BQ24040_16 Datasheet, PDF (23/38 Pages) Texas Instruments – Single Cell Li-Ion and Li-Pol Battery Charger With Auto Start

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bq24040, bq24041, bq24045
SLUS941F – SEPTEMBER 2009 – REVISED MARCH 2015
Typical Applications (continued)
Factors that can influence the measurement and calculation of θ_JA 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 ≉3.4V 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)
(9)
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.
9.2.1.2.3.1 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 negliable.
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