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BQ34Z100 Issue with I2C – not communication related

Kumar Krishnan
Prodigy 40 points
Other Parts Discussed in Thread: BQ34Z100, BQ77910A, BQ78PL114

We designed 5S 2P Li-Ion Battery pack with BQ34Z100 Gas Gauge and BQ77910A Protector. We got the design verified by Local TI. We have tested for charging, discharging, short circuit current, battery cut-off, etc.,

Now, we are facing below described problem and looking for clarification on few questions

  1. What is the advantage of using FET switch in Batt- instead of Batt+?
  2. Usually, power disconnect means, +ve / Phase terminal to be disconnected instead of –Ve / Neutral line. Right? But, in this case it is –Ve.
  3. Now, our specific problem is.
    1. Since, we are not disconnecting Batt+ terminal, the voltage appears till our DSP TMS320 after battery cut-off at 15.5V. Due to which, the battery gets closed path for current through I2C line. Under this condition, we observed 33mA is flowing out of battery which causes battery to drain further. We disconnected the I2C line and found no current flows (hardly, 0.3mA).

Kindly provide us the solution for this issue.

Further, in BQ34Z100’s datasheet, page no 4, figure 2 depicts that FET switch to be used on Batt+ terminal. How it is feasible with BQ77910A as it got N-channel driver and other connections required for driver (CCAP, CPCKN). We are confused with the signals to use P-Channel FET switch in Batt+ line. Kindly clear us. Hope, Using FET switch in Batt+ solves the issue we are facing.

  • 0
    TI__Guru 73830 points

    Kumar,

    Most of our devices place the protection FETs in the PACK+ power path. The bq77910A was designed for low-side protection FET placement and I am not sure why. We will investigate the issue that you noted in Item 3 above.

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 40 points

    Thanks...

    I am waiting for reply / final answer.

    Kindly let me know, why low side switch was preferred for this IC design? What is the advantage having low side switch?

    This is our second battery design with TI’s Gas gauge chips. The first design is with BQ78PL114. Based on TI’s recommendation we designed with BQ34Z100 and BQ77910A. But, we are facing this issues. So, looking for quick solution from TI.

    Please update.

    Kumar

  • 0 in reply to Kumar Krishnan
    TI__Guru 54715 points

    The bq77910A uses a low side switch because it is a standalone protector.  With a battery using a standalone protector which has only 2 wires in use, either can be switched and it looks the same to the system load.  The low side N-ch is relatively simple to drive internal to the IC.  An N-ch power FET will typically cost less than an equivalent P-ch.  The combination of these is attractive in a low cost protector system.  Adding extra signals to the battery for separate charge path, a "charger connected" signal and/or a temp sensing signal are usually manageable with the low side switch.

    When the '910A is used with a gauge such as the bq34Z100 and communication is desired in the protected state or connected during the protected state, the low side switching is undesirable and can lead to issues like you are observing. To avoid these and possible damage to the circuits, you may need to switch off or isolate the interface when the protector has switched off the pack. The circuitry to do this may be complex and require significant current depending on your system. A simpler approach may be to use a high side switch.  High side N-ch switching will require some external boost regulator(s) or charge pump drivers to provide a gate voltage for the FET and may not be practical in your system.  A high side P-ch is fairly simple to implement however and may be a good system solution.  The switch and driver you used on the PL114 design should work (or the one from the EVM) with minor adjustment for the higher IC output voltages.  The CHG and DSG pins of the '114 swing 0-2.5V while the outputs of the '910A swing 0-12V, so some adjustment to the switch circuit is needed.  (CPCKN should be connected to VSS when using the bq77910A CHG output as a drive signal for a P-ch switch.)

     

  • 0 in reply to WM5295
    Prodigy 40 points

    Thanks...

      We will change our schematics for design similiar to '114 and upload for review. We have done I2C line isolation work around for present design. still 1mA current flows out of battery after cutoff, which makes battery to die in month.

  • 0 in reply to Kumar Krishnan
    Prodigy 40 points

    Hi,

    We could not able to complete the design with high side switch due to usage of different pins to be used for low side switch for the same. Kindly provide us the reference design from TI.