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BQ25790: Vsys dip periodically when battery is absent

Howard Zou
Genius 12365 points
Part Number: BQ25790

My customer is using BQ25790 to charge 2cell battery from 5V input.

When the battery is absent, we could see the system voltage is as below, it dips from 7V to 3V, period is 15ms, and the load on SYS pin is about 300mA. 

We haven't changed the registers.

Then we cut all the load on SYS pin, and we get the waveform below: the period is still 15ms, but Vsys rises up. What may cause the issue?

  • 0
    TI__Guru*** 143950 points

    Hi Howard,

    The SYS and BAT overshoot without a battery and minimum recommended capacitors while charge enabled is expected.  The SYS or BAT hits OVP and the converter turns off.  To prevent:

    1. disable charge when no battery is attached.

    2. add more capacitance on BAT pin

    Regards,

    Jeff

     

  • 0 in reply to Jeff F
    TI__Genius 12365 points

    Jeff,

    Yes when we disable charge the problem is solved.

    And the interval is around 1.5s not 15ms as I said before. 15ms refers to the overshoot pulse width

    But my question is:

    why SYSOVP is triggered every around 1.5s?

    In the datasheet, it says SYSOVP will force the converter to stop switching, then why the SYS pin can still be stable at around 8V most of the time?

  • 0 in reply to Howard Zou
    TI__Genius 12365 points

    Besides,

    the SYS is powering the processor with a buck.

    If SYS will dip to 3V every 1.5s, then the processor will shutdown every time SYS dips.

    When the customer is loading program to processor, the loading duration will be longer than 1.5s, and it will cause the processor load fail.

    Since at the time there is no software running, it's not possible to disable charge with software.

    Then we can only pull up CE pin to disable charge.

    I wonder are there any way to minimize the dip, for example, we can accept that SYS decrease from 7V to 6.5V, not by disable the charge.

  • 0 in reply to Howard Zou
    TI__Guru*** 143950 points

    Howard,

    The SYS and BAT OVP are relative to the current system regulation and battery regulation voltages, respectively.  So, lowering the regulation point does not help.  Adding more capacitance is the recommended solution. 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 12365 points

    Jeff,

    Thank you.

    Could you please give some analysis for below question? since we see the same phenomenon on our EVM:

    why SYSOVP is triggered every around 1.5s?

    In the datasheet, it says SYSOVP will force the converter to stop switching, then why the SYS pin can still be stable at around 8V most of the time? My guess is SYSOVP is triggered, switching is stopped, then it restart(how long time later will it restart?), why this time SYSOVP is not triggered and then SYS stay at around 8V?

    Why 1.5s later it's triggered again?

  • 0 in reply to Howard Zou
    TI__Guru*** 143950 points

    Howard,

    When charging is enabled, after the battery voltage falls below BATUVLO (~2.5), the charger attempts to regulate the BATP pin to a fixed voltage (5V for 2S battery) for ~1.5 s to allow the pack protector FETs time to close.  After that time, the charger resumes normal charging.  In your plots, because there is no battery and only the minimum capacitance at BAT, the battery voltage overshoots after the 1.5 s, causing SYS to overshoot, hitting both OVP thesholds and turning off the BATFET and the converter.  If your customer cannot add the additional capacitance to BAT, please recommend that they use the /CE pin to disable charge when no battery is attached.

    Regards,

    Jeff