SLUSAM9E July   2011  – April 2020 BQ76925

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
  4. 4 Revision History
  5. 5 Description (Continued)
  6. 6 Pin Configuration and Functions
    1.     Pin Functions
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics: Supply Current
    6. 7.6  Internal Power Control (Startup and Shutdown)
    7. 7.7  3.3-V Voltage Regulator
    8. 7.8  Voltage Reference
    9. 7.9  Cell Voltage Amplifier
    10. 7.10 Current Sense Amplifier
    11. 7.11 Overcurrent Comparator
    12. 7.12 Internal Temperature Measurement
    13. 7.13 Cell Balancing and Open Cell Detection
    14. 7.14 I2C Compatible Interface
    15. 7.15 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Internal LDO Voltage Regulator
      2. 8.3.2 ADC Interface
        1. 8.3.2.1 Reference Voltage
          1. 8.3.2.1.1 Host ADC Calibration
        2. 8.3.2.2 Cell Voltage Monitoring
          1. 8.3.2.2.1 Cell Amplifier Headroom Under Extreme Cell Imbalance
          2. 8.3.2.2.2 Cell Amplifier Headroom Under BAT Voltage Drop
        3. 8.3.2.3 Current Monitoring
        4. 8.3.2.4 Overcurrent Monitoring
        5. 8.3.2.5 Temperature Monitoring
          1. 8.3.2.5.1 Internal Temperature Monitoring
      3. 8.3.3 Cell Balancing and Open Cell Detection
    4. 8.4 Device Functional Modes
      1. 8.4.1 Power Modes
        1. 8.4.1.1 POWER ON RESET (POR)
        2. 8.4.1.2 STANDBY
        3. 8.4.1.3 SLEEP
    5. 8.5 Programming
      1. 8.5.1 Host Interface
        1. 8.5.1.1 I2C Addressing
        2. 8.5.1.2 Bus Write Command to BQ76925
        3. 8.5.1.3 Bus Read Command from BQ76925 Device
    6. 8.6 Register Maps
      1. 8.6.1 Register Descriptions
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Recommended System Implementation
        1. 9.1.1.1 Voltage, Current, and Temperature Outputs
        2. 9.1.1.2 Power Management
        3. 9.1.1.3 Low Dropout (LDO) Regulator
        4. 9.1.1.4 Input Filters
        5. 9.1.1.5 Output Filters
      2. 9.1.2 Cell Balancing
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Support Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

8.3.3 Cell Balancing and Open Cell Detection

The BQ76925 device integrates cell-balancing FETs that are individually controlled by the Host. The balancing method is resistive bleed balancing, where the balancing current is set by the external cell input resistors. The maximum allowed balancing current is 50 mA per cell.

The Host may activate one or more cell balancing FETs by writing the BAL_n bits in the BAL_CTL register. To allow the greatest flexibility, the Host has complete control over the balancing FETs. However, in order to avoid exceeding the maximum cell input voltage, the BQ76925 will prevent two adjacent balancing FETs from being turned on simultaneously. If two adjacent bits in the balance control register are set to 1, neither balancing transistor will be turned on. The Host based balancing algorithm must also limit the power dissipation to the maximum ratings of the device.

In a normal system, closing a cell-balancing FET will cause 2 cell voltages to appear across one cell input. This fact can be utilized to detect a cell sense-line open condition, that is, a broken wire from the cell-sense point to the BQ76925 VCn input. Table 3 shows how this can be accomplished. Note that the normal cell-voltage measurements may represent a saturated or full-scale reading. However, these will normally be distinguishable from the open-cell measurement.

Table 3. Open Cell Detection Method

Kelvin input to test Method 1 Method 2
Turn On Measure Result Turn On Measure Result
Normal Open Normal Open
VC0 BAL_1 CELL2 CELL2 + 0.5 × CELL1 CELL2
VC1 BAL_2 CELL3 CELL3 + 0.5 × CELL2 CELL3
VC2 BAL_3 CELL4 CELL4 + 0.5 × CELL3 CELL4 BAL_2 CELL1 CELL1 + 0.5 × CELL2 CELL1
VC3 BAL_4 CELL5 CELL5 + 0.5 × CELL4 CELL5 BAL_3 CELL2 CELL2 + 0.5 × CELL3 CELL2
VC4 BAL_5 CELL6 CELL6 + 0.5 × CELL5 CELL6 BAL_4 CELL3 CELL3 + 0.5 × CELL4 CELL3
VC5 BAL_5 CELL4 CELL4 + 0.5 × CELL5 CELL4
VC6 BAL_6 CELL5 CELL5 + 0.5 × CELL6 CELL5

NOTE

The cell amplifier headroom limits discussed above apply to the open-cell detection method because by virtue of closing a switch between 2 cell inputs, internal to the device this appears as an extreme cell imbalance. Therefore, when testing for an open on CELL2 by closing the CELL1 balancing FET, the CELL2 measurement will be less than the expected normal result due to gain loss caused by the imbalance. However, the CELL2 measurement will still increase under this condition so that a difference between open (no change) and normal (measured voltage increases) can be detected.