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LDC0851: Stack coil issue

Cherry Zhou
Mastermind 22235 points
Part Number: LDC0851

Hi team,

Here's an issue from the customer may need your help:

When testing the LDC0851, the internal layers of the stack coils were reversed due to design errors, which resulted in the output state of the 0851 being reversed. However, it was found that the output could be converted high and low by moving metal close to and away from the reference coil for detection purposes.

1) Does this use affect the stability of switching transitions and temperature variations?

2) Also, can the coil be far away from the chip and connected with a wire? If so, the capacitor should be close to the chip or coil?

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Mastermind 18545 points

    Hello Cherry, 

    Can you clarify what you mean by the layers being reversed? Do you mean that the internal layers were swapped or that the circular coil direction was reversed? 

    The coil can be placed remotely and the capacitor should be placed next to the chip. If you are connecting via a wire, it is best to ensure that the traces are length matched as best as possible. it is also a good practice to keep the sensors on the same PCB as each other even when they are remote to the IC. 

    Best Regards, 
    Justin Beigel

  • 0 in reply to Justin Beigel
    TI__Mastermind 22235 points

    Hi Justin,

    Thanks for your support.

    Justin Beigel said:
    Can you clarify what you mean by the layers being reversed? Do you mean that the internal layers were swapped or that the circular coil direction was reversed? 

    "Layers being reversed" refers to that the two coils are oriented exactly the same, but the inner layers are in the opposite direction, that is being reversed. Since as per the manual, inner coil routing is consistent.

    Currently by adjusting the distance, initial detection can be implemented and the output state can be adjusted by the inverter. The customer just wants to double check if there is a problem with this?

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 40295 points

    Cherry,

    Justin is not available at the moment, but should be back online later today or tomorrow.
    In the mean-time, I have a few questions. 
    How many layers are used for each coil?
    Are the coils implemented as intended, except that all of the turns for each coil go in  the opposite direction that was intended?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Mastermind 22235 points

    Hello John,

    Thanks for your help here.

    John Miller - Sensing said:
    How many layers are used for each coil?

    The coil is 2 layers, 1mm thick.

    Both coils are identical, so the same inner layer rotation direction cannot be achieved by reversing it anyway.

    The customer has re-made both coils that meet the requirements. Now just to confirm if a non-compliant coil (which can implement the initial detection as mentioned above) will have an impact on stability and temperature variation? 

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 18545 points

    Hello Cherry, 

    By having the inner coil winding direction reversed, the current doesn't flow in a constant direction which causes magnetic field generated by each layer to be  oppositely polarized. This can decrease the sensing range of the coil.

    Since both of your coils are identical still, I would not expect it to be an issue as long as you are still seeing consistent switching behavior in your application. 

    Your design may be slightly more dependent to variation in the PCB manufacturing and I would recommend doing in depth testing on a few units with the reversed layers to ensure that you are not seeing any instabilities over temperature. If you verified this on a small batch of the reversed layer units, then I would expect the rest of them to continue to function normally. 

    Best Regards, 

    Justin Beigel