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BQ25703A: Replace input and output current sense resistors for higher currents

Ravid BY
Intellectual 330 points
Part Number: BQ25703A
Other Parts Discussed in Thread: BQ24650

Hi,

We are using BQ25703A in order to charge 4S11P lithium-ion battery pack.
Currently the input source is an 80W solar panel (Vmp=17.7V, Imp=4.52A) and it is working fine.
In the datasheet the recommended input current sense resistor is 10mOhm - up to 6400mA input charge current.
We are wondering if we can replace the input current sense resistor to a small value like 5mOhm so we can use higher current input source (bigger solar panel).
(Same question on the battery charge current sense)

BTW, if it can be done we will replace the mosfets and inductor to fit to the new design.

Thanks,

  • 0
    TI__Expert 5330 points

    Hello Ravid,

    In addition to being used for input current limiting, the input sense resistor is used for current-mode control of the switching regulator.  Reducing the resistance reduces the current sense signal and can lead to instability in the feedback loop of the regulator.  Our designers have determined that there is enough margin to reduce the input sense resistor (ACN/ACP) from 10 mOhm to 8 mOhm, therefore increasing input current form 6.4 A to 8 A.  Any further reduction in the resistance will reduce the SNR of the current feedback loop and may lead to instability in the regulation.

    The battery current sense resistor (SRP/SRN) is also used to regulate the battery charging current, but this regulation loop is less susceptible to noise than that of the switching regulator and could probably be pushed further.  Given that there is a lower limit of 8 mOhm on ACN/ACP input sense resistor and corresponding upper limit of 8A input current, can you tell me what target resistance you would have for the battery charging sense resistor (or the target charging current.  10 mOhm --> 8128 mA) and I will double-check with the design team?

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    Intellectual 330 points

    Hi Steve,

    Thanks for the quick reply!
    We would like to connect x2 80W panels in parallel, so the maximum input power will be about 160W.
    If the input power will be 160W and the battery will be fully discharged (about 10V) in means we would like to charge it up to 16A.

  • 0 in reply to Ravid BY
    TI__Expert 5330 points
    Hi Ravid,

    I talked to our team. These devices undergo extensive characterization and validation under the test conditions outlined in the datasheet, and to date, our team has not performed any of this testing with a sense resistor below 10 mOhms at BATFET SRP/SRN. There are quite a few monitoring and control loops within the device, and while there was no specific threshold for resistance identified as a concern, neither can we guarantee that the device will operate as intended outside of the range we have tested.

    We do have some devices that are characterized with charge currents up to 20A, including the BQ24650, which was designed specifically for solar charging applications.

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Intellectual 330 points
    Thanks Steve, I will look at BQ24650.