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BQ51221 pin CM_ILIM

Other Parts Discussed in Thread: BQ51221, BQ24166

I observed with our Qi Transmitter and the BQ51221 on the Qi Receiver side a much more stable charging process

if I pull-down the CM_ILIM pin on the BQ51221 with the internal pull-down of the BQ51221. Much more stable charging

process means that the charging doesn't stop in an unexpected manner (or with other words Qi Transmitter keeps

connection with BQ51221). Now my questions.

 

  • In the BQ51221 datasheet it is mentioned that the charger IC which is connected to the BQ51221 output

    needs a VIN-DPM feature. In our case we have the charger IC BQ24166 which has a on Pin 1 the input

    named VDPM. I assume that the BQ24166 VDPM input fulfills the needed VIN-DPM feature which is

    mentioned in the BQ51221 datasheet?

  • The BQ51221 output is adjusted in our case to 5 Volt. The voltage divider of the BQ24166 VDPM input

    which is connected to the BQ51221 5Volt output is dimensioned in a way that the 1.2Volt threshold of the

    VDPM input is reached if the BQ51221 output voltage decreases from 5Volt down to 4.4Volt. Now my

    question: Is there any further information regarding the needed VIN-DPM feature and how to dimension

    the voltage divider on the VDPM input of the BQ24166 in the right way? I’m asking this to avoid that the

    output of the BQ51221 will collapse if the communication current limit is enabled.

  • Will the BQ51221 EMC behavior (radiated emission and immunity) change if the communication current

    limit is enabled compared to the case of disabled communication current limit?  

 

I would appreciate it to get feedback.         

  • Frank,

    Yes, the bq24166 VDPM pin will work for preventing the bq51221 output from drooping too far if the bq24166 tries to draw too much current.

    I am not sure of the reaction time of the VINDPM which will impact how the bq24166 reacts to the communication current limit.  You have taken the right steps.  I believe experimentation with your setup will result in the best solution. Adding capacitance at the OUT pin of the bq51221 and the IN pin of the bq24166 is always a good idea for the transients.

    We have not done EMC testing based on the communication current limit.  

    Regards,

    Dick