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BQ76930: Noise on top cell reading

Erik Anderson41
Intellectual 925 points
Part Number: BQ76930


Hi,

I'm noticing an 8mV or so spike in cell reading 6 (top cell) periodically that I can't seem to figure out.  The clue I have is that I'm powering a PIC microprocessor off of the 3.3V regulator output and the problem reduces from 8mV spike to 2mV spike when I stop putting it into sleep periodically.  Can you see any reason why the current draw on the REGOUT would make an error on the top cell reading?  I've confirmed this spike in cell voltage with a DMM across cell 6.

Can you check my schematic again?  Note the microcontroller is not shown but is connected to the 3.3V rail in another schematic page. 

You can see that I'm using a diode into REGIN instead of a voltage follower transistor.  Perhaps the diode-based regulator has bad PSRR compared to the transistor circuit?

Thanks!

Erik

BatteryMonitor.pdf

Thanks,

Erik

  • 0
    TI__Guru* 81035 points

    Hi Erik,

    Normally I would expect the large filter circuit on BAT and VC5X to prevent the spikes you describe. I think your circuit looks okay and it follows the example for a 6 cell system outlined in the BQ769x0 Family Top 10 Design Considerations app note. Do you have the filter components placed close to the device? Is there a bypass cap on the VCC of the MCU?

    The transistor circuit on REGIN may improve performance if REGOUT is causing the voltage on the top cell to fluctuate, but hopefully there is a way to avoid making that change to your board.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 925 points

    Turns out it was my ammeter at the top of the pack which was creating a bit of IR drop. This is why pulling less current (microcontroller in sleep) made the issue almost resolve itself. The top cell reading is actually tricky because it has the capability of reading some overall pack current if the wire/connection resistances are not low enough.

    Anyways, thanks for the reply Matt!

    Erik