BQ25050 Datasheet, PDF (22/28 Pages) Texas Instruments – 1A, Single-Input, Single Cell Li-Ion Battery Charger with 50-mA LDO, External Power Path

bq25050
SLUSA33 – MARCH 2010
www.ti.com
Thermal Considerations
The bq25050 is packaged in a thermally enhanced QFN 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 (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 - T_A
P_D
(1)
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 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_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
(2)
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 22. 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
PCB Layout Considerations
It is important to pay special attention to the PCB layout. The following provides some guidelines:
• To obtain optimal performance, the decoupling capacitor from IN to GND (thermal pad) and the output filter
capacitors from OUT to GND (thermal pad) should be placed as close as possible to the bq25050, with short
trace runs to both IN, OUT and GND (thermal pad).
• All low-current GND connections should be kept separate from the high-current charge or discharge paths
from the battery. Use a single-point ground technique incorporating both the small signal ground path and the
power ground path.
• The high current charge paths into IN pin and from the OUT pin must be sized appropriately for the maximum
charge current in order to avoid voltage drops in these traces.
• The bq25050 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); this thermal pad is
also the main ground connection for the device. Connect the thermal pad to the PCB ground connection. Full
PCB design guidelines for this package are provided in the application note entitled: QFN/SON PCB
Attachment Application Note (SLUA271).
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