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For coil to be used for LDC1000

Masami Michino
Genius 5070 points
Other Parts Discussed in Thread: LDC1314, LDC1000EVM, LDC1314EVM, LDC1101


Dear, All

A Customer has a question about the coil to be used for LDC1000 and LDC1314.

① Please tell us about the coil of LDC1000EVM and LDC1314EVM.
a. inductor value
b. The thickness of the substrate and the interlayer distance
c. Thickness of PCB copper
d. the material of the substrate

② There is a difference in the following data.

- DataSheet (C version) 8.2.1.1 and 8.2.1.2.1

"Nominal sensor Inductance": 18uH
"Number of PCB layers for sensor": 62mils


- Webench calculation result of (Inductive PCB Coil Designer)

The value of the "Total inductance Circular" to match the 18uH is, "Space between 1st layer and 2nd layer (x12)" that may require us to 14mils.
At this time,
"Q factor": 33.6
"AC resistance (skin effect only)": 12.35 Ω


- Excel calculation sheet (LDC_Tools-ext22.xlsx)
e2e.ti.com/.../1742329
Calculation sheet by.

When I set the value of "Webench",
"Q factor": 40.68
"AC resistance (skin effect only)": 10.237 Ω


Which is correct?

Thanks, Masami M.

  • 0
    TI__Genius 13650 points
    Hello Masami,
    The inductor measurements for each EVM are described in the user guides. The EVMs are manufactured on standard 62mil FR4 PCBs and use 1oz copper thickness.

    The LDC131x/161x family has superior performance, lower power consumption, and easier system design than the LDC1000, and is recommended for most new designs except for metal type identification applications.
    The LDC1101 is a good alternative to the LDC1000 for high-speed sensing applications.

    I looked into the difference between Q calculations and found that Webench uses a slightly more pessimistic estimate for the skin depth, which results in a lower Q.

    Here are some links that you may find useful:
    LDC1000 user guide: www.ti.com/.../snau150a.pdf
    LDC1314 rev A EVM user guide: www.ti.com/.../snou133.pdf
    LDC1314 rev B EVM user guide: www.ti.com/.../snou135.pdf
    Inductive sensing FAQe2e.ti.com/.../295036
  • 0 in reply to Ben Kasemsadeh
    Genius 5070 points


    Hi, Ben-san

    Thank you for your reply.
    I understood about Webench uses a slightly more pessimistic estimate for the skin depth.

    However, the data sheet "PCB thickness" is to a 18uH at the time of the 62mils, in Webench and Excel sheet do not know the reason to become 18uH is when this value is 14mils.
    I want you to tell me the reason for this.

    In addition, the inductance value of the coil of LCD1000EVM has been described as the average 19.558uH in User's Guide.
    It may be considered 18uH is the target value?

    Thanks, Masami M.

  • 0 in reply to Masami Michino
    TI__Genius 13650 points

    Hello Masami-san,

    please regard both Webench and the excel spreadsheet as tools that simulate or estimate the response. They are based on models which will always have some degree of variation to actual manufactured coils. The 19.5uH is a measurement of the actual manufactured design.

    In almost all LDC designs, absolute inductance is not relevant to the system: usually, output codes from the converter are used to determine target position. Both LDC131x and LDC1000 can operate with coils that have much lower inductances than the 18uH (also refer to this blog).

    Is this difference difference in inductance between the models and the actual PCB causing any problems? Please let me know what issue you are trying to solve or what your concerns are with the inductance tolerance, and I may be able to suggest a better solution.

  • 0 in reply to Ben Kasemsadeh
    Genius 5070 points

    Hi, Ben-san

    Thank you for your reply.
    The customer is asked if it is sufficient to use any Tool to calculate for the board design.
    Does it may be considered in the following manner?
    a. The value of the datasheet is the target value of the design.
    b. Value of Webench and Excel sheet is the worst value of each Patameta.
    c. Webench are pessimistic calculated than Excel sheet.

    Thanks, Masami M.

  • 0 in reply to Masami Michino
    TI__Genius 13650 points
    Hello Masami,
    yes, either of the tools will provide a good estimate of the actual coil characteristics.

    a. the datasheet shows a typical example application. It is possible to use Inductors of a much wider range.
    b. Webench and Excel are based on slightly different models; they present an estimate of coil parameters.
    c. correct
    Sensor design is also covered in a lot of detail in this apps note: www.ti.com/.../snoa930.pdf
  • 0 in reply to Ben Kasemsadeh
    Genius 5070 points

    Hi, Ben-san

    Thank you very much for your answer.
    I answer it to the customer.

    Thanks, Masami M.