BQ24100_10 Datasheet, PDF (28/40 Pages) Texas Instruments – SYNCHRONOUS SWITCHMODE, LI-ION AND LI-POLYMER CHARGE-MANAGEMENT IC WITH INTEGRATED POWER FETs ( bqSWITCHER™)

bq24100, bq24103, bq24103A
bq24104, bq24105, bq24108, bq24109
bq24113, bq24113A, bq24115
SLUS606O – JUNE 2004 – REVISED MARCH 2010
THERMAL CONSIDERATIONS
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The SWITCHER 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). Full PCB design
guidelines for this package are provided in the application report entitled: QFN/SON PCB Attachment
(SLUA271).
The most common measure of package thermal performance is thermal impedance (q_JA) measured (or modeled)
from the chip junction to the air surrounding the package surface (ambient). The mathematical expression for q_JA
is:
q _(JA)
+
T_J
*
P
T_A
(25)
Where:
T_J = chip junction temperature
T_A = ambient temperature
P = device power dissipation
Factors that can greatly influence the measurement and calculation of q_JA 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
The device power dissipation, P, is a function of the charge rate and the voltage drop across the internal power
FET. It can be calculated from the following equation:
P = [Vin × lin - Vbat × Ibat]
Due to the charge profile of Li-xx batteries, the maximum power dissipation is typically seen at the beginning of
the charge cycle when the battery voltage is at its lowest. (See Figure 9.)
28
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