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BQ25895: VSYS Max Current Output

Mitchell
Expert 4915 points
Part Number: BQ25895

Hi Team,

My customer is seeing an issue with our BQ25895.During load testing on the VSYS output, we noticed that VSYS dropped significantly when loaded to the 5A maximum stated in the spec.  We have a 5V input on VBUS rated for 5A.

Below are our default I2C configuration settings:


REG07 = 0x8D (Disable WDT)
REG02 = 0x3C (Disable D+/D- Sense, Boost @ 500kHz)
REG00 = 0x7F (ILIM pin enabled, input current limit = 3.25A)
REG03 = 0x3E (OTG enabled, charger enabled, VSYSmin = 3.7V)
REG04 = 0x3E (Max charge current = 4A [3.968A])
REG05 = 0x10 (Precharge limit = 128mA, Termination current = 64mA)
REG06 = 0x5E (Charge voltage limit = 4.208V)


This issue occurs whether using a fully charged, discharged or no battery plugged into the charger. 

We found that by reducing the charge current, the voltage on VSYS was more stable and higher.  However, around 3.5A on VSYS (even with charge current set to zero) the VSYS voltage starts to drop well below the 3.7V minimum we set in the registers.  It then continues to fall steadily as the battery loses charge.  This is concerning, since we expect up to 4A on VSYS and while we can adjust charge current, we'd like to understand what our setup issue is so that we can maintain a stable VSYS voltage.

Thanks,
Mitchell

  • 0
    TI__Guru 57541 points

    Mitchell,

    1. BQ25895 is a 5-A charger. It can handle up to 5A total current, i.e. the sum of the charging current and the system current. If there's no SYS load, the maximum charge current can be 5A if the input power allows.

    2. The max input current limit setting is 3.25A. The input can't supply 5A input current.

    3. Please read all register values especially the charger status and fault registers. The charger should enter DPM indicated by REG13[7:6]. Please visit 8.2.6.2 Dynamic Power Management (DPM) on the d/s for the details.

    4. Please visit https://e2e.ti.com/support/power-management/f/196/t/874557 for BQ2589X: Schematic Review and PCB Layout Design Tips and https://e2e.ti.com/support/power-management/f/196/t/875386 for BQ2589x/BQ25898x FAQs. FAQ#3 should help with your questions.

    Thanks,

     

    Ning.