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BQ51050B - Selecting Cs and CD with WE Coil

Other Parts Discussed in Thread: BQ51050B, CC2650, BQ51003, TIDA-00668, BQ500212AEVM-550, TIDA-00762, BQ25100, BQ24090

Hi there,

how can i get the L's, Ls, fs and fd from a Coil?

I want to use the BQ51050B with the WE 760308103206 Coil.

The Coil specs are as followed:

And can someone please check my schematics if i missed anything? The C1 and C2 values muss still be calculated and depending on the values i will add parallel capacitors to achieve the exact calculated value.

  • Michael,

    The section of the bq51050B datasheet just above the formula you pasted (formula 6 in the datasheet) covers how Ls and Ls' are measured.  Ls is the measured inductance at 100kHz (fs).  Ls' is the same measurement, but with the coil place on a specific ferrite test fixture with the appropriate spacing.  Another way to estimate the Ls' is the measurement of the inductance when the coil is places on the transmitter platform intend for the receiver solution.  We recommend using as close to the final solution as possible (including spacing, batteries and any other metals that will be included in the final solution).  

    The section just below the equation discusses fs (100kHz) and fD (1MHz).

    If the Ls' cannot be measured, a good rule of thumb would be to start with Ls' about 20% higher than Ls.  This is NOT a final solution.  Once the system is operational with the chosen Ls' value, experiment with the changing the C1 value (from the equation in the original post) higher and lower.  Measuring the efficiency of your system at these values will enable you to find the optimum values for C1.

    Since all of the current in the receiver will be passing through C1, we highly recommend using 3 capacitors in parallel for the final solution.  Your schematic for C1 shows 470nF.  A better solution would be 2 capacitors of 150nF and one of 180nF (for 480nF total).

    The AC1/AC2, CLAMP1/2, BOOT1/2 and COMM1/2 capacitors are recommended to be rated to 25V or higher.

    What is your intended charge current?  Termination current?

    EN2 is left floating.  The internal resistance will pull low.  See the Adapter Enable section for details.  I recommend terminating the EN2 with a zero ohm resistor to ground if that is the intent.

    The FOD resistor (R5) and the Ros resistor (R1) may need to be tuned for Foreign Object Detection calibration when the final solution is ready.  The value for these resistances is based on the final system solution.  The datasheet has more information.



  • Hi Dick,

    i am working on a IOT project that uses ultra low power BLE CC2650 and display. The electronic should run on a single coin cell and i have found some reference design for a wireless power charge for it.

    i want to make a PCB exactly like the 50mA Wireless Chrager Booster Pack Reference design.

    A 50 mA current would be enougth. The Coil would be the same as the design.

    So the Space between the PCB and the Coil is important or the space between Coil and Ferrite?

    But if i design the Coil on another PCB and the space is way bigger, would the space between the RX and Coil matter?

    I want to use the BQ51050B instead of the BQ51003(Reference Design) becouse i want to pick and place the component myself, and i cant implement a BGA package myself.

    So if i only change this component, can i use the same C1s and C2s on the Reference design? 

    The transmitter i wish to implement this one: 

    Do you think the TX and the RX would work together? Or muss i change the coils?

    Thank you,


  • Michael,

    The coupling between the TX and RX coils is the key.  The coupling is impacted by the distances between the coil and ferrite (the coils should be on the ferrite with no spacing), the distance between the TX and RX coils, the metals and other surrounding materials, the coil inductance, size and many other factors.

    The PCB coils tend to result in lower efficiency.  The shielding is the most overlooked source of possible improvement.  The shield helps "direct" the magnetic field between the TX and RX coils.  It protects friendly metals from absorbing the magnetic field.

    The bq51050B and bq51003 will use the same C1 and C2 - since the calculations for these resonant capacitors is based on coil inductance.

    I have not tested the TIDA-00668 with the TIDA-00415.  Testing with multiple coils will yield different results.



  • Hi Dick,

    which TX have you tested on the TIDA-00668 that works?
    Which TX do you think i should use?
  • Martin,

    I did not generate TIDA-00668.  I was able to borrow a working sample from the developer, who has tested on multiple transmitters.

    I did some quick testing with the TIDA-00668 on the TIDA-00415, the TIDA-00762 and the bq500212AEVM-550 without issues.

    In a previous message you asked about using the bq51050B (since it is available in QFN).  You must be careful with the low charging currents you are using with the bq51050B. The device can charge at that level, but the variation in part to part may be quite large.  Additionally, and more important, is the termination current.  The bq51050B was designed to work with a much higher current (1A) and does not have the accuracy that the small currents for termination require.  

    Another option you have is to use the your own termination circuit.  You would have to use your system CPU to measure the current and use the EN2 pin to terminate charge.



  • Hi Dick,

    thank you for testing it =)

    So if i understood you right, i have to measure the current thats going to the battery.

    When EN2 is High the wireless Power mode is active and the battery start to be charged.

    Whenthe battery is  fully charged, i have to put EN2 Low to disable wireless Power. So it terminates the charging process.

    Is this correct?

    On the TIDA-00668 schematic there is the BQ25100 IC that is a battery chargerand the Current Termination can be programmed.

    So do i have to still use my CPU to look after the EN2 PIN? Or does the BQ25100 does the job for me?

    The BQ25100 comes also on a BGA Package, do you think if i use the BQ24090 would be a problem? He comes on a MSOP10 package and can also charge the battery at 50mA.




  • Michael,

    To set your own termination limit, you must measure the current into the battery.

    I mentioned to use EN2 to terminate charge on the bq51050B, but it is actually TS/CTRL.  This is a dual function pin that is TS (temperature sense) with the thermister - it is checked periodically.  In between tests (details in the datasheet), it is CTRL.  Pull CTRL low to  send a "fault" condition to the TX or pull CTRL high to send an EPT Termination to the TX (desired for this case).  Since this is a wireless charger, the bq25100 is not operating.

    The bq25100 has a CHG pin that indicates charge (and complete).