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BQ24616 Oscillation Problem

John Teel76
Prodigy 35 points
Other Parts Discussed in Thread: BQ24616

We're having an oscillation issue with the BQ24616.  What makes this oscillation unusual is that it doesn't happen every cycle.  We can cycle (charge and discharge) the charger multiple times with no problem, but then eventually one cycle will show an oscillation, and then the next cycle may be back to normal with no oscillation.

The primary issue is that this oscillation seems to be causing the charge cycle to terminate prematurely.  When a charge cycle shows this oscillation, the charge voltage oscillates while in the current regulation phase, and then the charge current oscillates once in the voltage regulation phase.  This doesn't make sense to me and I would think a stability issue would cause the charge current to oscillate when in the current regulation phase, and the charge voltage to oscillate when in the voltage regulation phase.

We're using Lo=6.8uH, and Co=20uF as recommended in the datasheet.

One thing we have a bit different is the value of the feedforward capacitor placed across the top resistor in the feedback resistor divider.  In the datasheet the R2=500k, R1=100k, and Cff=22pF, so 500k || 100k = 83.3k giving an RC of 83.3k x 22pF = 1.83e-6.  Whereas we have R2=165k, R1=24k, and Cff=22pF giving an RC of 20.95k x 22pF = 0.46e-6.

How critical is the RC time constant set by the feedforward cap?  To match the RC in the datasheet we need to increase Cff by 4x.  Any chance this difference could cause such an oscillation?

Attached is an image showing this oscillation and early charge termination.  This is charge cycle #365 and the previous 364 charge cycles had no issue.  The charge termination current is set to 152mA, but you can see the charge was terminated at about 400mA.

Any ideas?  This issue is preventing us from releasing the product to market so I would really appreciate a prompt reply.

Thanks,

John Teel (former TI'er)

Predictable Designs

  • 0
    TI__Expert 8690 points
    Hi John,

    This may be an issue with the placement of power stage components and routing of sensing lines on the PCB. In some designs, the Cff is not placed at all and causes no issues like this. With a scope, you should take a look into the HIDRV, LODRV, PH, BTST nodes for any excessive ringing, as well as VREF, VFB, ACP, ACN, SRP, SRN for any noise coupling from the power stage.

    I imagine you have already reviewed your schematic with our EVM user guide as well. If not, take a look. Can you send me the schematic?

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Prodigy 35 points
    Hi Steven, thanks for the helping us out. Is there a way for me to email you directly the schematic? Can I also send you the PCB layout (pdf or gerber) to take a look at?

    I believe the power components are placed correctly. However, the SRP/SRN and ACN/ACP sense lines via between multiple layers which I know is less than ideal. Also the input and output capacitors don't connect to their own ground before connecting to the chip ground as recommended in the datasheet. Do you think either of these could cause this problem?

    What makes the issue especially puzzling is it doesn't happen on all boards, and it doesn't happen on most charge/discharge cycles.

    Thanks,
    John
  • 0 in reply to John Teel76
    TI__Expert 8690 points
    John,

    Probably because the caps are not grounded right, you are getting varied responses based on the fab and assembly of your boards. I will send a message to you.

    Regards,
    Steven