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BQ25713B: Power supply management for a smart speaker

Samir Yazane
Prodigy 80 points
Part Number: BQ25713B
Other Parts Discussed in Thread: BQ25713, BQ77915, PMP40441

Dear colleagues,

I am working on a smart speaker project where we need to use battery+BMS powered by USB-C PD.

We identified the BQ25713 which seems a good circuit for such application. Combined with 3S battery @ approx 4Ah.

Use case 1: speaker is connected to USB supply. The product will need 20V output to operate correctly.

Question1: How do we configure USB-PD charger to send 20V instead of 5V ? ideally with a stand-alone circuit instead of a copy-pasted obscure chunk of soft on a µC.

Question2: Is it OK to connect the system directly to VBUS, instead of VSYS ? Is it OK to not use VSYS or does it needs some load ?

Use case 2: The battery is charged and the user disconnect the charger. The speaker needs to stay ON during the transition.

Question3:  Is OTG fast enough to maintain VBUS at 20V and keep device ON ? Is there a minimum capacity value needed ?

Use case 3: Speaker runs on battery. Because the speaker circuits needs 20V and 3S Li-ion is only 3*3.7V = 11V, we need to boost it up. This is why we want to connect the system to VBUS. 

Question4: Correct me if i am wrong but in battery mode, VSYS can only go as high as battery voltage, right ?

Question5: Overall, is such a concept a good usage of the BQ25713 circuit or not ? 

 

Use case 4: Speaker runs on battery and user decides to connect back the charger.

Question6: Is it safe for both the charger and BQ25713 or do wee need to put a PMOS switch in between ? 

Question7: Also, in PMP40441 ref design (buck-boost USB-PD charger) we see a BQ77915 circuit for cell protection. Do you recommend using it or we can use BQ25713 only ?

Thank you guys in advance for your help.

Best regards,

Samir

  • 0
    TI__Mastermind 25800 points
    Hey Samir,

    Use Case 1 -
    Question 1: For USB PD, you will need some extra PD controller to arbitrate the 20V VBUS rail. On the charger side, if you need the system to be powered from that 20V input (assuming you have gone through the PD controller/firmware to handshake that voltage), you would likely need to connect the system directly to the input of the charger and not use the regulated SYS pin of the charger. This SYS pin will only operate up to 19.5V in the buck-boost operation. The followup leads to question 2.

    Question 2: Yes, you can connect your system directly to VBUS but you won't be able to use the input current regulation feature of the charger (namely IINDPM that reduces charge current in order to keep the input current from surpassing a threshold, usually give to you by the PD port and programmed separately into the charger.

    Use Case 2-
    Question 3: This typically does not work unless your load was very light. It takes the OTG approximately 15ms to start up from the input removal. So your output capacitors (in forward mode, these are the input capacitors) must sustain the system load for 15ms, which typically translates to much too high capacitance, especially in a portable speaker application.

    Use Case 3-
    Question 4: You are correct, which is why you were looking at connecting the system directly to the input.

    Question 5: The concept is one that customers have brought up to us before but the OTG turn on limitation of the charger makes it difficult for them to achieve, unless your system can guarantee a very light load when the input is removed. In your application, this could mean much lower volume. I imagine the 20V rail is to drive the amplifier.

    Use Case 4-
    Question 6: What do you mean by the "charger"? The BQ25713 would ideally be your charger, and this already has an external PMOS between the system rail and the battery rail. Are you talking about the PD input source?

    Question 7: I cannot speak to the protection device added in that reference design as this post focuses on the charging controller. But in general, we will always recommend a protection circuit as a primary protection for unprotection Li-ion cells. The BQ25713 should only be considered as a secondary protection to a device like that.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 80 points
    Hi Joel,

    First thank you for your quick response, it is much appreciated !

    To continue point by point:

    Q1: Such circuit must already exist, can you recommend a ref design to handle this handshake for 20V supply ?
    Q2: I guess we will have to skip this feature and rely on the external USB charger.

    Note: let's use "charger" to refers to BQ25713 circuit, and "external charger" for the AC/DC wall connected USB-PD supply.

    Q3: The amp will drive a speaker around 30W, so 1.5A (max). Do-able ?

    Q6: Yes, the PD input source, the external charger. I am not sure about the conflict between BQ25713 and external charger when the user connected it back. I want to be sure that the charger recognize the external charger and:
    A) stop using battery to power the amp
    B) start charging the battery

    Thank you !

    Best regards,
    Samir
  • 0 in reply to Samir Yazane
    TI__Mastermind 25800 points
    Hey Samir,

    For Q1: I don't have a particular reference circuit in mind that will perform the USB PD handshaking, but you can reach out the USB Interface e2e forum to get their feedback on what controller and reference design you can start with.

    You may want to take a look at these two reference designs showcasing the downstream port, but I do not believe they implement a PD controller.
    www.ti.com/.../PMP40312
    www.ti.com/.../TIDA-01523

    For Q3: This will not be do-able for the OTG delay time. The capacitance you would need would be very very large and unreasonable.

    For Q6: If you are operating off of OTG mode from the battery, (assuming you can handle the system brown out condition from Question 3), and you suddenly apply an input, you will likely need some external circuit and load switch as you mentioned to isolate your amplifier rail from the input and also disable OTG mode. It will also not automatically start charging the battery. This register must be programmed.


    Regards,
    Joel H