TIDUF46 October   2023

 

  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 Isolated UART Interface to MCU
    3. 2.3 Highlighted Products
      1. 2.3.1 BQ79616
      2. 2.3.2 TMUX1308
      3. 2.3.3 TMUX1574
      4. 2.3.4 TMUX1102
      5. 2.3.5 TPS22810
      6. 2.3.6 ISO7742
      7. 2.3.7 TSD05C
      8. 2.3.8 ESD441
      9. 2.3.9 ESD2CAN24-Q1
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Cell Voltage Accuracy
      2. 3.3.2 Temperature Sensing Accuracy
      3. 3.3.3 Cell Voltage and Temperature Sensing Timings
      4. 3.3.4 Cell Balancing and Thermal Performance
      5. 3.3.5 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 16 Negative Temperature Coefficient (NTC) thermistors to one BQ79616. The BQ79616 uses three GPIOs (GPIO5, GPIO6, and GPIO7) to address the 8 NTC thermistor channels of the TMUX1308 and two GPIOs (GPIO1 and GPIO2) to read the common output pin from two TMUX1308 devices. This means 5 GPIOs can switch 16 NTC thermistors. If more thermistors are required, 6 GPIOs can switch 24 NTC thermistors.

GUID-20230925-SS0I-4S6C-V2SN-WX2MWTV3KQ1K-low.svg Figure 2-2 Strategy of Reading all Thermistors and Cell Voltages

Although the number of NTC thermistors can easily be increased using the TMUX1308 or a different multiplexer, the system still needs an efficient switching strategy to connect all 16 NTC thermistors in a safe time defined by regulation.

The loop of NTC thermistor switching consists of a broadcast write to all the stacked BQ79616 GPIO5 to 7 pins and a broadcast read of the TSREF and GPIO1 to GPIO2. The design needs 8 loops to read the temperature data from 16 NTC thermistors. Supposing the number of stacked BMUs is N, and the design uses a BQ79600 base device (not counted when determining N), then one loop spends 14 + 4 × 2 × N_BMU - 1 + 60   μ s to perform the broadcast write to the GPIO5 to7 pins on all devices. A broadcast read of the TSREF and GPIO1 to GPIO2 takes 14 + 4 × 2 × N_BMU - 1 + 60 + 14 + 4 × 2 × N_BMU - 1 + 90 × 2 × N_BMU   μ s.

If the BESS rack voltage is 1500 V, and one rack consists of 470 pieces of battery cells in series, then use 15 BMUs (30 BQ79616 devices) to monitor all the battery cells. Performing one loop to read temperature data from the stacked BQ79616 devices takes 4.11 ms and polls 2 out of the 16 NTC thermistors on each BQ79616 in the stack. Reading temperature data from all 16 NTC thermistors on each BQ79616 in the stack takes 32.880 ms. Following the NTC thermistors data reading, 11.706 ms are required to read the cell voltage (VCELL) data of all the stacked BQ79616 devices. The total time spent gathering temperature and VCELL data for a 1500-V rack is about 44 ms, which meets GBT34131-2023 standards (100 ms for VCELL and 1 s for NTC thermistors).