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Setting external MPPT of BQ25570 using VRDIV

MJ
Intellectual 950 points
Other Parts Discussed in Thread: BQ25570, BQ25504

Hello,

I am using the BQ25570 to harvest from RF source. The RF source has a non-continuous carrier wave, so if the MPPT sampling occurs when there is no transmission a wrong value is used. Since there is no way to disable the MPPT function I want to set a fixed voltage (although tunable by resistor ratio) at the VOC_SAMP port.

Since my design uses the BQ25570 as only power source, I would like to use the VRDIV port as reference voltage.  Is it possible to connect another resistive divider from VRDIV to GND, with the middle point connected to VOC_SAMP? If so, should I also obey the recommendation to keep the sum of the used resistors between 11 and 15 Megaohm? And is the voltage provided by VRDIV equal to VBIAS (=1.21V nominally) or should I calculate my reference using another expression?

Best regards,
Michel Jansen

  • 0
    TI__Guru**** 166076 points

    VRDIV is not a constant reference voltage.  As shown in datasheet figure 40, it only has a voltage on it every 64ms for a brief time and that voltage varies.  bq25504, the core from which bq25570 was derived, can have MPPT sampling disabled.   

  • 0 in reply to Jeff F
    Intellectual 950 points
    I am too far in my current design to switch to the BQ25504, but that would indeed be a solution.
    I have decided to connect it to VOUT with total sum of resistors 20 Megaohm (even though it is a kind of dirty fix).
    Thanks for your clarification anyway!