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BQ25731: BQ25731 VBUS voltage spike upon switching startup

Part Number: BQ25731


I am looking for some direction in diagnosing a custom PCB using the BQ25731.

The issue I am having is upon switching startup to charge the attached battery the input voltage spikes drastically above the supply voltage for a significant amount of time. See the below VBUS trace (Yellow) with respect to the Q4 MOSFET (Blue).

VBUS spikes from 5V to 9.4V over 5ms. I have seen spikes up to 23V from a 5V input from multiple sources.

My hardware configuration:

  • The BQ25731 is setup in its 400kHz using a 4.7uH inductor and all of the recommended compensation component values.
  • The BQ25731 has a 14.6V nominal 4S battery that it is charging.
  • I have 134uF of effective capacitance on the VBUS side of the BQ25731 accounting for DC bias of the caps I have chosen.
  • I have 156uF of effective capacitance on the VBAT side of the BQ25731 also accounting for the DC bias of the chosen caps.

My register/firmware configuration

  • All POR default settings.

I seem to get the max voltage spike of 23V consistently when switching starts by toggling the CHRG_INHIBIT register with the 5V already plugged in.

I can provide schematics/design files if we can move this to a non public thread.