Data Sheet

bq24072 , bq24073 , bq24074 , bq24075 , bq24079
www.ti.com SLUS810K –SEPTEMBER 2008–REVISED MARCH 2015
12.3 Thermal Considerations
The bq24072/3/4/5 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). The power pad
should be directly connected to the V
SS
pin. Full PCB design guidelines for this package are provided in
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:
θ
JA
= (T
J
- T) / P
where
• T
J
= chip junction temperature
• T = ambient temperature
• P = device power dissipation (10)
Factors that can 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
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 ≉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 verified, with the system and a fully discharged battery, by plotting temperature on the bottom of the
PCB under the IC (pad should have multiple vias), the charge current and the battery voltage as a function of
time. The fast charge current will start 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 non typical situations such as hot environments or
higher than normal input source voltage. With that said, the IC will still perform as described, if the thermal loop
is always active.
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