that when choosing your inductor, always check for the saturation current rating of the inductor to prevent damaging/saturating it.

    To have a guided reference for your circuit design, I suggest you to try the LTpowerCAD. Here is the link FYR.

    LTpowerCAD | Analog Devices