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BQ77904: Can I use this chip's UV feature without removing the load?

Richard Cooke
Expert 1530 points
Part Number: BQ77904
Other Parts Discussed in Thread: BQ77905, BQ77915

Hi Folks,

Our application has a non-removable 8S4P LiFePO4 battery pack and I'm in charge of designing the BMS for the system.  The BQ77904 chip looks promising.  I'd use two of them to handle the 8 cells.  I've looked at the datasheet but I can't seem to figure out how to set up the circuit so a UV fault can be recovered by plugging in the charger and raising the cell's voltages.  We can't use the remove-the-load resetting method.

Another question - When do you recommend using external mosfets to control the balancing current? 

Thanks,

Richard

  • 0
    TI__Expert 7215 points

    Richard,

    Per section 8.3.2.2. of the datasheet, the UV fault can recover when the cell voltage in fault is above UV Threshold + UV hysteresis for a time of UV delay. These values can be found in Table 1 (for BQ77904) and Table 2 (for BQ77905). For the BQ7790400, the voltage would have to be 2.6V for a time of 2 seconds, and the fault will recover. 

    If you are looking for a part with cell balancing, you might want to check out the BQ77915 here:

    https://www.ti.com/product/BQ77915

    It has all of the features the BQ77904 has, plus some additional ones. 

  • 0 in reply to Shawn Hinkle
    Expert 1530 points

    Thanks Shawn. 

    The BQ77915 chip looks just what we're looking for.  I've done a quick review of the datasheet and it looks like in section "9.3.2.2 Operation in UV" states that the chip will recover from a UV fault without having to remove the load (something that our end users can't do).  But I'm not 100% sure if there is a setting to enable that behavior.  Can you point me to the section in the datasheet where it describes how to set the chip to use just the cell voltages to recover from UV fault?

    Do you recommend using external balancing mosfets?  I think I can get away with using the 50ma balancing current available from the internal mosfets but if you think that this puts too much stress (or heat) on the chip I'll go with external mosfets.

    Now I just have to figure out which version of the BQ77915 chip I need.

    Thanks again,

    Richard

  • 0 in reply to Richard Cooke
    TI__Expert 7215 points

    Richard,

    We currently do not offer a version of the BQ77915 that can recover from UV without load removal. If you would like us to create a version of this part that meets your needs, please contact a local sales representative. Otherwise, you can use a current version and short the LD pin to mimic the act of load removal. However, this will affect some of the other features on the device. 

    We recommend using external balancing MOSFETs when you require a cell balancing current higher than 50mA. If you would like a cell balancing current less than 50mA, refer to section 10.2.2.1 of the datasheet. This can be adjusted by changing the value of Rin. 

  • 0 in reply to Shawn Hinkle
    Expert 1530 points

    Hi Shawn,

    Thank you for the quick reply.  It's a bit frustrating to learn that TI isn't making a version of the BQ77915 that works without load removal. 

    I'm also a bit confused as the datasheet isn't clear on that.  On page 37 of the datasheet below figure 26 there is the following text:

    "To ensure load removal is detected properly during a UV fault, TI recommends to use 3.3 MΩfor RGS_CHG (instead of a typical1 MΩ when load removal is NOT required for UV recovery). RCHG can stay in 1 MΩ as recommended when using CHGFET protection components.The CHGFET rise time impact is minimized, as described in Protecting the CHGFET. On a stacked configuration, connect the LD pin as shown in Figure27 if  load removal is used for a UV fault recovery. If load detection is not required for a UV fault recovery, a larger value of RGS_CHG can be used (that is, 10 MΩ), and the LD pin on the upper devices can be left floating."  

    Another example of confusing text - On page 41 in section 10.2.2.1 towards the bottom of the list there is this text:

    "Load removal for fault recovery is required. Recovery by connecting the charger is acceptable."  <== That's the behavior we need

    On page 42:

    "Because a charger connection for UV recovery is acceptable, the condition in Using Load Detect for UV Fault Recovery is not a concern. A 1-MΩRGS_CHG can be used for the schematic."

    Should those references I highlighted be removed from the datasheet?  That's why I thought the BQ77915 chip supported non-removal of load to recover from UV faults.  Figure 27 on page 38 is also a bit confusing.

    If TI currently isn't offering a battery protection chip that doesn't require load-removal I'll have to go with plan B (which isn't known at this time).  Our application probably won't be enough business to get TI to whip out the proper variant of the BQ77915 chip.  I'm also behind schedule to get this product designed and built.

    Thanks again for your help.

    Richard Cooke

  • 0 in reply to Richard Cooke
    TI__Expert 7215 points

    Richard,

    I understand your frustration. The BQ77915 is a new device, and we will have more versions in the future as the business cases present themselves. 

    The paragraph you mentioned on page 37 is in reference to a version that doesn't require load removal, so that paragraph will apply to future devices. 

    The scenario on page 41 is an appropriate solution for the current devices we offer. If you are able to connect the charger, that will allow the device to recover from a UV condition and you can use one of the devices we currently have. The protections for those devices are listed in the device comparison table on page 3 of the datasheet. We hope you find a device there that meets your needs. Continue to refer to the design example 10.2.2.1, especially the parts concerning UV recovery. You will have to change RGS_CHG to 1M ohm. Refer to section 9.3.5 for charger connection.

  • 0 in reply to Shawn Hinkle
    Expert 1530 points

    Thanks Shawn.  So you've given me hope.  I'll keep digging into the BQ77915 datasheet but if it'll recover from a UV fault by plugging in a charger I think we can use the chip.

  • 0 in reply to Richard Cooke
    Expert 1530 points

    Hi Shawn,

    I have one more question for you - can the BQ77915 chip handle LiFePO4 battery cells?  For our application we need a UV limit of 2.1V and a OV limit of 3.85V. I'm guessing that the required chip hasn't been released yet.  Even though the datasheet does show on the front page:

    Voltage protection(accuracy±10 mV for OV,±18 mV for UV) – Overvoltage: 3 V to 4.575V– Undervoltage: 1.2 V to 3 V

    Is there a way to get to these numbers with the current chip?

    Thanks again,

    Richard Cooke

  • 0 in reply to Richard Cooke
    TI__Expert 7215 points

    Richard,

    The chip should be able to handle LiFePO4 cells. 

    You are correct, we have not released a version that has 2.1V UV and 3.85V OV yet. All of our current configurations are listed in the Device Comparison table on the datasheet. The first page of the datasheet shows the limits for future devices. This means that it is possible for us to make a version of the device that has up to 4.575V OV or as low as 1.2V UV. 

    The thresholds of these chips are fixed, so a new version would have to be created. We are constantly working on creating more configurations, so I encourage you to keep checking the datasheet for new releases.