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BQ25731EVM: VBAT voltage will be pull down

Part Number: BQ25731EVM
Other Parts Discussed in Thread: BQ25730EVM, BQ25731, BQ25730

Dear Teams,

Customer now is evaluating our EVM, and they found following problems list here. Schematic diagram show below as well.

 

  1. With Vin power supply, and the battery is not connected, they found the VBAT voltage will pull down to 4V.
  2. After they start up the battery, they load is working properly when it with Vin and appropriate configuration. Once they remove the battery, VBAT voltage will be pull down by the load

Do you have any suggestions?

Thanks!

Dylan

  • Hi Dylan,

    Thanks for reaching out.

    First of all I want to point out that is not the schematic of BQ25731EVM and it is instead the schematic of BQ25730EVM.

    For the BQ25731EVM, there is no Q5 and VBAT is pretty much VSYS (there is only a sense resistor in between). The BQ25731 is not a BATFET power path part and the behavior is very different. If they are trying to correlate the results from the BQ25731 EVM with the BQ25730 EVM schematic, it will not make sense. The easiest thing to do is to check whether Q5 is installed on their EVM.

    In the absence of battery, the charger will try to regulate to the value set in the Charge Voltage register (0x05/0x04) and that is why they see 4.2 V at the BAT pin. 

    In case they are confused by the registers 0x0C/0x0D (minimum system voltage) in the bq studio GUI for bq2573x, they need to ignore it because they are not used in BQ25731 (as shown in the datasheet, they are not there in the register list).

    I would recommend redoing their intended tests and look at the correct schematic/datasheet. There is no MOSFET between VBAT and VSYS and therefore the behavior is different.

    Thanks,

    Peng

    *If my answer solves your question, please press "This Resolved My Issue" on the thread. 




  • Hi Zhu,

    The test waveforms of the last reply are all tested with a good board.
    The following figure shows the SW output waveform after damage:

    The damages tested here are basically damaged when the button is pressed. 
    The probability is very high and it is not recoverable.
    The picture below is the one-key switch circuit we currently use.
    Please help locate the possible reasons and improve them. Measures, thank you!


    Thanks!
    Dylan
  • Hi Dylan,

    What are the issues? I never see any detailed problem descriptions from you.

    This is what you mentioned in your original post:

    1. With Vin power supply, and the battery is not connected, they found the VBAT voltage will pull down to 4V.

                Peng - I answered it in my original reply. VBAT will go to 4.2 V

            2. After they start up the battery, they load is working properly when it with Vin and appropriate configuration. Once they remove the           battery, VBAT voltage will be pull down by the load

                Peng - What do you mean by VBAT pulled down by load? If you mean it gets pulled to 4.2 V, then my answer is the same as my last reply.


    In your original screenshot:

     
    Peng - Once again, this is the wrong schematic. You cannot load VBAT without loading VSYS as well for BQ25731.


    Your reply just now:

    The test waveforms of the last reply are all tested with a good board.

    Peng - There were no test waveforms or detailed problem descriptions available in the original post.


    Please let me know what the concerns for the customer is. I need to what:

    1. test conditions (input voltage, output voltage, output power). What are they testing?
    2. What do they expect to see.
    3. What are they actually seeing.
    4. Is that an extra circuit they added to the EVM? What are they trying to achieve with the circuit?
    5. What IC are they using? BQ25731 switches at 400k/800k and it is quite strange to see it goes to 1.5 Mhz. 1.5 MHZ is a very common fsw for many other ICs.

    Thanks. Feel free to email me if necessary.

    Peng



  • Hi Peng,

    Sorry for I replying to wrong post.

    Thanks!

    Dylan

  • Hi Dylan,

    Yes. No problem.

    Thanks,

    Peng

    *If my answer solves your question, please press "This Resolved My Issue" on the thread.