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BQ25190: Dips on SYS when external power removed

Sara Glick1
Prodigy 170 points

Part Number: BQ25190

I'm using the BQ25190 in a battery-powered system. I've seen on some devices that when I remove external power, the SYS rail will dip for 20ms and then recover. See scope shot for example. This causes the LDO rail (2V8) to also dip and the system processor to reset. There is a 10uF Murata GRM155R61A106ME11 cap on pin D5. I tried increasing the D5 capacitance value to 30uF, but it didn't make a difference. What could cause this behavior on some devices but not others? 

 SYS 30uF0.png

  • 0
    TI__Genius 12935 points

    Hi Sara,

    Thank you for reaching out.

    This is strange behavior. Supplement mode should allow the transition from input power to battery power to happen very quickly. It looks like something is disconnecting the battery from the device when this happens. It's most likely the BATFET but it could also be a battery protector IC.

    I'd be curious to see what VBAT looks like during this event. Would you be able to share another capture with VBAT included?

    Also, how repeatable is this issue? Is it more repeatable on a specific board? How many boards out of how many are showing this?

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 170 points

    Hi Alec,

    I am working on getting the requested scope shot with VBAT.

    In the meantime, I'm wondering if this issue could be caused by the PCM short circuit protection (83mA) tripping when an inrush current spike out of the battery (16mAh) occurs when the BATFET is turned on. The issue is repeatable on about 50% of my devices, so perhaps the current spike value is right on the edge of the PCM limit, and is tripping on devices that have passives at max or min tolerance. 

    To test this theory, I want to adjust the capacitance on VBAT, SYS (pin D5), and LSLDO1. Is there a max capacitance value recommended for the VBAT pin? 

    Thanks, Sara 

  • 0 in reply to Sara Glick1
    TI__Genius 12935 points

    Hi Sara,

    That's a good thought. Is the battery always fully charged when this happens (charging disabled=BATFET off)? From the capture you shared, it does look like the battery is around 4.2V. Does this behavior still occur if VIN is removed while the battery is charging?

    It sounds like this could be what's happening: When VIN is connected and charging is complete, the BATFET is off. Then if VIN is removed, the BATFET turns on and there is an inrush current spike out of the battery that trips the PCM short circuit protection.

    As you've mentioned, a larger BAT pin capacitor should help. We don't have a max recommended capacitance value for the BAT pin. You can try larger values to see what helps.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 170 points

    Hi Alec, 

    I see the same issue (on the failing devices) while the battery is still charging in CC mode. 

    Increasing the BAT pin capacitance to 22uF seems to resolve the issue. Are there any concerns with increasing the BAT capacitance that much? Will it interfere with any BQ25190 functionality? 

    Best, Sara

  • +1 in reply to Sara Glick1
    TI__Genius 12935 points

    Hi Sara,

    Thanks for the update. I'm glad to hear that increasing the capacitance helped. I don't have any concerns with using 22uF on BAT.

    The battery is already a large source of charge. However, the battery has internal resistance, and there is additional resistance and inductance between the battery terminals and the BAT pin of the IC due to the battery connector and PCB traces. These parasitics can limit how fast the battery responds to transients, so the purpose of the BAT pin capacitor is essentially to provide transient current close to the IC.

    Best regards,

    Alec