BQ76905: Charger detect functionality

Part Number: BQ76905

Tool/software:

Hello Experts ,

I am going through the external charger detect and wake-up  functionality of BQ76905.

Is this functionality can be used for wake-up from shipping (Shut down) mode  ? During this mode, both charge and discharge MOSFET's are OFF and we have isolated ground from charger.

One more point is e is if we use a level shifter for charge and discharge gate drive to increase the gate driver strength then also charger detect circuit will work ?

Explanation will help us.

thanks

Regards

Vasu

  • Hello Vasu,

    The part does not have a charger detect functionality by default, although circuits can be implemented for wake-up of the device when a charger is attached.

    There is an app note for an older family generation that shows some circuits that could implement a charger wake-up:

    The BQ76905 would be similar, but you have to ensure that the TS pin does not go above its abs max values. Either that or you could use the VC0 pin for wake-up. Here are examples on how you could wake up with VC0 or TS using a button:

    Using this and the circuits of the application note, you should be able to implement a charger wake-up function.

    Regarding trying to enhance the gate drive, why are you doing this? The MOSFET turn-off should be fast enough for most applications. You also do not want to make it too fast, otherwise you may have large inductive transients.

    Best Regards,

    Luis Hernandez Salomon

  • Hello Luis,

    Thanks for update ,  My question is based on below topic in datasheet.  External charger detect through Gate source resistor (high value )  connected to CHG pin of IC.

    Can you please check on this topic ?   We need to detect the charger when BDS is off condition and wake the BQ76905 on charger connect condition.

    To answer to more gate drive strength  : We have used MOSFET with higher gate capacitance  (22000pF) used 2 in parallel  , which needs higher drive current and faster turn off during faults .

    Thanks for application note , How this circuit can bw used for charger detect if we are not using push button, and low side FET is off i.e Ground open condition .

    Thanks regards

    Vasu

  • Hello Vasu,

    This is a detection feature that detects if there's a high voltage on the CHG pin. It could be used to detect loads/charger with some considerations, but it would still not wake the device.

    If you want to wake the device from SHUTDOWN you have to use external circuitry to apply a pulse to TS or VC0. The application note I shared shows circuits that could be used to apply a pulse when a charger is attached and PACK- is disconnected from BAT-/Vss due to protection FEs. This same circuitry could be used to apply a pulse voltage to the BQ76905 device. See Figure 4. Pack Terminal Boot Circuit and Figure 5. Pack Boot Switch From REGSRC.

    So instead of a push-button you'd use a circuit as shown in that app note.

    For FET drive, 22-nF should be more than okay with our device, turn-off is still very fast during faults with our drivers and we have tested the drivers with up-to 6 FETs in parallel for both CHG and DSG (For 12 FETs total). Fall time for DSG with 6 MOSFETs in parallel using a ~83-Ohm resistor on the DSG pin was ~25-us with our FET driver. Each MOSFET here had a 9.25-nF of gate capacitance.

    Additionally, it is very easy to enhance the turn-off speed by using a simple turn-off loop circuit using a BJT and resistor, this we also have tested. With 6 FETs in parallel, the turn-off speed of both CHG/DSG was ~16-us.

    If you have any questions on that, please let me know! 

    Best Regards,

    Luis Hernandez Salomon

  • Hello Luis

    Thanks for explanations, It really helps us.

    You mentioned that you have test results for driving 12 FET (6 parallel path )  using BQ76905 , Can you please share the test results for the same ?

    Thanks

    Regards

    Vasu

  • Hello Vasu,

    These will eventually be put in an application note that I will be writing. To make it easier for you, here is the waveform capture of this test:

    The above waveform shows the capture for when there is a ~90-Ohm resistance on the CHG/DSG pins and 12 FETs total (6 CHG/6 DSG).

    Similarly, here is the waveform of the turn-off when using a BJT local turn-off circuit for both CHG and DSG in the same conditions:

    Hope these helped and let me know if you have any other questions Slight smile.

    Best Regards,

    Luis Hernandez Salomon

  • Hello Luis,

    Thanks for clarifications , 

    It is really support me .

    I want to have one to one discussion with you.  Please let me know how to reach you directly .

    Thanks

    regards

    Vasu

  • Hello Vasu,

    Glad to help!

    By any chance are you in contact with any person from TI local to you? They should be able to help set-up a meeting to have a more in-depth discussion!

    Is there anything in particular that you would like to talk during the discussion Slight smile?

    Best Regards,

    Luis Hernandez Salomon

  • Hello Luis,

    Thanks for reply, I am in discussion with my Local TI contacts  for commercial.

    One to one mail discussion is also ok for me to have some clarifications .

    thanks

    Regards

    Vasu

  • Hello Vasu,

    I will reach out by e-mail, I will also loop in the field team that should be able to provide support locally as well Slight smile.

    Closing this thread.

    Best Regards,

    Luis Hernandez Salomon

  • Thanks for support.

    Agree to close this thread.