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BQ51221: Modualtion depth regarding to the operating frequency

Part Number: BQ51221

Hi,

We are evaluating WPC/PMA operation based on bq51221 EVB. During the test, we found one weired symptom regarding to the modulation depth on TX coil when the operatng frequecy moved.

We found that modulation depth is getting smaller when we move to the higher frequency on the Duracell PMA pad. Sometimes. even more, it looks reversed when there is more load (500mA)  at 280kHz or higher.

I think modulation depth can be reduced by the frequency getting higher but I can't understand why the reversed waveform exist in the higher frequency.

I can't see any abnormlly arround 230kHz but it showed the problematic symptom at over 280kHz. Do we check the resonant frequency and related params for the system?

Would you explain the the symptoms or give some advices for the further debugging?

Thanks in advance,