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Original question:

BQ27Z561: Not responding over I2C (sometimes).

BQ27Z561: Resolution to I2C issue

mark watson
Prodigy 40 points
Part Number: BQ27Z561

This is a response to my original post about the fuel gauge becoming unresponsive over I2C: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1037566/bq27z561-not-responding-over-i2c-sometimes. My post was locked by TI, so I am posting our solution here to try and help future people who may be experiencing this issue.

After reviewing the datasheet and schematic extensively we couldn’t find any real issues, until extensive googling found this: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/983346/bq27z561-i2c-communication-issue

Here, the customer reports an I2C communication lockup similar to ours, depending on the state of VBAT. Although worded incorrectly, another user stated that the CE line must be 75% of VBAT to enable the device. The datasheet implies that only 1.15V is required, so driving it from a 2V IO expander should have been fine:

The simplest fix is to tie CE to VBAT. A board that was consistently locking up was modified in this way and no longer exhibited the problem.

  • 0
    TI__Expert 8315 points

    Hi Mark, I will test this behavior in the lab and update the datasheet accordingly. Thanks for highlighting this issue.

  • 0 in reply to Jackson DiGiovanna
    Prodigy 40 points

    Hi Jackson. Any update? You should find there is a relationship between the voltage on the CE pin and Vbat. The statement in the datasheet that the minimum voltage on CE is 1.15 V is not true (at least according to our findings). We were trying to enable the fuel gauge using an output pin from a two volt I/O expander (PCAL6524HEAZ). The fuel gauge was powered by a 4.2 V Lithium-ion battery. The fuel gauge comms over I2C were inconsistent until we tied CE to Vbat. I hope that makes sense and helps someone who might experience a similar issue.

  • +1 in reply to mark watson
    TI__Expert 8315 points

    Hi Mark,

    After testing this in the lab I can confirm that 1.15V is not sufficient as the minimum voltage on the CE pin. However, the statement that CE must be 75% of VBAT is correct as I was able to power the device until CE < 1.35V. Once power is lost however, it took near 1.8 V in order to "revive" the device.

    This is a typo in the datasheet so I will go in and make the change so the info is correct.

    Thanks again for bringing this up!

    Best,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 40 points

    Thank you very much