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BQ25890: Current Pulsing during charging

Adam Bierman
Intellectual 710 points
Part Number: BQ25890

Tool/software:

I have a BQ25890 that I'm using in a portable application and I'm seeing abnormal current waveforms during charging. The current is pulsing, almost like the charge keeps being turned on and off, I'm also seeing this with VBUS disconnected. I have a very light load, nothing that would be pulling current in this fashion. I was hoping someone could help me understand why I'd be seeing waveforms like this?

With VBUS Connected (5V): 

Without VBUS Connected: 

Here's my schematic: 

  • 0
    Intellectual 710 points

    I wanted to note that I've completed a charge cycle, my STAT pin stays low, but is noisy, and the STAT pin releases when charging is complete. So it looks as if it's functioning the way it's supposed to, but it also looks like it keeps turning on and off rapidly based on that waveform, I don't see that change in magnitude in the voltage.  Just to clarify the oscilloscope is showing the waveform off a current probe, so that's the current into the battery that's being shown.

  • 0
    TI__Expert 7920 points

    Hello Adam,

    Please help to answer a few questions about your system:

    Adam Bierman said:
    I have a very light load, nothing that would be pulling current in this fashion.

    It would be helpful to know more about the load. Is the load pulsing? If so, how frequently is it pulsing? How much current is your load pulling?

    You mentioned that you observe this behavior whether or not VBUS is connected; I would not expect to see current flowing into the battery without VBUS present.

    Are you using boost mode? If not, I would recommend debugging without VBUS present, since you are observing this behavior with the converter off.

    Best regards,
    Alec

  • 0 in reply to Alec Lehman
    Intellectual 710 points

    Hi Alex, 

    Thanks for reaching out. There's no pulsing load on this device. There are two LT3154 DCDC regulators on the SYS output, one at 5V and the other at 3.8V. Downstream of the 5V it feeds a 3.3V and 1.8V regulator. Normally there'd be a raspberry pi cm3+ on the 5V, 3.3, and 1.8V loads, and a cellular modem on 3.8V. However for the purposes of debuggin I've removed the CM3+ and the cellular modem. There's almost nothing on the lines now except a couple of quiescent ICs. This is the current waveform from the battery in this case 

  • 0 in reply to Adam Bierman
    TI__Expert 7920 points

    Hi Adam,

    Thanks for providing more details about your system.

    It seems that the purple signal in your scope capture is battery current. Can you please clarify what each signal is?

    Adam Bierman said:
    There are two LT3154 DCDC regulators on the SYS output, one at 5V and the other at 3.8V.

    I see that these are buck-boost converters. If those have a light load (i.e. PFM switching), then it is very likely that they are creating a pulsed load. I recommend that you disconnect all downstream components from SYS, including the DC/DC converters, and check if the battery current becomes stable. These buck-boost converters operating with a light load are most likely the cause of the current pulsing that you are observing.

    Best regards,

    Alec