BQ25101H_15 Datasheet, PDF (25/32 Pages) Texas Instruments – bq2510x 250-mA Single Cell Li-Ion Battery Chargers, 1-mA Termination, 75-nA Battery Leakage

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11.2 Layout Example
bq25100, bq25101, bq25100A, bq25100H, bq25101H, bq25100L
SLUSBV8C – AUGUST 2014 – REVISED NOVEMBER 2014
Figure 28. Board Layout
11.3 Thermal Package
The most common measure of package thermal performance is thermal impedance (θ_JA ) measured (or
modeled) from the chip junction to the air surrounding the package surface (ambient). The mathematical
expression for θ_JA is:
θ_JA = (T_J – T) / P
(3)
Where:
T_J = chip junction temperature
T = ambient temperature
P = device power dissipation
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 ≉3.4 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.
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)
(4)
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