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LDC0851: Improving the sensing distance

POUJAUD Guillaume
Prodigy 30 points
Part Number: LDC0851
Other Parts Discussed in Thread: LDC1612

Hello,

I have understood that the sensing distance is directly linked to the outer diameter of the coil. I would like to know, if by using SMT coils (not an 0402 type, but a "wound coil") it would be possible to increase the sensing distance.

Thanks for you help !

  • 0
    TI__Genius 13335 points
    Hello,
    The sensing range of the LDC0851 is determined by both the coil diameter AND the inductance matching between the LREF and LSENSE sensors. For example, if the inductors are perfectly matched then you could get around a 40% switching distance using an ADJ setting of 1. If you mismatch the inductors by 5%, this distance could drop down to 25% or less. This is the reason we suggest PCB inductors because they can be very well matched. Theoretically a wire-wound inductor with a ferrite core would get a longer sensing range, but the part-to-part matching of the inductors would end up hurting you more than helping.
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Prodigy 30 points

    Hello Luke,

    Thank you very much for this accurate answer. This confirms what I was thinking. Also, between the two criteria (coil diameter and matching), which one, according to you, would be the most important. Do you think that it would be possible, by choosing the biggest coil as possible for my application, to "by-pass" the matching criterion. I mean to make the diameter criteria much more efficient than the matching one. I don't expect a "yes" or "no" for this last question, but do you think it would be a huge waste of time to perform this kind of experimental study ?

    Thank you for your support,

    Guillaume

  • 0 in reply to POUJAUD Guillaume
    TI__Genius 13335 points
    Hello Guillaume,
    This is an interesting question, but I believe the answer will be that you need both a high level of matching and a large coil diameter to achieve the longest sensing range. A compromise with wire-wound solutions may work for one or two units, but once you start to hit the part-to-part variation of the inductors you will start to see switching distance variation. Additionally, the temperature characteristics of the wire-wound inductor is not going to match the PCB coil so this is another factor to consider for the matching/switching distance. If you had to use this approach (for a small number of units) you may be able to purposely pick non-matched LREF inductor so that accounting for all of the tolerances it is higher than than the LSENSE inductance, then use the ADJ settings to "calibrate" the distance. Note that the ADJ setting effectively allows you to subtract a fixed amount of inductance from the LREF sensor. However, this may be impractical on a large-volume scale. I've posted more info on this approach in the following thread: e2e.ti.com/.../2090005
    This is primarily why we point people towards PCB inductors because all of these factors are well controlled and don't pose issues over temperature or part-to-part.
    Alternatively, if you wanted more control over your switching distance or a longer sensing distance you could use a high resolution device like the LDC1612 which has an I2C interface and converts the oscillation frequency to a 28 bit value. You could use a micro-controller and processing to determine a switching threshold in software. You can refer to the following TI Design which uses this device in a button application: www.ti.com/.../TIDA-01102
    Regards,
    Luke LaPointe
  • 0 in reply to Luke LaPointe
    Prodigy 30 points

    Hello Luke,

    Thank you very much for your answer. I think you provide me a good starting point for a first design. I was interesting in the LDC0851 because it provides a simple push-pull output, no programming interface. Thank you for the link to the thread and for the TI design. I will now go deeper in the datasheet and the several application notes provided on TI website.

    Best regards,

    Guillaume