SLUS968G January   2010  – August 2015

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Description (continued)
  6. Device Options
  7. Pin Configuration and Functions
  8. Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Dissipation Ratings
    6. 8.6 Electrical Characteristics
    7. 8.7 Typical Characteristics
      1. 8.7.1 Power Up, Power Down, OVP, Disable and Enable Waveforms
      2. 8.7.2 Protection Circuits Waveforms
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1  Power-Down or Undervoltage Lockout (UVLO)
      2. 9.3.2  UVLO
      3. 9.3.3  Power-Up
      4. 9.3.4  Sleep Mode
      5. 9.3.5  New Charge Cycle
      6. 9.3.6  Overvoltage-Protection (OVP) - Continuously Monitored
      7. 9.3.7  Power Good Indication (PG)
      8. 9.3.8  CHG Pin Indication
      9. 9.3.9  CHG and PG LED Pull-Up Source
      10. 9.3.10 IN-DPM (VIN-DPM or IN-DPM)
      11. 9.3.11 OUT
      12. 9.3.12 ISET
      13. 9.3.13 PRE_TERM - Pre-Charge and Termination Programmable Threshold
      14. 9.3.14 ISET2
      15. 9.3.15 TS
    4. 9.4 Device Functional Modes
      1. 9.4.1 Termination and Timer Disable Mode (TTDM) -TS Pin High
      2. 9.4.2 Timers
      3. 9.4.3 Termination
      4. 9.4.4 Battery Detect Routine
      5. 9.4.5 Refresh Threshold
      6. 9.4.6 Starting a Charge on a Full Battery
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Calculations
          1. 10.2.2.1.1 Program the Fast Charge Current, ISET:
          2. 10.2.2.1.2 Program the Termination Current Threshold, ITERM:
          3. 10.2.2.1.3 TS Function
          4. 10.2.2.1.4 CHG and PG
        2. 10.2.2.2 Selecting IN and OUT Pin Capacitors
      3. 10.2.3 Application Curves
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
    3. 12.3 Thermal Considerations
      1. 12.3.1 Leakage Current Effects on Battery Capacity
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Third-Party Products Disclaimer
    2. 13.2 Related Links
    3. 13.3 Trademarks
    4. 13.4 Electrostatic Discharge Caution
    5. 13.5 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

12 Layout

12.1 Layout Guidelines

To obtain optimal performance, the decoupling capacitor from IN to GND (thermal pad) and the output filter capacitors from OUT to GND (thermal pad) should be placed as close as possible to the bq2409x, with short trace runs to both IN, OUT and GND (thermal pad).

  • All low-current GND connections should be kept separate from the high-current charge or discharge paths from the battery. Use a single-point ground technique incorporating both the small signal ground path and the power ground path.
  • The high current charge paths into IN pin and from the OUT pin must be sized appropriately for the maximum charge current in order to avoid voltage drops in these traces
  • The bq2409x 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); this thermal pad is also the main ground connection for the device. Connect the thermal pad to the PCB ground connection. It is best to use multiple 10mil vias in the power pad of the IC and in close proximity to conduct the heat to the bottom ground plane. The bottom ground place should avoid traces that “cut off” the thermal path. The thinner the PCB the less temperature rise. The EVM PCB has a thickness of 0.031 inches and uses 2 oz. (2.8 mil thick) copper on top and bottom, and is a good example of optimal thermal performance.

12.2 Layout Example

bq24091 bq24092 bq24093 bq24095 bq24090 bq2409x_layout.pngFigure 26. PCB Layout Example

12.3 Thermal Considerations

The bq2409x family is packaged in a thermally enhanced MSOP 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 application note entitled: Power Pad Thermally Enhanced Package Note (SLMA002). 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 2. 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:

  • Whether or not the device is board mounted
  • Trace size, composition, thickness, and geometry
  • Orientation of the device (horizontal or vertical)
  • Volume of the ambient air surrounding the device under test and airflow
  • 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 ≉3.4V 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, 3.4V 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 :

P = [V(IN) – V(OUT)] × I(OUT) + [V(OUT) – V(BAT)] × I(BAT)

The thermal loop feature reduces the charge current to limit excessive IC junction temperature. It is recommended that the design not run in thermal regulation for typical operating conditions (nominal input voltage and nominal ambient temperatures) and use the feature for non typical 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.

12.3.1 Leakage Current Effects on Battery Capacity

To determine how fast a leakage current on the battery will discharge the battery is an easy calculation. The time from full to discharge can be calculated by dividing the Amp-Hour Capacity of the battery by the leakage current. For a 0.75AHr battery and a 10μA leakage current (750mAHr/0.010mA = 75000 Hours), it would take 75k hours or 8.8 years to discharge. In reality the self discharge of the cell would be much faster so the 10μA leakage would be considered negligible.