SLUS940D September   2009  – May 2021 BQ24050 , BQ24052

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings (1)
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions (1)
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Switching Characteristics
    8. 6.8 Typical Characteristics
      1. 6.8.1 Power Up, Down, OVP, Disable and Enable Waveforms
      2. 6.8.2 Protection Circuits Waveforms
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Power Down, or Undervoltage Lockout (UVLO)
      2. 7.3.2  Power Up
      3. 7.3.3  D+, D– Detection
      4. 7.3.4  New Charge Cycle
      5. 7.3.5  Overvoltage Protection (OVP) – Continuously Monitored
      6. 7.3.6  CHG Pin Indication
      7. 7.3.7  CHG LED Pullup Source
      8. 7.3.8  Input DPM Mode (VIN-DPM or IN-DPM)
      9. 7.3.9  OUT
      10. 7.3.10 ISET
      11. 7.3.11 TS
      12. 7.3.12 Termination and Timer Disable Mode (TTDM) -TS Pin High
      13. 7.3.13 Timers
      14. 7.3.14 Termination
      15. 7.3.15 Battery Detect Routine
      16. 7.3.16 Refresh Threshold
      17. 7.3.17 Starting a Charge on a Full Battery
    4. 7.4 Device Functional Modes
      1. 7.4.1 Sleep Mode
    5. 7.5 Programming
      1. 7.5.1 PRE_TERM – Precharge and Termination Programmable Threshold
      2. 7.5.2 ISET2
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 BQ2405x Charger Application Design Example
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Program the Fast Charge Current, ISET
          2. 8.2.1.2.2 Program the Termination Current Threshold, ITERM
          3. 8.2.1.2.3 TS Function
          4. 8.2.1.2.4 CHG
          5. 8.2.1.2.5 Selecting IN and OUT Pin Capacitors
        3. 8.2.1.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Thermal Considerations
      1. 10.3.1 Leakage Current Effects on Battery Capacity
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Support Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary

10.3 Thermal Considerations

The BQ2405x family is packaged in a thermally enhanced MLP package. The package includes a thermal pad to provide an effective thermal contact between the IC and the printed circuit board (PCB). The power pad should be directly connected to the VSS pin. Full PCB design guidelines for this package are provided in the QFN and SON PCB Attachment Application Report. The most common measure of package thermal performance is thermal impedance (θJA ) measured (or modeled) from the chip junction to the air surrounding the package surface (ambient). The mathematical expression for θJA is:

Equation 3. θJA = (TJ – T) / P

Where:

TJ = chip junction temperature
T = ambient temperature
P = device power dissipation

Factors that can influence the measurement and calculation of θJA include:

  1. Whether or not the device is board mounted
  2. Trace size, composition, thickness, and geometry
  3. Orientation of the device (horizontal or vertical)
  4. Volume of the ambient air surrounding the device under test and airflow
  5. Whether other surfaces are in close proximity to the device being tested

Due to the charge profile of Li-Ion and Li-Pol batteries the maximum power dissipation is typically seen at the beginning of the charge cycle when the battery voltage is at its lowest. Typically after fast charge begins the pack voltage increases to ≉3.4 V within the first 2 minutes. The thermal time constant of the assembly typically takes a few minutes to heat up so when doing maximum power dissipation calculations, 3.4 V is a good minimum voltage to use. This is verified, with the system and a fully discharged battery, by plotting temperature on the bottom of the PCB under the IC (pad should have multiple vias), the charge current and the battery voltage as a function of time. The fast charge current will start to taper off if the part goes into thermal regulation.

The device power dissipation, P, is a function of the charge rate and the voltage drop across the internal PowerFET. It can be calculated from the following equation when a battery pack is being charged:

Equation 4. P = [V(IN) – V(OUT)] × I(OUT) + [V(OUT) – V(OUT)] × I(OUT)

The thermal loop feature reduces the charge current to limit excessive IC junction temperature. TI recommends that the design not run in thermal regulation for typical operating conditions (nominal input voltage and nominal ambient temperatures) and use the feature for nontypical situations such as hot environments or higher than normal input source voltage. With that said, the IC will still perform as described, if the thermal loop is always active.