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LM61495: Inductor Selection

Robin Yuan
Prodigy 80 points
Part Number: LM61495

Tool/software:

Hi,

I am currently in the process of designing a buck converter using LM61495 to step down my 8s Li-ion pack voltage from 29.6V to 5.3V. The rated current is around 2A and the peak current is about 7A. I will assume a 10A of max current so we have some safety margin. 

Then, I put my requirements into the TI Power Designer to get some initial ideas and I was recommended the following topology where a 560nH output inductor was used:

I then double-checked the datasheet of LM61495 and encountered equations 5 and 6 which are both used to determine the required output inductance.

 

Assuming Fsw = 2.21MHz, K = 0.3 and Imax = 10A, we get L ~= 660nH using Equation 5. If we use  K =0.25, then it becomes 790nH.

However, my typical current draw is about 2A which means I would need a inductance of 2uH based on eqn 6. Given there is a such large discrepancy, which equation should i use to find my required output inductance?

Thank you so much in advance!

  • 0
    TI__Genius 11385 points

    Hi Robin,

    The inductor is selected based on the max rated current of your design. In your case that will be 10A (or 7A). 

    You can also refer to this e2e post: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1320123/lm61495-inductor-value-selection

    Furthermore, designing with LM61495-Q1 for a max current of 10A at 2.2MHz may lead to thermal limitations. Please look at the junction temperature rise in WEBENCH to get an understanding (alternatively - input the max ambient temperature of your design requirements and check the device performance). 

    You can also use the Design Calculator for this device (https://www.ti.com/tool/download/SNVR498).

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Prodigy 80 points

    Hi Taru,

    Thank you for your reply and suggestion!

    I am well aware that I need to pick an inductor based the max rated current of my circuit but I also need to know what inductance I need for my circuit. Eqns 5 and 6 are giving me conflicting results. The post you referenced mentioned that, "Usually for current mode control devices, subharmonic oscillation happens when steady state operation duty cycle is above 50%." Does that mean I can ignore eqn 6 and only use eqn 5 to determine my required inductance since my application will be stepping down from 24V~34V to 5V, which will have a duty cycle less than 50%?

    I have also checked Power Designer simulation results and there will be a 75C IC temp raise when running at 10A, which leaves me a lot of margin before thermal shutdown kicks in. Plus, I will only be running at 7A max for a few seconds. Do you think that will cause any problem?

    Thank you so much in advance!

    Best,

    Robin

  • 0 in reply to Robin Yuan
    TI__Genius 11385 points

    Hi Robin,

    Robin Yuan said:
    I am well aware that I need to pick an inductor based the max rated current of my circuit but I also need to know what inductance I need for my circuit. Eqns 5 and 6 are giving me conflicting results. The post you referenced mentioned that, "Usually for current mode control devices, subharmonic oscillation happens when steady state operation duty cycle is above 50%." Does that mean I can ignore eqn 6 and only use eqn 5 to determine my required inductance since my application will be stepping down from 24V~34V to 5V, which will have a duty cycle less than 50%?

    Yes. 

    For your use case - 


    As mentioned earlier, please use the design calculator - it shall make things simpler for you. 

    Robin Yuan said:
    I have also checked Power Designer simulation results and there will be a 75C IC temp raise when running at 10A, which leaves me a lot of margin before thermal shutdown kicks in. Plus, I will only be running at 7A max for a few seconds. Do you think that will cause any problem?

    This is completely dependent on your use case.

    A 75 degC rise in temperature leaves you with a max ambient temperature of 75 degC (Max Junction Temperature - Temperature Rise in the IC; 150degC - 75 degC). If this works for your application - you can definitely go ahead. Your board thermals will also play a large part in it. From the perspective of the device - as long as you do not exceed the max junction temperature - you should be okay!

    Thanks,

    Best Regards,

    Taru