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BQ25756: i2c communication issue

adam czerwiec
Prodigy 25 points
Part Number: BQ25756
Other Parts Discussed in Thread: TPS25751, TPS26750

Tool/software:

Hello everyone,

We're currently working on our engineering thesis project involving the TPS25751 and BQ25756 components. Project is a smart battery charger with bidirectional usb pd power up to 100W. We carefully followed the datasheets for our schematics and PCB layout, and we're using an EEPROM for storing the boot configuration required by the TPS25751.

The TPS25751 is functioning properly and successfully communicates via I2C with our ESP32 microcontroller. However, we have encountered significant difficulties with the BQ25756 component. Despite ensuring the device is powered correctly, it remains unresponsive to all I2C commands.

We've tested various configurations, especially regarding the CE pin settings, and have followed numerous troubleshooting steps outlined in the datasheet, but the issue persists. Currently, our TPS25751 can only negotiate 5V output, as the BQ25756 does not respond or provide the necessary voltage feedback.

Given the complexity and critical nature of this issue for our thesis, we'd be extremely grateful if anyone here could share their experience, resources, or offer advice that might help us resolve this issue.

Thank you very much for your help!

schemat.pdf7711.PCB.pdf

  • 0
    TI__Mastermind 31975 points

    Hello Adam,

    Thanks for working on this. I'm the apps engineer for the BQ25756.

    I'm looking at your schematic and I see that you are using GaN FETs for the switch FETs. The BQ25756 can't work with GaN FETs. The gate leakage is too high and the BQ25756 doesn't have any protection for the GaN FETs.

    You'll also need diodes for BTST pins. The BQ25756 doesn't have integrated BTST diodes.

    Just for reference, here's the FAQ page for the BQ2575X part family and here's the schematic and layout checklist for the BQ25756.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Thank you for your prompt response. In addition to replacing the transistors and incorporating BTST diodes, are there any other design aspects that we should address? Our project is due at the end of April, so we would be very grateful for any advice regarding schematics or components or PCB layout.

  • 0 in reply to adam czerwiec
    TI__Mastermind 31975 points

    Hello Adam,

    I have a few more suggestions:

    • I would recommend installing spots for an RC filter in parallel with ICHG and ILIM_HIZ. The BQ25756 needs an RC filter here when the ILIM_HIZ or ICHG resistors are greater than 5kΩ.

    • Make sure DRV_SUP has 4.7µF cap close to the pin

    I have a few suggestions for the layout:

    • Make sure HIDRV, SW, and LODRV traces are at least 20mils thick.
    • Route HIDRV and SW next to each other. SW is the return trace for HIDRV. Route LODRV next to GND. GND is the return for LODRV.
    • Put a few vias underneath the IC pad to provide better electrical and thermal conduction to the bottom layer.
    • Try to route as much as you can on top so the GND plane on the bottom can be bigger. You might want to go to a 4 layer board to make routing easier.
    • Make sure the SW pads are large to help conduct heat. You may want to go to a 300kHz switching frequency to also help reduce heat.

    Also, we have a reference design using the BQ25756 and the TPS25751 that may be helpful.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Hello Ethan,

    I have reviewed your suggestions and made the following changes to the schematic and PCB: I have added BTST diodes, changed the GaN FETs to MOSFETs, corrected the trace width, and added thermal vias.
    Please note that the PCB was already a 4-layer design, with middle layers designated as GND planes.

    I would greatly appreciate your review of the updated schematic before proceeding with the order for the PCB and components.


    Best regards,
    Adam CzerwiecPCB+SCHEMATIC.PDF

  • 0 in reply to adam czerwiec
    TI__Mastermind 31975 points

    Hello Adam,

    Thanks for being patient with this. I'll review this schematic / layout early next week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Thanks! I'm looking forward to your review.

  • 0 in reply to adam czerwiec
    TI__Mastermind 31975 points

    Hello Adam,

    Once again, thanks for being patient with this. The schematic looks good. Can you send me the part number for the inductor? I want to check that the inductor will work with the circuit.

    Here are my suggestions for layout:

    • I recommend moving the ACN/ACP and SRN/SRP filter circuits closer to the IC.
    • What are these half moon shapes in the circuits? I haven't seen these before. They should be fine for the circuit.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Hello Ethan,

    Thank you for your swift response.I will ensure that the current sense filters are positioned in as close proximity to the IC as is feasible.

    The inductor used was: https://www.mouser.pl/ProductDetail/710-7443641000B I acknowledge that this may be considered excessive, however, the original intention was for the circuit to be based on the TPS26750 and to support 240W Power Delivery, thus the inductor selection. Should this inductor be incompatible with the circuit, I would be most grateful if you could recommend an alternative.

    The half-moon-shaped components are solder bridges, which are either defaultly connected or disconnected.

    Best regards,

    Adam Czerwiec

  • +1 in reply to adam czerwiec
    TI__Mastermind 31975 points

    Hello Adam,

    Thanks for letting me know about the solder bridges.

    The inductor has too low of a DCR to work with the BQ25756. The DCR needs to be between 1.75mΩ and 60mΩ. Would an inductor like the IHLP6767GZER100M01 work for you circuit?

    You've probably already found this, but we have a 100W USB-PD reference design that may help: www.ti.com/.../PMP41062

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Thank you so much! I'll be sure to let you know when the PCB arrives and if everything works.

    Best regards,

    Adam Czerwiec