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Original question:

BQ76940: BQ76940

BQ76940: BQ76940

user5247366
Prodigy 250 points

Part Number: BQ76940

Dear WM:

     I have some questions about BQ76940. Connect to 10 cells,BAT-VC10x,VC10x-VC5x,VC5x-Vss,the voltage is normal,but CAP3-VC10x ,CAP2-VC5X ,the voltage is not 3.3v,

CAP1-VSS is normal.TS1,TS2,TS3 act as extertal temperature sensor, must connect to  extertal temperature sensor? If  do not connect ,CAP3-VC10x ,CAP2-VC5X may be  not normal?

  • 0
    TI__Guru 54715 points
    Hi User,
    You are correct, the part must have a resistance from TS3 to VC10X and TS2 to VC5X to operate. Without this resistance the upper groups may not start as you have observed. A resistor can be used if temperature measurement is not needed, 10k is typical but higher or lower values may be used.
    If thermistors are connected by a connector, thermistors or resistors should be used during board test to pull the inputs to the appropriate reference for proper operation.
  • 0
    Prodigy 250 points
    Dear WM:
    Thanks for your reply.I have some questions.
    1: If BAT-VC10x,VC10x-VC5x,VC5x-Vss ,the voltage is not equal to the sum of battery voltage of reach group , BQ76940 go bad? Can I say that?
    2: If thermistors are connected by a connector, thermistors or resistors should be used during board test to pull the inputs to the appropriate reference for proper operation. Why? Can you explain further?
    3: What's the range of thermistors?
    Thanks!
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    1. No, the group supply voltage must be in the acceptable range. If you exceed abs max the bq76940 can go bad. Also note the diodes from the top group input to power, these avoid forcing the inputs far above power to cause damage although there is no specification. But if the sum of the voltages is far off there must be some large voltage drop from something damaged or not connected.
    2. The TS2 and TS3 must be pulled to their reference for the part to start properly. TS1 must be low so that it can receive a rising edge, if it is already high it can not get a rising edge to boot. If the thermistors are attached at a later assembly step, when the board is tested some substitute resistances must be connected. If thermistors are not used in the system some resistors should be placed on the board to allow the part to operate.
    3. The 103AT thermistors are 10k nominal at 25C and range from 188k at -40C to 2.2k at 70C, see the thermistor data sheet if interested. The FAQ for the bq769x0 suggested 1k to 100k for a substitute resistor. If you are using only some thermistors you may want to pick a value which matches the thermistor resistance at the normal operating temperature of your system. Mismatched resistances will cause a small load difference on the cell groups, in most cases it won't matter. For example if you use only TS2 for temperature measurement and you know the system will normally be 30C, the thermistor will be 8.3k. You might select a resistor value which is similar for the unused thermistors.
  • 0 in reply to WM5295
    Prodigy 250 points
    Dear WM:
    Thanks for your reply.I have some questions.
    1: Reference schematic diagram of BQ76940, C0 connect BATT-( Reference ground of board), and then pass through RC network connect reference ground of board, why?
    2: Plug in the battery's sampling line to 76940 first, then power up mcu, wake up 76940, Sometimes it happens XREADY=1, then mcu set XREADY=0, it is OK, why?
    Thanks!
  • 0 in reply to user5247366
    TI__Guru 54715 points
    Hi User,
    1. The bq76940 senses the bottom of the lowest cell with input VC0. That can be connected at the cell or on the board to reduce the number of connections to the board. If connected at the cell be cautious of transients in the interconnect. The RC will help with transients. The VC0 pin is also used for cell balancing, the pin pair pulls together to balance. See figure 5 of the application note www.ti.com/.../slua775 . The RC filter helps stabilize the input as it returns to normal for measurement. The RC should not be too big or it will still be settling when measurement begins. It is a design tradeoff.
    2. If transients are present on the TS2 or TS3 lines during connection those groups may have power and set the XREADY error when the bq76940 is booted. Similarly if a transient occurs on TS1 which starts the IC before power is connected on the upper cells, XREADY will set.