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help bq2031 (problem with charging current )

hiep dang
Prodigy 30 points
Other Parts Discussed in Thread: BQ2031

hi chris and jeff !

i'm a student . Now, i'm using bq2031 for my project, but with my circuit, i can have charging current from 2.5A to 5A. In fact, it just is 2A, and i don't  kown why . I changed resistor sense, but nothing change. Could you help me please ! i inserted my file in this page. thank you so much !7418.BQ2031.rar

  • 0
    TI__Guru**** 150930 points

    I am unable to open your files.  Can you post a pdf of the schematic?

    How are you measuring the current?  What is the battery voltage at the different currents?  Is it at the maximum voltage?  Have you confirmed that your input power supply to the bq2031 is not limiting?  What is your input voltage and cell count?

    We have an EVM available, so you can order that and be using a known good circuit and layout.

  • 0 in reply to Chris Glaser
    Prodigy 30 points

    hi chris ! thank for reply. i measure charging current by measuring the voltage on RSNS,and use the equation imax=0.25/Rsns to caculete. My max charging voltage is 13V ( because my circuite have a buck ) . Besides,my battery have 6 cells/5Ah and i use a supply with 20v and 14617.This is my schematic.doc0A. I inserted my schematic by file word (.doc ). please help me !

  • 0 in reply to hiep dang
    TI__Guru**** 150930 points

    First, you need to measure the charging current with a current meter.  There are tolerances in the resistor and 250 mV voltage that will affect the real current flowing to the battery.  Also, you would need to measure precisely across just the sense resistor so as to not pick up any other voltage drop from PCB board traces.  You need to measure the current with a current meter.

    Also, I don't understand your schematic.  Q1 is in the wrong place for a buck converter.  It needs to be between R8 and L1/D3 so that it can switch on and off and chop up Vin so that the output filter can average it and produce a reduced voltage.  As it is now, there is no way to stop the current from flowing to the battery from the input.

  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Hi chris ! thank you so much. I will do the actions that you advice me. However, i want to show you a issue: Q1 in my circuit is a N-FET ( not P-FET), so would you like see again for me ? thanks

  • 0 in reply to hiep dang
    TI__Guru**** 150930 points

    Using this topology with an NFET is more difficult because you need a higher gate voltage than your input voltage.  That is why the EVM design use PFETs.

    In any case, with NFET or PFET, a FET in the location where you have it doesn't do anything.  You cannot open the path between your input supply and the battery.  So, the bq2031 has no control over the charging current.

    You should order the EVM and/or stick to its schematic which is built and tested.