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BQ76920: Designing BMS

Shibin Varghese
Expert 1130 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ77915, BQ78350-R1, BQ78350, BQ2947, BQ7718, BQ7716, BQ77908A, BQ76200

Hi Team,
I am looking for a 12V BMS(3*4.2V=12.6V) for a Li-ion battery pack.
Hence I came across BQ77915. It is stand alone.
But since we cannot store the Li-ion battery for a long time being fully charged, we are looking for another solution for the same and thus got the chip 
BQ76920 with I2C interface.
After reading the datasheet, I've some doubts and I hope TI can help me with it.
1) External cell balancing is not recommended for this chip. But I believe it will be very helpful in my application. Would someone be able to help me with a reference design with external FET for balancing for a 3 cell battery pack?. Or can I just follow the design procedure in fig 9.3 on page number 22 of the datasheet of BQ77915 and also considering table 24 on page 41 of the BQ76920s datasheet. 
2) Output current at REGOUT pin.
3) As per the table of recommended operating conditions the maximum voltage across a cell {VCn-V(Cn-1)} is 5V but my battery voltage is 4.2V maximum. Can I program this voltage through I2C?.
4) What is the need for BQ78350DBT IC?. or can I use any other controller to control the BQ76920 chip through I2C?. Can you suggest to me?.
5) I believe I can use the pin of the controller instead of the push button. 
6) Is the controller connected to the REGOUT pin is power during the ship mode?.
7) I'm planning to use BQ24610RGER as the charger IC, I hope there won't be any problem with this.

  • 0
    TI__Guru 54715 points

    Hi Shibin,

    1. Yes, external balancing can be used with the BQ76920.  Since the BQ76920 can take higher internal balance currents than the other family members it is not expected to be used frequently, but is possible.  The application note https://www.ti.com/lit/pdf/slua810 may be helpful. When reducing the cell count of a circuit like figure 5, use the connections and capacitor placement for VC1 as recommended in the data sheet. 

    2. REGOUT can support up to 20 mA, see the specification of VEXTLDO_DC in the data sheet.  With higher currents a separate filter for REGSRC may be desired.

    3. Yes, you can enter an OV voltage threshold through I2C.  The part can operate with up to a 5V cell voltage, but it is not required.  See the data range for limits of the threshold.

    4. The BQ76920 does not operate standalone, a controller is needed.  BQ78350-R1 (or other versions) are a controller for the the part with gauging included.  You can use your own MCU.  The MCU must do temperature limits, turn on FETs, recover from voltage faults and provide a balancing algorithm.

    5. The application note https://www.ti.com/lit/pdf/slua769 may be interesting.   A pushbutton is not needed, but you will need some way to wake the part.

    6. I'm not sure I understand.  There are 2 common ways to design a system, one to power the controller from the REGOUT (as is done with BQ78350 versions).  The BQ76920 is booted and the controller runs until it shuts down the system.   The other way is to use a separate power supply for the controller which then decides when to wake the BQ76920 and when to shut it down, but the controller remains in its operational or sleep mode as needed to monitor system interfaces.  Select the design method which meets your needs.

    7. No issue is expected.  If communicating outside the battery to the controller be sure to provide a suitable interface if using low side protection.

  • 0 in reply to WM5295
    Expert 1130 points

    Hi,

    Thank you for your reply.

    Sorry to bother you again and again.

    During ship mode, or in POR almost all of the blocks are disabled. Does this include external LDO too. If yes, how can I send wakeup commands to the chip since the controller is powered from the REGOUT LDO output of the chip.

    Can you suggest me a secondary protection IC for the same circuit.

  • 0 in reply to Shibin Varghese
    TI__Guru 54715 points

    Hi Shibin,

    Yes, the LDO is disabled during ship mode.  If the MCU is powered by the BQ76920 send a boot pulse to TS1 to wake the BQ76920 which will power the controller.  If the voltage of the cell stack is below VPORA the BQ76920 will not boot. 

    If the MCU is powered by a separate regulator it can wake the BQ76920, the same voltage limits apply for the BQ76920.  This can take more standby current, and the MCU must decide what to do if it wakes up but the BQ76920 has too low of a voltage to boot to provide cell voltages.

    The BQ7718, BQ7716 and BQ2947 are secondary protector families which may be suitable.  Features, present thresholds, and packages vary between the families.

  • 0 in reply to WM5295
    Expert 1130 points

    Hi,

    Can you close one of the thread.

    If I got it right, the TS1 is an interrupt to wake up the whole system.

    The whole system I'm planning works from this power source so an external regulator to power the mcu is beyond our concern.

    Also, if the boot pin is toggled using the mcu, that too won't be a solution. I think the better way is to not use the ship mode. Will it effect the performance or lifetime of the device.

  • 0 in reply to Shibin Varghese
    TI__Guru 54715 points

    Hi Shibin,

    From the other thread, "control signal for the external LDO from the VSTUP/POR block"  Yes boot is shown as an input to the block and starts the regulator.

    See above for secondary protector suggestions.

    Yes, TS1 is the boot signal for the simple system with REGOUT powering a controller.  You could think of it as an interrupt for a shut down system.  The part does not boot from I2C.

    You are correct, if REGOUT powers the MCU the MCU can't provide the boot signal.  Some systems will find it attractive to use a switching regulator to power the MCU since it is more efficient than the linear regulation of REGOUT and would leave the MCU always on.  Then the MCU can shutdown and wake the BQ76920 as desired.  It takes more circuitry, but is a system consideration.

    Leaving the REGOUT on (not using ship mode) does not change the lifetime of the device. Normal operating mode with the LDO on does consume more current than ship mode even if the CC, ADC, and FETs are turned off.

  • 0 in reply to WM5295
    Expert 1130 points

    Hi Team,
    Thank you for your reply.
    Your answer is very helpful. 
    3.85-4.6V is the overvoltage range of all secondary protection chips you've provided. 
    But my battery is 3.2V. Can you suggest to me a chip with over-voltage around 3.2V. 
    It will be very helpful if it is a leaded package.
    The only chip I've seen is bq77908A which does the same function as BQ76920 Chip.
    Is there any other solution other than a separate power supply?.(I think it will increase the BoM cost)
    If no, Can you suggest to me a switching power supply to power the MCU?.
    The input voltage will be in the range of 12.8V.

  • 0 in reply to Shibin Varghese
    TI__Guru 54715 points

    Hi Shibin,

    Check your cell data sheet or with the supplier, many 3.2V cells charge at a higher voltage, 3.6V in some cases. Since there are no OV protectors defined at around 3.7V it may be that this function is provided by some other method in industry or no one has had a sufficient quantity need to develop a configuration at 3.85V which might be suitable.

    The BQ7790521 has a 3.7V OV threshold, it is a much more complex part than just an OV protector.   It is a protector, so there is no data output, and has no regulator output to support  a MCU.

    The BQ77908A is also a protector with no data output.  It is user programmable instead of factory programmable.  It does have the regulator output for use by other circuitry.  It does not have the same function as BQ76920 which provides voltage and current readouts and protection thresholds are register based and must be programmed by a MCU.

    If the concern is how to boot the BQ76920, see the application note https://www.ti.com/lit/pdf/slua769 for circuit ideas.  Design something appropriate for your system.

    For DC-DC converters there are many choices, see selections starting at http://www.ti.com/dcdc 

  • 0 in reply to WM5295
    Expert 1130 points

    Hi Team,
    Sorry for replying late.
    The charge limited voltage of the battery is 3.65V. So I think none of the above will work in my case.

  • 0 in reply to Shibin Varghese
    Expert 1130 points

    Hi Team,
    I just completed reading the datasheet of  BQ77908A and BQ7790521.
    Since the overvoltage threshold of BQ7790521 is 3.7V, it is also beyond our scope.
    The BQ77908A seems like a complex system than just a secondary protection IC.
    Its overvoltage threshold can be programmed through I2C (3.2V in my case).
    But I can't find a chip with cell overvoltage detection rated just above 3.2V (The charge limited voltage is 3.65V).
    1) So is there any problem in using BQ77908A as a secondary protection IC along with BQ76920?.
    2) The datasheet says the cell balancing of the BQ77908A can be disabled. Is there any problem if both the ICs balance the cell at the same time (I'm planning to provide external cell balancing for BQ76920)
    3) If there is no problem in using it as a secondary protection device, I'm planning to write a 3.4V as the over threshold voltage of the BQ77908A  IC and 3.3V as the over threshold voltage of the BQ76920 IC and the under voltages also in the similar pattern. Will it help my performance.
    4) Since charging/discharging MOSFETs and Rsense are used in the BQ76920 IC is there any option for me to use to avoid those in BQ77908A IC or my BoM cost will be very high.
    5) The datasheet of BQ77908A says it is standalone (section 9.1.8) By stand alone I think the controller is needed only to configure the values initially and can run further without MCU. Can I rewrite the values that are already written?.
    6) There is an alert pin for the chip BQ76920 which can be used to alert the MCU and it can be also driven by the external secondary protection IC (like the out pin of BQ7718 IC). Is there any option available with BQ77908A  when used as a secondary protector?.
    7) Some documents related to BMS prefer high side MOSFETs over low side FETs, So I'm planning to use bq76200 for BQ76920 or BQ77908A. Will this arise any problem in my system?.

  • 0 in reply to Shibin Varghese
    TI__Guru 54715 points

    1. If the BQ77908A is available and meets your requirements it may be suitable.

    2. The BQ77908A has a behavior where balancing can limit the range of the OCD and SCD protection, see the data sheet. BQ77908A balancing also operates by selecting the highest voltage cell and balancing it down for a time.  While operating balancing continues to do this, so it will take a perfectly balanced pack and imbalance it in its process of maintaining a relative balance.  Otherwise, with separate filters to the cells balancing could operate on both parts simultaneously, each balance algorithm would see and respond to the voltage differences caused by any common path resistance. For example if one part sees a cell as high and starts balancing it, then the other device measures, the balance algorithm of the second part may see a slightly reduced voltage and pick a different cell to balance.  Remember the BQ76920 has no balance algorithm on its own, it must be implemented by the MCU.

    3. Some margin between primary and secondary protection thresholds is commonly expected.

    4. The FET drive outputs are outputs and do not need to be used for FETs if your system does not need them.  The levels are generally suitable for FET gates, use a voltage limiter if sending them to a low voltage input such as a MCU IO pin. Connect any sense pins to the proper level if not used.

    5. Yes, program then operate. The recommended "EEPROM number of writes" is 3 maximum

    6. The BQ77908A is intended to drive a FET on when normal and off when in fault.  This is opposite a common BQ7718 configuration.  If using the signal directly you will need some circuit.  You could send the signal to the MCU for decision making which will also take some circuit for level shift/limiting.

    7. BQ76920 load present is on the CHG pin and won't work with high side switching, detect a load with the MCU.  Check BQ77908A inputs you may intend to use 

  • 0 in reply to WM5295
    Expert 1130 points

    Hi team,

    Thank you for such a quick reply.

    I've some more doubts. Hope you wouldn't mind.

    1) I think I can use BQ77908A as a secondary protection IC along with BQ76920.

    2) I'm planning to disable balancing feature of BQ77908A and planning to provide external fet balancing with bq76920. Seperate RC filter for both ICs. And their over voltage and under voltage thresholds will be slightly different

    3) What is the best balancing algorithm for bq76920?.

    4) In the datasheet of bq76200, it says that it can be used with bq76920 family ic. So my system will have a low side fet from bq77908a (if used) and a high side fet from bq76920 through bq76200. I hope no problem will be with this configuration.

  • 0 in reply to Shibin Varghese
    TI__Guru 54715 points

    Hi Shibin,

    3. This is your choice.  Balancing one cell (the high cell) at a time is simplest.

    4. Low side protection FETs have a challenge with simple ground (PACK-) referenced communication.  If your battery is self contained with no communication protecting both sides should be fine.  Use an isolated communication path if needed.