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BQ24085_17 Datasheet, PDF (30/42 Pages) Texas Instruments – 750-mA Single-Chip Li-Ion and Li-Pol Charge Management IC With Thermal Regulation
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bq24085, bq24086, bq24087, bq24088
SLUS784E – DECEMBER 2007 – REVISED DECEMBER 2015
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11.3 Thermal Considerations
The bq2408x family 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 note entitled: QFN/SON PCB Attachment Application
Note, SLUA271.
The most common measure of package thermal performance is thermal impedance (θJA ) measured (or
modeled) from the chip junction to the air surrounding the package surface (ambient). Use Equation 13 as the
mathematical expression for θJA:
q(JA)
=
TJ - TA
P
where
• TJ = chip junction temperature
• TA = ambient temperature
• P = device power dissipation
(13)
Factors that can greatly influence the measurement and calculation of θJA 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
PowerFET. Use Equation 14 to calculate the device power dissipation when a battery pack is being charged:
P = [V(IN) – V(OUT)] × I(OUT)
(14)
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, Figure 8.
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.
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