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Smallest coil in watch?

Bruce li
Genius 5580 points
Other Parts Discussed in Thread: BQ51050B

Hi Wireless power team,

One of my cutosmer kicked off a smart watch project, they want to add wireless charger function.Here is the question, could you kindly help? Thanks so much.

What is the smallest coil that is supported by the TI design?

We need to fit a 15mm diameter coil if at-all possible, but need to ensure that it will work with BQ51050B, and this can be produced and provide reliable charging.

  • 0
    TI__Expert 7730 points

    We have not used 15mm diameter coil with bq51050B. However, that doesn't mean it won't work. If you are using a standard Qi compliant TX which has much bigger coil size, you may encounter coupling issue. The coupling and efficiency may be higher when you also customize the TX coil to match the small size. The Cs, Cd and the COMM cap will also need to tuned to make it work.

    Thanks,

    Wenjia

  • 0
    Genius 5580 points
    Hi Wenjia,
    Thanks a lot for your support. I'll suggest customer to take your advice.
  • 0 in reply to Wenjia Liu
    Prodigy 70 points

    Hello Wenjia,
    To tune Cs and Cd cap we take into account only the capacity of inductances ? the caracteristics like diameter doesn't intervene ? (Ls represents the transmitter inductance ? )
    And for COMM cap on the datasheet we find only a value, how can we modify it ?

    As regards,
    Pierre

  • 0 in reply to Pierre GACHET
    TI__Genius 12485 points
    Hi Pireere,

    To tune the Cs and Cd caps you need the inductance Ls and Lsp(inductance on stack).

    For the COMM caps, have you tried using the datasheet value? The rough ball park is it could be anywhere from 1/3rd to 1/10th of the Cs capacitance.

    This value varies based on the system.

    Regards,
    Gautham Ramachandran
    Apps. Engineer- Texas Instruments
  • 0 in reply to Pierre GACHET
    Intellectual 310 points

    The required modulation is mentioned in paragraph 6.2 of the WPC specification. You can measure the signal at the AC pins and tune the COMM caps to meet the requirements. Higher capacity = higher modulation index, but also higher losses.