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BQ51050B: How RFOD and Ros affect the FOD mechanism ?

Nicolas Law
Prodigy 10 points
Part Number: BQ51050B

Hello,

I'm actually designing an IoT product which include a wireless charging solution following the WPC 1.1 standard. The BQ51050B has the advantage of having an integrated Li-ion charger so I choose this one. My goal is to recharge a Li-ion battery with a "fast charge" current of 1A.
Following your datasheet, application note and EVM (bq51050BEVM-764), I understand and found the different values ​​of the calibration components (Resonant capacitor, max current output resistor, termination resistor ...) but I don't understand one feature: how Ros and RFOD calibrate the FOD mechanism ?

I understood that those resistors must have an particular attention and must be "calibrated" to assure the WPC v1.1 Compatibility. I found your associated tool (Foreign Object Detection (FOD) Calibration/Tuning Tool) and I have some question about it:
1) How the model error curve is established ? What parameters determine this curve and how Vos change this model ?
2) By manipulating Vos (I suppose the voltage in the resistor Ros) to fix the model error, does this value impact on the electrical characteristic of the resistor ?
3) Electrically speaking, how Ros change the offset and RFOD the slope of the Ploss curve ?
Is there a kind of equation about this relationship ?
I guess one of those resistor (RFOD ?) represents components that influence received power (part 6.2 of User Manual) and the voltage (VFOD) must be a part of this equation.

Sorry for those questions but I would like to have at least few informations about FOD mechanism before building the PCB.
The goal of the project is to have a proof of concept so we probably won't need the AVID tool for now. I want to be sure that with the EVM value (Ros = 42.2k and RFOD = 200), I will have a functional device.

Thank you

Nicolas LAW

  • 0
    TI__Guru 65716 points
    Hi Nicolas
    Also take a look at the FAQ section I have a presentation on wireless power that may help.
    The RX will report how much power it has received, this info is set to the TX as the received power packet. The TX know how much power is sent to the RX and RX has just told it how much it received and the difference is loss. The loss is assumed to be going into a foreign object and when it exceeds a threshold set by the user a fault is declared.

    Looking only at FOD in the RX--The RX will measure received power as a function of output current. The FOD pin is part of the ILIM ckt. The ILIM pin will generate a current that is proportional to the output current. FOD Resistor will sample this current and voltage will be converted into Received Power. The combination of R-FOD and R-OS are used to generate a curve that represents received power across load.

    Received Power also needs to represent the loss in metals in the phone, behavior of the coil and other factors. As part of the FOD calibration the received power is measured center on the coil and at 4 locations off center. The readings are then used to calculate an acceptable value for R-FOD and R-OS using the spread sheet provided.