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BQ25504: BQ25504 charging current control

John Hsu
Prodigy 30 points
Part Number: BQ25504

Tool/software:

Dear Support Team,

I am currently utilizing the BQ25504 as a WiFi energy harvesting charger IC to charge a Li-ion battery. The circuit is divided into two main sections, and I have conducted the following verifications:

1.  Rectifier (C311, C310, C315, D63, D62, C314)

    a. Input WiFi signal at C310 pin 2 with 5dBm so the rectifier is able to output DC 2.7V with a load resistor 10k ohm which is placed parallel with C314.

    b. Input WiFi signal at C310 pin 2 with 22dBm so the rectifier is able to output DC 0.68V (BQ25504 cold start voltage is 0.6V) with a load resistor 47 ohm which is placed parallel with C314.

2. Charger 

   I use a external power supply to provide the input voltage to BQ25504 then oberseve the input current and output charging current with 4.2V Li-ion battery.

However, when I connect these two sub-circuits together and input 5dBm WiFi signal, the rectfier output DC voltage will drop down to 0.45V only. Looks like the battery charging current, probably 1x mA, is higher than the rectifier capacity.  So my question is there any mechanism to reduce the charging current by adjusting the external resistor?

Your assistance in addressing these questions would be greatly appreciated.

Best regards,

John

  • 0
    TI__Mastermind 40325 points

    Hi John,

    Please see Section 8.3.1 Maximum Power Point Tracking, the output will try and drag the input (VIN_DC) as low as possible while stopping at the MPPT point. If the open circuit voltage of your input source is too small, then 71% (your voltage divider for VOC_SAMP) of the input may drive the input too low while loading the circuit.

    It is not possible to adjust the charge current with resistor. Please attach an input source capable of supplying power to the input without crashing it under load.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 30 points

    Hi Mike, 

     If I reduce the percentage from 71% to like 40% or lower than that, would it increase the input voltage? 

    Best Regards, 

     John

  • 0 in reply to John Hsu
    TI__Mastermind 40325 points

    Hi John,

    You need to regulate the input through the divider to where you want the input to pass no lower than, so ideally higher. If the input is still then lower than that, your input cannot handle the required load.

    Regards,

    Mike Emanuel