BQ25040 Datasheet, PDF (19/25 Pages) Texas Instruments – 1.1A, Single-Input, Single Cell Li-Ion Battery Charger With 50mA LDO and 2.3A Production Test Support

bq25040
www.ti.com ...................................................................................................................................................... SLUS910B – MARCH 2009 – REVISED MARCH 2009
Thermal Considerations
The bq25040 is packaged in a thermally enhanced SON 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 note entitled: QFN/SON PCB Attachment Application
Note (SLUA271).
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:
J_JA = ^TJ
-
P_D
T_A
(3)
Where:
T_J = chip junction temperature
T_A = ambient temperature
P_D = device power dissipation
Factors that can greatly influence the measurement and calculation of θ_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_D, 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_D = (V_IN – V_OUT) × I_OUT
Due to the charge profile of Li-Ion batteries the maximum power dissipation is typically seen at the beginning of
the charge cycle when the battery voltage is at its lowest. See the charging profile. If the board thermal design is
not adequate the programmed fast charge rate current may not be achieved under maximum input voltage and
minimum battery voltage, as the thermal loop can be active, effectively reducing the charge current to avoid
excessive IC junction temperature
Copyright © 2009, Texas Instruments Incorporated
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