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BQ25756: Device operation in Standalone Mode

Andy Grier
Prodigy 20 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25750, BQ25856-Q1

Tool/software:

The device datasheet states that the BQ25756 can be operated as a 'Standalone' device.

This logically implies that the device can be configured and then run in either charge mode (SW1 to SW2 power transfer direction), or in discharge mode (Reverse Mode SW2 to SW1 power transfer direction), or both automatically without an I2C connection out to a microcontroller.

Datasheet section 8.3.2 states that the device can be powered up from the battery side (using SRN as the supply input line) and '..the converter is ready to start operation in reverse mode'.

Datasheet section 8.3.6 states that Reverse mode operation can be enabled if 4 conditions are valid, one of which is that the EN_REV bit is set to 1 in register REG0x19. 

One of the device FAQ questions asks:  Can I enable EN_REV=1 using only hardware?  To which the answer is:  This is not currently possible.  So, to programme the EN_REV bit you need to fire up the I2C interface.  This could be done on initial board setup.  However, there is another question in the FAQ section that reads;  Will the BQ2575X IC remember the register when the IC is powered off?  The answer to the question is No - the register bit values will be lost if the device is powered off.

This is very confusing.  You need a micro or PC on the end of the I2C to programme the chip for Reverse Mode, but Reverse mode is autonomous if the device is powered off a battery.

My questions are these:

1)  Do I need to programme the EN_REV bit at all? 

2)  If I have an enternal supply connected, then the device will charge the connected battery if the /CE input is asserted (logic 0).  If the input power fails, then the device will automatically re-power up via SRN using battery power and go straight into Reverse mode and allow discharge of the battery.  Is ths correct?

3)  If the /CE pin remains asserted during input power loss, will this prevent Reverse Mode from firing up?

4)  If a fault is detected during Reverse mode that would negate EN_REV (e.g. Temp sensor outside limits, section 8.3.4.7.2), Reverse Mode would be halted.  If the temperature then came back within limits, would Reverse mode automatically re-start, bearing in mind that EN_REV has been cleared?

Hope all the above makes sense.

  • 0
    TI__Mastermind 31035 points

    Hello Andy,

    Thanks for working on this.

    1. No, you do not need to program the EN_REV bit if you are not using reverse mode.

    2. The device will not go into reverse mode without an I2C command.

    3. The /CE pin being HIGH will not prevent reverse mode.

    4. No, reverse mode will not automatically come back.

    If you need reverse mode to activate automatically, I recommend using the auto reverse mode feature of the BQ25750 or the BQ25856-Q1. You'll need an I2C command to set EN_AUTO_REV=1, but the IC will automatically activate reverse mode when the VAC power fails. In the case of the BQ25856-Q1, you'll also need an I2C command to get the device out of reverse mode when the VAC power is good again.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 20 points

    Good Morning Ethan,

    Thanks for you swift response.

    We will be using Reverse Mode, so presumably we do need to programme the EN_REV bit.  However based on the answer to the Power Off question, the bit state is not stored in the device when unpowered so would have to be re-programmed to a 1 each time the unit powers up, which we would not be able to do as we are using the device in standalone mode - no micro attached to the I2C bus.

    Your answer to my Q2 seems to be in conflict with the datasheet section 8.3.2 statement.

    My Q3 was about the /CE remaining asserted (logic 0) during input power loss (our /CE control logic is powered from our battery, so always powered).  If /CE is logic 0 would this prevent Reverse mode?

    Q4 - understood.

    Overall, the datasheet appears to be seriously inconsistent.  The device is NOT a standalone bidirectional device if it needs an I2C command to enable the Reverse mode.  My I suggest raising an Erratum to capture and clarify these inconsistencies?

    Regards

    Andy

  • 0 in reply to Andy Grier
    TI__Mastermind 31035 points

    Hello Andy,

    Thanks for your comments. I'll get to your questions later this week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 31035 points

    Hello Andy,

    Thanks for being patient with this.

    Andy Grier said:
    If /CE is logic 0 would this prevent Reverse mode?

    The /CE pin being 0 does not prevent reverse mode.

    Andy Grier said:
    My I suggest raising an Erratum to capture and clarify these inconsistencies?

    I'll talk to the datasheet writers about clarifying this. The BQ25750 and BQ25756 do charge batteries in standalone. More in-depth features, like MPPT or reverse mode, require a microcontroller to set. The registers of the BQ25750 or BQ25756 lose their value when the IC loses power. There's also not an external way to control and regulate the reverse mode voltage with resistors for the IC.

    Also, just for reference, we have an FAQ for the BQ2575X family.

    Let me know if you have questions about this.

    Best Regards,
    Ethan Galloway