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BQ76940: BQ76940

user5247366
Prodigy 250 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ76930, TIDA-00792, BQ76200, BQ78350, BQ76920

Dear All: 

I am using the BQ76940 to design the 13S BMS ,can you answer the  following questions？

1:The doument of " bq76930 and bq76940 Evaluation Module  User's Guide"  ,Figure 33 ,What's the role of  C4、C5、E1、E2、D4、D5？

2：The doument of " bq76930 and bq76940 Evaluation Module  User's Guide"  ,Figure 35，What's the role of R60、R61、R62、R79？

thanks for your reply!

  • 0
    TI__Genius 10830 points
    Hi ,
    Your post has been assigned to one of our BMS experts.

    Regards,
    David
  • 0 in reply to David Hien
    TI__Guru 54715 points
    Hi User,
    C4 and C5 are capacitors to provide a low impedance path past the FETs for ESD currents. C6 and C7 are similar capacitors to provide a low impedance path between PACK+ and PACK-.
    E2 is spark gap to shunt ESD current between PACK+ and PACK-. E1 is a spark gap across the FETs, typically this is undesired since it is a single point fault which can bypass the FETs
    D4、D5 are fast recovery diodes used for flyback or freewheel diodes to clamp the PACK- voltage to PACK+ with inductive response. They are alternate footprints and current ratings.

    R60, R61, R62 and R79 are current shunt resistors to allow calculation of current from a voltage measurement during operation for evaluation. These were provided for a consistent resistance value for easier calculation. With the final selection of the Rf resistors (R57-59) as 1k the math is easier on those.
  • 0 in reply to WM5295
    Prodigy 250 points
    Hi WM:
    What is the maximum output current of the pin CHG and DHG of BQ76940 ? I use it to drive the mos，wonder if driving power is enough?
    3Ks!
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    The bq76940 has approximately 5k internal pull up resistors to the regulated FET on output voltage. It is not shown in the data sheet but could be calculated from the rise time. When in regulation this will allow about 2.2 mA maximum drive current.
  • 0 in reply to WM5295
    Prodigy 250 points
    Hi WM:
    1: BQ76940 from SHIP mode to normal mode, the device must follow the standard boot sequence by
    applying a voltage greater than the VBOOT threshold on the TS1 pin, external circuit how to realize ? (Except the way the switch is connected. )
    2: REGSRC pin, Can use an external power source to power it,instead of BAT?
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    1. See the boot switch alternatives ap note in the bq76940 product folder technical documents tab. See also the TIDA-00792 for an example and test data with high side protection FETs.
    2. You can use an external regulator to provide REGSRC. That regulator would still normally come from the battery voltage. Without REGSRC you can not operate the part correctly.
  • 0 in reply to WM5295
    Prodigy 250 points
    Hi WM:
    1：If bq76940 is SHIP mode , how to wake up it when external load is inserted or the charge is inserted or the communication line is accessed ? Do you have a similar circuit reference? Can the main microcontroller have no power when bq76940 is SHIP mode?
    2: TIDA-00792 refer to "Sleep and Wake-up Circuit", refer to" After some time with the battery off,PACK+ approaches PACK– potential", why?
    3: Systems which have higher charge currents or parallel FETs may require a driver. Do you have a recommended driver?
    4: If use low-Side switching,Do I need to be isolated from external and mcu communications to bq76940 ?
    Thanks for your reply!
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    1. We do not have a reference circuit which provides all those functions. Typically a circuit must be powered to function and experience some change to detect a system condition change. If it biased PACK+ to some known level it could detect if it moved up or down.
    2. The real voltage will depend on leakages. If the esd capacitors were to leak or the FETs were to leak they may counteract the leakage of the PACK- and DSG pins to GND which do not have a specification but will tend to pull toward VSS. If you are monitoring the pin the instrument will pull down the net. If you want a defined voltage when off you may need to add a circuit to define the voltage when the BMS is off.
    3. We do not have a recommended driver. Various techniques common to FETs can be applied. With the bq76200 the device function is low current specific function for battery protection FET control, it may not support all circuit topologies.
    For a low side driver various circuits could be used. If an external driver is used for the charge FET realize it must float below VSS of the bq76940, so design appropriately.
    4. It depends on your system, but typically you will need some isolation either physically, electrically to cut off the interface such as is shown in the bq78350 application note, or electrically to operate through an isolated communication interface.
  • 0 in reply to WM5295
    Prodigy 250 points
    Hi WM:
    1:If a piece of 76940 and a piece of mcu make up a systerm, When charge and discharge current is not detected , 76940 and mcu are they all SHIP model? If ok, When the load is attached ,how to wake up?
    2:The bq76940 refer to"Short circuit in discharge delay setting. A 400-μs setting is recommended only in systems using maximum cell measurement input resistance, Rc, of 1 kΩ (which corresponds to minimum internal cell balancing current or external cell balancing configuration),why is 400us ？a shorter time is ok? Can you explain it in detail?
    3: 76940 has "Load Detection" function , to do this, need to turn on ADC model?
    Thanks for your reply!
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    1. You need to determine how to wake the system and design an appropriate circuit.
    2. The filter capacitors have a charge, when shorted the cells will pull down through the filter resistors. See section 3 of the ap note www.ti.com/.../slua749a.pdf
    3. The ADC does not need to run. The load detection comparator is enabled when CHG is off.
  • 0 in reply to WM5295
    Prodigy 250 points
    Hi WM:
    1: when 76940 is SHIP mode ,is the load detection useful?
    2: if 76940 is SHIP mode , mcu is wake up , How to detect external loads ?
    Thanks for your reply!
  • 0 in reply to WM5295
    Prodigy 250 points
    Hi WM:
    1: If 76940 is SHIP mode, can it start load detection mode?
    2: In the document of "bq76920, bq76930, bq76940 AFE FAQ", refer to " Will load detection work with high-side FET configurations?
    No. The load detection is sensed through the CHG pin. With typical high-side FET applications, there is no path to pull up the CHG pin for the load detection circuit to operate. If a load detection is desired, TI suggests that the microcontroller read the pack voltage." the microcontroller read the pack voltage, Why do you say that? How to design a circuit?
    3：If the communication ground connects the battery ground, do not connects the PACK-,is it OK? if OK, do you need isolation?
    Thanks for your reply!
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    1. No in ship mode all power is off except the boot detector.
    2. Use a level shifter to an ADC input or comparator input of the MCU. Use any protection or signal condition needed for the input, use a switch to sample the voltage if needed to reduce current.
    3. If the communication signals are referenced to the BAT- and it comes out of the pack, some industries may allow this, others will not. You may need something to either break the communication signals during fault, isolate the signals, or use high side protection switching so that the communication signal reference is not lost during fault.