SLUS968G January 2010 – August 2015
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).
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:
Factors that can influence the measurement and calculation of θJA include:
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 :
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.
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.