LM60430: Inductor selection for wide output current

Hi Ganesh,

You want to design your inductor so it has the worst case inductor current ripple, which will be at full load 2A. The inductor ripple ratio should be between 20%-40%. I usually design the an inductor ripple ratio of 30%. Therefore, at max load the ripple will be 600mA (2A * 30%) from 2A. At 1A load, the ripple will be 300mA from 1A.

My calculation of the inductor value at 5V to 3.3V, load current of 2A, designing with 30% ripple, Fsw = 1MHz, is 4.29uH.

Ben

Hi Ben,

Thanks for your response.

Yes, I do also use the 30% of Load current as inductor current, But my Inductance value shown 1.87uH for same case where you shown 4.29uH.

Please let me know how you arrived 4.29uH inductance.

I am still not satisfied for inductor selection in wide range output load current.

I calculated inductance value for 1 A , 1.5A and 2A load with 30% ripple current the inductance value are  3.7uH for 1A Iout , 2.49uH for 1.5A Iout and 1.87uH for 2A Iout.

Please elaborate which should be consider like min load or max load.

If used inductance as per Max Load then how this will be suitable for min Load?

Thanks

Ganesh V

Hi Ganesh, 

My mistake, I had entered the wrong numbers. You are correct with the inductor values for 1A, 1.5A and 2A.

When selecting an inductor, the Isat rating is an important spec that needs attention. If the current through the inductor exceeds the Isat rating, the inductance of the inductor (depending on what type of core material is used in the inductor) will drop to the point the inductor may have very to no inductance. This is the reason, when selecting an inductor, you want to use the max IOUT current so all the other loads will be below the Isat rating.

Ben

Thanks for the response,

I understand the inductor current rating I will follow this when I am choosing inductor part.

But in my case, 3.7uH for 1A Iout , 2.49uH for 1.5A Iout and 1.87uH for 2A Iout.

2A is Iout max current for 2AIout the calculated inductance is 1.87uH can I use this ?

Or I need use the worst-case inductance ? example for 1A calculated inductance is 3.7uH.

If I choose 1.87uH with 3A ratting current, this will work for my max current case then How about min current case how this inductance satisfy this since the calculated inductance for min current is 3.7uH.

Thanks

Ganesh V

Hi Ganesh,

Ganesh V said:

2A is Iout max current for 2AIout the calculated inductance is 1.87uH can I use this ?

Yes, you can. Since 1.87uH is not a standard inductor value, I suggest using a 2.2uH inductor.

Ganesh V said:

Or I need use the worst-case inductance ? example for 1A calculated inductance is 3.7uH.

What is the typical output current for the application? You can use a 3.3uH inductor for the 1A IOUT and the inductor will work for 1.5A and 2A IOUT. Remember, using a larger inductor value means the size of the inductor will be physically bigger and the inductor has to meet the Isat rating for the application.

Ben