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BQ40Z50-R2: Low power idle mode

Victor Hansen
Prodigy 80 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ25713, TPS65988

Tool/software:

Hi, I am designing a powerbank (4s2p) that must support OTG and I have chosen the BQ40Z50-R2 as the BMS IC. I wanted to implement a form of low power mode as I can see the input voltage can go as low as 2 V. Therefore, instead of powering the BMS with the entire battery stack and consume more power, I wanted to use either a LDO or low power switch mode to lower the input supply voltage to e.g. 3.3V. However, as I understand the datasheet, the primary power supply input pin BAT and secondary power supply input pin VCC must be within 2.2 V of each other. This would mean that both VCC and BAT should be supplied from the output of either the LDO or switch mode. Assuming I can get the switching circuitry to work this way, is there any issues with this approach I am not aware in this text?

See the drawing for a visualization of what I want to achieve, and please ask if anything is uncertain, it was made slightly on-the-fly.

As a side question, even though I know it might be considered bad style, could an alternative to the above approach be to route the output power of the low power circuitry to the PBI pin?

Thanks in advance.

Kind regards, Victor

  • 0
    TI__Mastermind 28480 points

    Hi Victor,

    I believe adding components to the power path of the device could present issues to the gauging functionality of the device. Please allow us to confirm this.

    Regards,

    Anthony

  • 0 in reply to Anthony
    Prodigy 80 points

    Hi Anthony.

    No problem, looking forward to your response Slight smile

    Kind regards,
    Victor

  • 0 in reply to Victor Hansen
    TI__Mastermind 28480 points

    Hi Victor,

    After getting consultation, we do not recommend doing this since it would effect the top of stack voltage that the gauge would be expecting at this time. Can you please go more into depth about the goal of this type of setup to see what we can do?

  • 0 in reply to Anthony
    Prodigy 80 points

    Hello Anthony and apologies for the late reply.

    Thank you very much for investigating my inquiry.

    The goal of this setup is to have a 3s3p Li-ion battery application including the BQ40Z50-R2, BQ25713 and TPS65988 configured as DRP for a 100Wh battery pack. One of the requirements of the battery pack must be that it can be stored for an extended period up to 9 months and still be recoverable that is, not discharged by the system to a degree where the BMS will not allow charging. While in storage I want the battery to operate in sleep mode, until lowest cell voltage is below 3V where I will go into shutdown.

    Therefore, my idea with using a LDO / low power switch mode was to have the battery enter a low power scenario where BMS and TPS65988 could be powered from the LDO, BQ25713 completely off. The reason for supplying the TPS65988 is to be responsive to sink applications being connected to the pack. Source applications is not a problem as they can power the IC over VBUS. Furthermore, I plan to use the TPS65988 as I2C master in my battery system.
    The LDO could bring the voltage down to 3.3V which is needed for the TPS65988. Furthermore, having access to 5V would allow me to lower the power consumption of the BMS from 75uA @ nominal 10.8V to 75 uA @ 3.3V, which would be quite a reduction in the power consumption.

    But I see your point with how the BMS gauge will be affected by having an LDO connected in the way I showed above. Is it then possible for me to move the LDO to the system side of the BMS, for supplying the TPS65988, and still have the BMS in sleep mode?
    I have made a simplified circuit sketch below for illustration, where the grayed out parts of the circuit are unused for the specified mode. Let me know if anything is unclear.

    Thanks in advance

    Kind regards,
    Victor

  • +1 in reply to Victor Hansen
    TI__Mastermind 28480 points

    Hi Victor,

    Below are the conditions for the gauge to exit sleep mode. As long as the current does not break the sleep current threshold or any of the other conditions aren't met, then I believe device would still stay in sleep at this time.

    However, I believe this could be an issue when the gauge enters shutdown. The exit condition to leave shutdown is based on the voltage seen on the pack pin, so if this threshold is lower than the 3.3V, the gauge would not be able to be in shutdown at this time.

    Regards,

    Anthony