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bq51050B Ls and Ls'

Tom Saluzzo
Expert 1280 points

Other Parts Discussed in Thread: BQ51050B, BQ25100, BQ51003, TIDA-00318

In the bq51050B datasheet (SLUSB42D), page 25, paragraph below Figure 29, it states:

"The receiver coil, as it will be placed in the final system (e.g. the back cover and battery must be included if the system calls for this), is to be placed on top of this surface and the inductance is to be measured at 1-V RMS and a frequency of 100 kHz. This measurement is termed Ls’. This measurement is termed Ls or the free-space inductance."

This is confusing for defining Ls and Ls'. Is it saying Ls' is the inductance measured on top of the surface at 1-V RMS and 100KHz and Ls is the free space inductance?

 

Also, page 3 of the datasheet show a typical application schematic (Figure 3). The current is not defined for all pins. For the external resistors on the outputs, what is the required power dissipation required? Can we use 0201 package resistors?

 

  • 0
    TI__Genius 14890 points

    Tom,

    Ls is definitely free space inductance at 1-V RMS and 100kHz.

    Ls', per the WPC specification, is measured with the 50mm x 50mm x 1mm ferrite.  However, since the actual inductance changes based on its environment, the closer you have the coil to its end configuration, the more accurate you will have the Ls'.  I suggest trying a few different configurations to see how the inductance changes.  You can then enter those values in the calculation for the series capacitors to determine the range of capacitance you may have and make a judgement from that.

    The resistors will see very little power.  Please see www.ti.com/tool/TIDA-00329 for an example of a (non-Qi) small solution (it can be built with the bq51050B or the bq51003).  It uses many 0201 resistors.  Additionally, while you're there, check out TIDA-00318 for a Qi solution which included the bq51003 and linear charger bq25100.  Keep checking back as we're adding new reference designs regularly.

    Regards,

    Dick