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BQ25798: Vsys resetting even after same register value

Pratham M
Prodigy 40 points
Part Number: BQ25798


Tool/software:

Hi there, 

We are using the BQ25798 and need higher voltage at Vsys for system and battery both.

According to our configuration, which uses a single-cell battery, we have set the registers through the PROG pin, and we correctly get 3.7 V at VSYS, as expected.

Since our entire system depends on VSYS, we need a higher voltage when VBUS is connected. To achieve this, I programmed the REG00_Minimal_System_Voltage register with values corresponding to 9 V (0x1A) and 12 V (0x63). The VSYS voltage does rise to the desired value, but only for a few seconds before dropping back to 3.7 V.

Interestingly, when I read back REG00, it still shows the value I set for 9 V. I have also disabled the watchdog timer, but the issue persists.

Could you please help us understand what might be causing this behavior? Is there something we might have missed?
This issue is critical for our design.

Best Regards,

Pratham

  • 0
    TI__Guru**** 167726 points

    Hi Pratham,

    The MINSYS voltage cannot be set higher than VREG.  MINSYS is only intended to provide a minimum SYS voltage until the battery charges above that voltage.  Then SYS follows the battery voltage up. You will need to add a boost converter following VREG.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi  ,

    Thanks for the prompt response,

    Please check the image from the datasheet. It mentions that VSYSMIN can be configured to any value. Could you please explain what that statement means? Does it apply under specific conditions?

    We observed that when we set the VSYSMIN value, the VSYS voltage changes accordingly to the set value, but it resets after some time. We want to understand why it resets — if the condition is not met, then ideally, it should not be set, right?

  • +1 in reply to Pratham M
    TI__Guru**** 167726 points

    HI Pratham,

    That statement needs to be clarified to say any value less than VREG.  All of our NVDC chargers require that MINSYS (VSYSMIN) be set lower than VREG in order for the battery to terminate correctly.  Operation with MINSYS>VREG has not be tested and is not supported.

    Regards,

    Jeff