BQ24070_14 Datasheet, PDF (24/33 Pages) Texas Instruments – System Power-Path Management IC

bq24070, bq24071
SLUS694G – MARCH 2006 – REVISED DECEMBER 2014
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11.3 Thermal Considerations
The bq2407x is packaged in a thermally enhanced MLP package. The package includes a QFN thermal pad to
provide an effective thermal contact between the device and the printed-circuit board (PCB). Full PCB design
guidelines for this package are provided in the application note entitled QFN/SON PCB Attachment (SLUA271).
The power pad should be tied to the VSS plane. 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:
q_JA
+
T_J
*
P
T_A
where
• T_J = chip junction temperature
• T_A = ambient temperature
• P = device power dissipation
(9)
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, is a function of the charge rate and the voltage drop across the internal power
FET. It can be calculated from Equation 10:
P + ^ƪǒV_IN * V_OUT^{Ǔ ǒ}I_OUT ) I_BAT^Ǔƫ ) ^ƪǒV_OUT * V_BAT^{Ǔ ǒ}I_BAT^Ǔƫ
(10)
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 2. Typically the voltage of the Li-ion battery
quickly (< 2 V minutes) ramps to approximately 3.5 V, when entering fast charge (1-C charge rate and battery
above 3 V). Therefore, it is customary to perform the steady-state thermal design using 3.5 V as the minimum
battery voltage because the system board and charging device does not have time to reach a maximum
temperature due to the thermal mass of the assembly during the early stages of fast charge. This theory is easily
verified by performing a charge cycle on a discharged battery while monitoring the battery voltage and chargers
power pad temperature.
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