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LMR38020: What is the correct footprint - the datasheet has two conflicting footprints

Ian Rogers
Prodigy 20 points
Part Number: LMR38020

Tool/software:

Hello

I am trying to design the footprint for the DDA package, and the Datasheet for the LMR38020 has two different stencil aperture dimensions on tables in Pages 38 and 41

LMR38020 4.2-V to 80-V, 2-A, Synchronous SIMPLE SWITCHER Power Converter with 40-µA IQ datasheet (Rev. E) (ti.com)

How do I know which one is correct?

Thanks

Ian 

  • 0
    TI__Guru* 78400 points

    Hi Ian,

    Thank you for designing with our device.  You asked a good question.  Since the expert of this device is currently out of office, let me check with other team members to get a clarification.  I will get back to you ASAP.

    Best Regards,

    Youhao

  • 0
    Prodigy 20 points

    Just to add to this, the DDA footprint document on the TI Footprint  site, has two pages in the document, a front page and a disclaimer, but no footprint information at all

    Find Product by Package | Texas Instruments (ti.com)

    Package Drawing: https://www.ti.com/lit/pdf/MPDS092F 

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Any update on this?

  • 0 in reply to Ian Rogers
    TI__Guru* 78400 points

    I am still waiting for the answer.  Hopefully later today.

  • 0 in reply to Youhao Xi
    TI__Guru* 78400 points

    Hi Ian,

    Sorry for the delay. The two drawings in the datasheet because the device is dual sourced, namely from two different assembly sites, and you will get both devices in the long run.  Our experts advise to design for the worst case, namely to have a PCB footprint to cover both versions.  To to so, you need to overlap the two foot print and takes the larger dimensions, including the solder pad for the pins, for the under-pad, as well as the stencil apertures.   

    Hope this helps.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    If the product is dual manufactured then the datasheet should still account for this and still only provide ONE recommended footprint for designers to follow.  

    What is the impact of putting a package designed for the smaller dimensions (especially the under pad) on the larger dimensions you have recommended? will we have issues with excess solder causing shorts.

  • 0 in reply to Ian Rogers
    TI__Guru* 78400 points

    Hi Ian,  I agree with you and I also suggested the same to our team yesterday. By the way, I heard there would be a PCN issued soon, but I haven't seen it yet. You should receive it once it is issued. 

    I think a larger dimension may allow the IC to shift to a side, which you wish to more centered.  However, a maller dimension may have some reliability issue for the solder joint form, so smaller dimension of the aperture is less favorable. You can tell that all PCB solder pad is bigger than the physical part to be soldered onto it.   

    Your concern of causing short is reasonable, but if you cross check the dimensions of the two versions--and I believe it was already checked by our experts-- it should be safe by doing a combined footprint.

    Thanks,

    Youhao