BQ24232HA Datasheet, PDF (31/36 Pages) Texas Instruments – USB-Friendly Lithium-Ion Battery Charger

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bq24232HA
SLUSCG4 – MAY 2016
10.3 Thermal Considerations
The bq24232HA 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). The power pad must be
directly connected to the Vss pin. 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 (R_θJA ) measured (or modeled) from the chip junction to the air surrounding
the package surface (ambient). The mathematical expression for R_θJA is:
R_θJA = (T_J – T) / P
where
• T_J = Chip junction temperature
• T = Ambient temperature
• P = Device power dissipation
(10)
Factors that can greatly influence the measurement and calculation of R_θJA include:
1. Whether 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 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 about 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. This is easy to verify, with the system and a fully discharged battery, by plotting temperature on the
bottom of the PCB under the IC (pad must have multiple vias), the charge current and the battery voltage as a
function of time. The fast-charge current starts to taper off if the part goes into thermal regulation.
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) + [V_(OUT) – V_(BAT)] × I_(BAT)
(11)
The thermal loop feature reduces the charge current to limit excessive IC junction temperature. It is
recommended that the design not run in thermal regulation for typical operating conditions (nominal input voltage
and nominal ambient temperatures) and use the feature for nontypical situations such as hot environments or
higher than normal input source voltage. With that said, the IC still performs as described, if the thermal loop is
always active.
Copyright © 2016, Texas Instruments Incorporated
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