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BQ51003: Coil/Passive values to charge a 3V, 17mAh coin cell battery

Luke B
Prodigy 35 points
Part Number: BQ51003
Other Parts Discussed in Thread: TIDA-00668, TIDA-00318, , BQ25100

Dear support,

The TIDA-00668 which I copied values from uses a 19m diameter coil while I will be restricted to a maximum size of 15mm, due to availability and cost I am considering the 12mm WR121210-27M8-ID which is 8.32uH. I intend to use a off the shelf transmitter such as the TYLT Vu Solo and then explore cheaper options from there. I don't fully understand how to pair or design this Rx stage but I do not need very much power transfer since the end goal is to recharge a little 17maH coin cell battery. For the BQ51003YFPT I followed multiple TI dev platforms (TIDA-00318 and TIDA-00668) which varied in capacitor values going from the coil to AC1, is this fine tuning that is dependent on the Rx coil? 

Some examples include use of a battery management IC (such as BQ29702DSER) and others do not, is this strongly recommended when using these other TI components?

The BQ25100AYFPT connected to the output of the BQ51003 charges the 3V 17mAh coin cell, the highest recommended R_ISET value is 13.3k which sets the charge current to 10mA, is that low enough to safely charge such a small battery? is a slightly higher R_ISET value allowable with this part? Lastly, it is very unclear to me what to set the PRE-TERM resistor value to for this application, please advise.

I have a schematic of all of this but first I have to find out how to add an attachment to this post Sweat smile

Thank you very much

  • +1
    TI__Guru 65726 points

    Hello

    Updated reply

    1.)  I am considering the 12mm WR121210-27M8-ID which is 8.32uH. --- Bill J -- This coil is high risk, size is small and L value low, will be hard to start due to low ping voltage.

    2.) I intend to use a off the shelf transmitter such as the TYLT Vu Solo and then explore cheaper options from there. --- Bill J -- good choice.

    3.) I don't fully understand how to pair or design this Rx stage but I do not need very much power transfer since the end goal is to recharge a little 17maH coin cell battery. For the BQ51003YFPT I followed multiple TI dev platforms (TIDA-00318 and TIDA-00668) which varied in capacitor values going from the coil to AC1, is this fine tuning that is dependent on the Rx coil? ----Bill J - For tuning see data sheet " Series and Parallel Resonant Capacitor Selection" section ( for BQ51003 9.2.1.2.2).  The unit is tuned with the coil L and C1 to value 100kHz.  This is the most important one.  Also note that the L value used is the measured L value of coil in case with PCB and battery while on TX.

    4.) Some examples include use of a battery management IC (such as BQ29702DSER) and others do not, is this strongly recommended when using these other TI components? --Bill J - Yes

    5.) The BQ25100AYFPT connected to the output of the BQ51003 charges the 3V 17mAh coin cell, the highest recommended R_ISET value is 13.3k which sets the charge current to 10mA, is that low enough to safely charge such a small battery? -- Bill J -- Do not know this is a question for the battery manufacture.

    6.) is a slightly higher R_ISET value allowable with this part?---Bill J -- Yes but you will need to evaluate the performance and verify that it meets your requirements.

    7.) Lastly, it is very unclear to me what to set the PRE-TERM resistor value to for this application, please advise.-- Bill J -- Typical value is 6k ohms for 20% precharge and 10% termination, BQ25100

  • 0 in reply to Bill Johns
    Prodigy 35 points

    Dear Bill, Thank you so much for you help, I feel as though I owe you lunch ha! I will press on and let you know how things go or if I get in another Jam. Based on another thread you answered, for the 12-15mm coil size I should be ideally aiming for >20uH correct?

  • +1 in reply to Luke B
    TI__Guru 65726 points

    HI Luke -- not problem glad to help.

    "ideally aiming for >20uH correct?" -- Bill J -- yes, also L value over 15uH.  

    Some things that will help with a smaller coil say 15mm to 20mm, higher L value, good alignment with the TX coil, smaller coil to coil to distance.