BQ2040: Current fluctuations with the BQ2040.

Hello,

This question has been assigned within the team and will be reviewed and followed up with a application engineer when possible. In the meantime please attach any associated .log/.gg files associated with the projects

Thank you,
Alan 

Is this on your own design or on a TI EVM? Please make sure that the voltage on SR is stable (check for noise on this pin).

Hi Dominik, thanks for getting back!

I monitored the noise on the BQ2040 SR pin (pin 9), using a Fluke 287 DMM.  I set it to Peak min / max mode and observed it for about 10 minutes.  The max reading was 25.47mV, the min was 24.67mV, with an average of 25.18mV.  So, the peak to peak delta was 800uV.   (However, the fluctuations appear transient.  I would think the filtering would pretty much eliminate small spikes.)  Anyway, with approximately 1A flowing, the 800uV translates to about 37mA of fluctuation.  But what is really breaking our test process, is the larger fluctuations in the BQ current readings.  During this test, the BQ current went from exactly 913mA to 996mA and back, seemingly at random intervals.

I’m not sure what to try next.

Any ideas?

The process inside the bq2040 is fairly simple: It integrates the voltage on SRP-SRN over a short period of time and delivers this as an integer conversion result to the FW, which then converts this to mA units.

If you see a jump, then there's either something wrong with the voltage on SRP-SRN or the gauge itself is damaged. About the voltage on SRP-SRN: There's a chance of RF interference that you may not be able to measure easily. We've seen excessive current errors when placing gauges close to RF sources (e.g. "walkie-talkie tests"). I'd try to shield the gauge in a test to check if that's the problem.

Hi Dominik.  I placed the entire assembly inside a silver ESD bag and held it closed while the test ran.  My intent was to create a Faraday Cage.  Unfortunately, there was no change in behavior.

Between the two of us, it seems like we’ve covered everything but the layout of the board.  I will have a closer look at “best practices” as they apply to this PCB layout.

Thank you very much for your input Dominik!

Darren W.

How many units that show this behavior do you have? If it's only one then it could be a damaged gauge.

Hi Dominik, I hope your day is going well. 

Turns out all units are showing this behavior.  A senior engineer inspected the layout.  It looks like the guidelines re placing Rsns as close as possible to the SR input (pin 9) were not followed.  The sense resistor is about 24mm away from pin 9, and the filter components are on the opposite side of the board, connecting through a via.

Could this be the cause of the BQ current fluctuations we are seeing?

And if so, is there a way to verify that this is the issue??

One way to verify this is to short out SRN/SRP right at the gauge and check if the current is stably close to 0mA.

I shorted Vss (Pin 8) to SR (Pin 9).  Yes, the current is very stable at 0mA.

This points to some noise on your sense lines - which gets shorted out with this test. While it's not 100% proof, it shows that the CC ADC can convert a 0V input with stable results.