TIDUFB6 December   2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Multiplexer Network and Switch Strategy
      2. 2.2.2 Cell Balancing
      3. 2.2.3 Stacked AFE Communication
      4. 2.2.4 MCU and CAN Interface
    3. 2.3 Highlighted Products
      1. 2.3.1 BQ78706
      2. 2.3.2 TMUX1308
      3. 2.3.3 TCAN1044-Q1
      4. 2.3.4 MSPM0G3519
      5. 2.3.5 LMR51406
      6. 2.3.6 ISO7731
      7. 2.3.7 UCC33420
      8. 2.3.8 UCC33421
      9. 2.3.9 TMP61
  9. 3Hardware, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Daisy Chain
      2. 3.3.2 Cell Voltage Accuracy
      3. 3.3.3 Temperature Sensing Using TMP61
      4. 3.3.4 Temperature Sensing Timings
      5. 3.3.5 Cell Balancing and Thermal Performance
      6. 3.3.6 Current Consumption
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

2.2.1 Multiplexer Network and Switch Strategy

Figure 2-2 shows the strategy of reading all thermistors and cell voltages. Two TMUX1308 devices are used to multiplex 14 thermistors and 2 constant resistors TS_R1 and TS_R2 to one BQ78706. The BQ78706 uses three GPIOs (GPIO9, GPIO10, and GPIO11) to address the 8 thermistor channels of the TMUX1308 and 2 GPIOs (GPIO7 and GPIO8) to read the common output pin from 2 TMUX1308 devices. This means 5 GPIOs can switch 16 thermistors.

TIDA-010279 Strategy of Reading all Thermistors and Cell Voltages Figure 2-2 Strategy of Reading all Thermistors and Cell Voltages

Although the number of thermistors can easily be increased using the TMUX1308 or a different multiplexer, the system still needs an efficient switching strategy to connect all thermistors in a safe time defined by regulation.

The loop of thermistor switching consists of a broadcast write to all the stacked BQ78706 GPIO9 to GPIO11 and a broadcast read of the GPIO7 and GPIO8 configured as ADC and OTUT inputs (ratiometric). The design needs 8 loops to read the temperature data from 14 thermistors and 2 constant resistors.

If the BESS rack voltage is 1500V, and one rack consists of 416 pieces of battery cells in series, then use 8 BMUs (32 BQ78706 devices) to monitor all the battery cells. Performing one loop to read temperature data from the stacked BQ78706 devices takes a longer time, which is likely to meet GBT34131-2023 standards (1s duty cycle for all the thermistors).