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BQ25756: TPS25751D with BQ25756 – Power Path & Supercap Balancing Questions

Mohammad Kireany
Prodigy 30 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25750

Tool/software:

Dear TI Support,

I am designing a USB Power Delivery (PD) application as a 100 W sink, powered from a USB Type-C PD adapter (controlled by the TPS25751D) and charging a 7-series supercapacitor bank (2.7 V each) using the BQ25756.

In my design:

  • The TPS25751D negotiates 20 V/5 A from the PD source.

  • The BQ25756 is used to charge the 7S supercap bank.

  • The system load (board) should normally be powered from the USB-C PD adapter, but must also operate from the supercaps for a few seconds if the input power is suddenly removed.

I selected TPS25751D + BQ25756 because this pairing is fully supported together in the USB PD Application Customization Tool (GUI Composer).

However, I have two concerns and questions:

  1. Power-Path Management
    Since the BQ25756 does not have integrated direct power path, what is the recommended approach for switching the system load between adapter power and the supercap bank?

    • Do you have a reference design showing this kind of power-path control with BQ25756?

    • If not, can you recommend a switching circuit (e.g., ideal diode controllers, power multiplexers, or FET-based load sharing) that TI recommends for this use case?

  2. Supercapacitor Balancing
    For the 7-series supercap bank, do you recommend using a balancing IC to maintain voltage distribution across the caps?

    • If so, can you suggest a TI balancing IC that would best complement the BQ25756 charger in this design?

    • Ideally, such an IC could also simplify integration with the charger.

I would greatly appreciate your guidance or reference material for implementing TPS25751D + BQ25756 in this configuration.

Thank you in advance for your support.

Best regards,
Mohammad Kireany

  • 0
    TI__Expert 6755 points

    Hi Mohammad,

    1. There are a couple of approaches that might work, but the best/easiest approach is using an ideal diode controller. Are you looking for back-to-back NFETs here or does a single FET work?
    2. I can loop in our battery monitor/balancer team to provide a recommendation to help with this question. They will have a better answer than I can provide.

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    Prodigy 30 points

    Dear Michael,

    Thank you for your reply. To clarify my design intent:

    • I am looking to implement back-to-back NFETs for power-path control using an ideal diode controller.

    • My system must be powered primarily from the USB-C PD adapter (20 V / 5 A negotiated via TPS25751D), and then switch seamlessly to a 7-series supercapacitor bank (charged by BQ25756) for a few seconds of backup operation if the adapter is suddenly removed.

    Could you please let me know:

    1. If TI has a reference design, application note, or EVM example that shows this type of adapter + backup source power multipath using back-to-back NFETs with LM74xxx ideal-diode controllers (or a recommended alternative)?

    2. If not, could you point me to the closest available schematic or reference material I can adapt for this configuration?

    This would greatly help me finalize the power-path implementation with TPS25751D + BQ25756 in my design.

    Thank you again for your support.

    Best regards,
    Mohammad Kireany

  • 0 in reply to Mohammad Kireany
    TI__Expert 6755 points

    Hi Mohammad,

    I think the LM74xxx ideal diode controller will work for your application. Based on the requirement, I think you will need 2 ideal diode controllers. One to block the battery voltage from reaching the VAC pin and one for separating the supercapacitor from the system voltage.

    1. I am not aware of any reference design, application note, or EVM that shows this system requirement.
    2. The closest material that we have would be the power path on the BQ25750. I don't think that the power path on this device will be able to perform the function required by the system, but the ideal diode controller should be able to replace the power path FETs.

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    Prodigy 30 points

    ear Michael,

    Thank you for clarifying the role of the LM74xxx ideal diode controllers.

    I would also like to ask about the BQ25750. Initially, I was considering this device instead of the BQ25756, because it has an integrated direct power path function. My thought was that this could help reduce BOM count and simplify the design, since the power-path control would be built in rather than implemented with external controllers.

    Could you please clarify why you don’t recommend the BQ25750 for my case? Is it a limitation with how its power-path block operates with a supercapacitor bank (instead of a battery), or is there another system-level reason that makes the BQ25756 + LM74xxx approach more suitable?

    Understanding this trade-off would help me decide whether I should continue with the BQ25756 + LM controllers, or re-evaluate the BQ25750 to simplify my design.

    Best regards,
    Mohammad Kireany

  • 0 in reply to Mohammad Kireany
    TI__Expert 6755 points

    Hi Mohammad,

    The BQ25750 could be possible, but there may be some difficulty with the inrush current through the BATFETs when using the power path of the device. Another way to use it would be to use EN_AUTO_REV which can be good for uninterruptable power supply. It's not quite the power path functionality that you have described.

    The BQ25750 has a current protection limit for the power path switch-over. The limit protects the FETs and the IC from large inrush currents during the power path switch over. We have an FAQ that may help with this.

    EN_AUTO_REV on the BQ25750 will transfer power back through the converter to the system load automatically which stops the power path switchover. This could be useful, but like I mentioned, it may not be exactly the operation mode of the supercapacitor backup that you had in mind.

    Also, there might be an answer on this thread that may be helpful to your solution as well:

    Best regards,
    Michael Bradbourne