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 System Overview

The design uses two BQ79616 devices to monitor each cell voltage, the temperature of a 32 cells battery pack, and to protect the pack against all unusual situations, including cell overvoltage, cell undervoltage, and overtemperature. In Figure 3-14, the top BQ79616 device is the BQ79616-A2 and the bottom BQ79616 device is the BQ79616-A1. The forward daisy-chain communication direction is from the BQ79616-A1 device to the BQ79616-A2 device.

Each BQ79616 has 8 GPIO pins for temperature sensing and 16 VC pins for voltage sensing. To monitor the temperatures for all the VC channels with fewer GPIO pins, two TMUX1308 multiplexers are used. The multiplexers expand temperature-sensing capabilities of one BQ79616 from 8 channels to 16 channels. The TMUX1574 is used to expand GPIO4 through GPIO7 of BQ79616 to an external SPI EEPROM that restores pack information.

To run diagnostics for the TMUX1308, each channel of the TMUX1308 can be connected one at a time to the TSREF output pin of BQ79616. TSREF is set at a high or low voltage level, according to the BCU command. The BCU then polls the MUX channels and determines if the voltage reported on the MUX channel connected to TSREF matches the TSREF pin output. This diagnostic method can show if the MUX is stuck on a specific channel or reporting voltages corresponding to incorrect channels.

In this design, the GPIO8 pin is reserved for a humidity sensor interface.

The CVDD pin on the BQ79616 is used to supply power to the TMUX1308, TMUX1574, external EEPROM, and humidity sensor. Since CVDD provides a constant-on power supply, the pin can cause leakage current to external loads when the BQ79616 is in SHUTDOWN mode. TPS22810 is enabled by GPIO3 of the BQ79616, and the device is used to switch the power supply output from CVDD to prevent unintended leakage current.

BQ79616 supports the UART interface to the MCU. An ISO7742 is placed between the BQ79616 and MCU to isolate the TX and RX pins of each device. Since the TX and CVDD pins of the BQ79616 are a constant-on power supply, these pins can cause leakage current to the ISO7742 in SHUTDOWN mode. A PMOS, TMUX1102, and optocoupler are used to block the leakage current. To initiate UART communication, the MCU needs enable the optocoupler to switch on the PMOS and TMUX1102 from the MCU side.

This design also uses a BJT network for external passive cell balancing. The internal passive cell balancing resistors can support up to 100 mA of balancing current. The voltage across the internal passive cell balancing resistors are used to switch ON the external BJTs for internal passive cell balancing.

To isolate communication, the design uses two high-voltage capacitors for daisy chain communication between two BQ79616 and two transformers in daisy chain communication between the BMUs or the BCU.