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[FAQ] BQ25756: What do I need to know about reverse mode or source mode for the BQ2575X family of parts?

Part Number: BQ25756
Other Parts Discussed in Thread: BQ25750

There are 4 registers for the BQ2575X family that control reverse mode, EN_REV, VAC_REV, IAC_REV, and SYSREV_UV. For the BQ25750 specifically, these registers are EN_REV, VSYS_REV, IAC_REV, SYSREV_UV, and EN_AUTO_REV.

Here's a brief description of each of these registers:

EN_REV will turn reverse mode ON and OFF as long as the conditions are met.
VAC_REV is 5V by default. VAC_REV can be set from 3.3V to 65V through I2C commands.
IAC_REV will limit the amount of current the charger will give in reverse mode.
SYSREV_UV = 0 will turn off reverse mode when the reverse voltage drops below 80%. SYSREV_UV = 1 will turn off reverse mode when the reverse voltage drops below 3.3V.

For the BQ25750, VSYS_REV is the equivalent of VAC_REV and EN_AUTO_REV will enable reverse mode automatically when VBAT is less than the VSYS_REV register.

Also, for a USB application, reverse mode might be referred to as source mode by a USB controller.

Can I charge a battery in reverse mode? If so, how does it work?
Yes, the BQ2575X family can do this. The BQ2575X family of parts are capable of CC and CV mode in reverse mode. For the register settings, you'll need to set IOUT_REG for the charge current and VAC_REV to the battery charge voltage. You'll also need to set VSYS_REV=0 for when the battery is lower than 80% of the charge voltage.