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BQ24295 - Circuit Verification & Initial Startup

Jagdish Gundu
Intellectual 390 points
Other Parts Discussed in Thread: BQ24295

Hello,

I have attached my schematics, I would like to verify the correctness of schematics and also i have below following question.. I would need some help in understanding few basic things. thx

1. I am using 2X 3.7V@5000mA Li-ion Batteries (both Batteries are in Parallel)

2. After i power up the circuit without batteries, what should be my voltage reading at BAT1/2 Pins

3. Can use this circuit without any I2C commands as standalone or does someone need to initiate some I2C commands to start the charging for very first time.

4. As i am using 5000mA capacity batteries is R8=330 is enough limit to charge the batteries?

Regards,JG

  • 0 in reply to Jagdish Gundu
    TI__Mastermind 25800 points
    Hello Jagdish,

    Firstly, in regards to your schematic, I have a couple of comments and questions:

    1) It seems like you are not using the D+/D- pins. We recommend shorting those two pins together so that the charger uses the ILIM pin setting for maximum input current.

    2) You may want to decrease R11 from 24k unless your have using an NTC resistor. Are you using the TS function? If not, R11 is too high, and your TS voltage will disable charging for the device.

    Secondly, in regards to your questions:

    2) When you power up the charger without a battery and only VBUS, you will see a fluctuating BAT pin voltage due to the decoupling caps on BAT. They will charge and discharge similar to a battery, but much faster.

    3) You will not need I2C if you are comfortable with the default register and charger settings . These are settings like charge current, termination current, DPM, etc. are adjusted through I2C.

    4) R8 will define what the input current limit of the charger will be (if you short D+/D-). This does not determine the charge current. In standalone mode, the default charge current is 1A. Now, if your input current limit is too low, the charge current will be reduced if the input limit is ever reached (this is called IINDPM). If your input current limit is too high, AND your adapter cannot support the current demanded by the charger, the charger will go into VINDPM, where adapter is being overloaded and the current is causing the current to drop. The charger places the device in VINDPM to limit this effect and prevent the adapter from crashing/failing.

    To determine the best R8, what is your expected input supply? USB or dedicated charging adapter?

    Just a final comment about your charge current; you may want to consider I2C interfacing to max out the charge current. Two 5000mAh batteries in parallel means 10000mAh battery. The max charge current for the bq24295 is 3A (3000mA), effectively 0.3C charge rate in your battery's eyes. This is the best case scenario if you want to charge the batteries in a reasonable amount of time. Just keep in mind this also means you need an adapter that can support the input current required for 3A charging.


    Hope this helps.
    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Intellectual 390 points

    Thanks Joel,

    Yes, I am not using D+ & D- and i have shorted both PINs to remove the Input current limit or the Charging current Limit.

    I have also replaced R11=10K, R10=5.1K which eventually removed the battery from being suspended mode (no charging). Now I am able to see the charging functionality, however when Plug DC adapter and charge the battery both in Parallel... I am measuring 4.71V on PMID Pin which should Ideally 5.1V or 5.0V. During this check, i have also removed DC Adapter to check the PMID voltage which shows 5.1V

    I am suspecting Internal RBFET is not turning ON. Is it?

    Please let me know is something i am missing.

    Regards,

    JG

  • 0 in reply to Jagdish Gundu
    TI__Mastermind 25800 points

    Hey JG,

    To answer your question, I need some more information.

    What is the rating of the adapter/power supply you are applying at VBUS? (i.e. what is its current limit)

    If the current limit is much lower than the charger's programmed current limit (either from the register setting or the ILIM pin resistance), you could be overloading the power supply. When that happens, the charger's VINDPM setting (default of 4.76V), instead of the IINDPM setting (same as input current limit setting) would decrease the charge current to prevent the charger from crashing below 4.76V.

    The RBFET would still be on. Any load on PMID and VSYS would be applied to the battery.

    So I suggest checking the rating/current limit of the supply

    I also suggest reading the I2C register settings to see what the charger sees. 

    Last comment, because you have pulled OTG high, the device will default to OTG mode when you remove VBUS.

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Intellectual 390 points

    Thanks Joel for the reply.

    I am using well rated adapter >3A. My problem is only when i plug adapter, i see only 4.7V but when i remove adapter/dc bus .. i can see Vboost as 5.01V. I am having trouble in writing I2C registers information, however i can read registers.

    Any other issue that you can predict with the circuit which i posted. Appreciate your support. thx

    JG

  • 0 in reply to Jagdish Gundu
    TI__Mastermind 25800 points

    Hey JG,

    If you can read the registers, and reply with them here, that would help in investigating the cause of what your're seeing.

    The only other question I have while I look into this is if you have a load at PMID's output when you plug in the adapter?

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Intellectual 390 points

    Hello Joel,

    Thanks for the reply, it has been while since i replied. I have made my final circuit and was testing it further as per application requirement.

    I have two strange behaviors, which i thought would share with you to know more about it.

    1. When i remove my DC adapter from the charger board, the change over time is taking around appx 30ms to switch over from DC to Battery boost mode, which is cause my application circuit to restart. I have attached the scope trace to explain about this.  

    2.Registers of Battery charger which were set (i.e. 0x00,0x05,0x09)are getting changed, their functionality defines the 0x00(source voltage and  Current) ,0x05(Charge Timer and termination) and 0x09( Fault registers) in  Battery Mode.Unable to set or write the registers while applications is running.

    Above circuit which is attached is the final release of what we have.

    Regards,

    JG

  • 0 in reply to Jagdish Gundu
    TI__Mastermind 25800 points

    Hey Jagdish,

    In reference to question one, this 30ms delay is spec'd in our datasheet. This is normal operation of the charger. The internal logic, once VBUS is removed, typically takes this amount of time to start the boost converter. Please refer to Section 8.3.1.3 Boost Mode Operation from Battery from the datasheet..

    In reference to question two, you said this was under Battery Only mode. What did you set the registers to before, and what did they read after? What did you change/remove when the changed occurred.

    Regards,

    Joel H 

  • 0 in reply to Joel Hernandez II
    Intellectual 390 points
    Thanks Joel,

    Can a bulk capacitor at output side should help atleast the dip down to maintain >2.5V?. My problem is the load connect to this board is getting restarted and causing user annoyed.

    Regards,
    JG
  • 0 in reply to Jagdish Gundu
    TI__Mastermind 25800 points
    Hey JG,

    It may depend on your load current at moment. How much current would you be pulling from PMID? Please be aware that you should be using

    You must consider the Capacitance value which will equal ( Load Current x 30ms ) / delta V that can maintain your system (5V - SystemMin).

    If this capacitance is too large (i.e. a little over a an order of magnitude larger than the PMID cap), you could cause issues with the converter regulation.


    Regards,
    Joel H