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BQ27Z746: What conditions must be true for CHG and DSG to be enabled?

Robert Poor1
Prodigy 180 points
Part Number: BQ27Z746
Other Parts Discussed in Thread: BQSTUDIO

This is a follow on from https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1122472/simply-enable-both-chg-and-dsg-fets/4165037#4165037

We are using the BQ27Z746 as part of a battery backup system.  The basic question is "how do we ensure the CHG and DSG FETs are enabled when the system requires it?"

Here's a crude block diagram to describe the system:

Under normal circumstances, Power Over Ethernet is available to power an RPi4.  When power is available, it keeps the LiION battery charged as well.

However, when POE fails, the system needs to run the RPi4 from the LiION battery.  

There is a bootstrapping issue: If the RPi is completely powered down, it won't be able to provide I2C commands to configure the BQ27Z746, so the BQ27Z746 must be "self sufficient".  In particular, if either of the FETs are switched off, the RPi will not get any power from the battery.  So we need to be able to guarantee "under normal circumstances" that the FETs will be enabled.

The issue I'm having is figuring out all of the conditions that make it possible for the BQ27Z746 to wake up with the FETs enabled.

  • 0
    Prodigy 180 points

    Update:  I should have mentioned that, although the chip is mounted on our custom PCB, I have brought out the I2C interface and have it communicating with BQStudio, so it should be easy to examine and modify any registers in the device as needed.

  • 0 in reply to Robert Poor1
    Prodigy 180 points

    Here's a snapshot of the current state of the registers.  (Pardon the squinty eye chart -- if there's a better way to convey the info, please let me know!)  My 'scope tells me that at least one of the FETs is off -- perhaps because the chip is in SLEEP mode?

  • 0 in reply to Robert Poor1
    TI__Guru 61006 points

    The bq27z746 is self-sufficient in the sense that once it powers up and if it was configured in the past, it will run without the need for configuration. The FETs are controlled by a complex rule set (see TRM https://www.ti.com/lit/ug/sluuca6/sluuca6.pdf, Chapter 4) which are too many to discuss in general on E2E.

    Your register screen shot shows that both FETs are off (XCHG and XDSG are red). 

    Please check the setting for FET Options (4.3.13). The gauge will not in all cases disable FETs in sleep. You can control behavior with this register. What are your settings?