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www.ti.com
SLUS549C − DECEMBER 2002 − REVISED AUGUST 2004
APPLICATION INFORMATION
THERMAL CONSIDERATIONS
The bqTINY-II is packaged in a thermally enhanced MLP package. The package includes a 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 Application Note (TI Literature
No. SLUA271).
The most common measure of package thermal performance is thermal impedance (θ ) measured (or modeled)
JA
from the device junction to the air surrounding the package surface (ambient). The mathematical expression for θ
is:
JA
T * T
J
A
q
+
JA
(5)
P
Where:
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T = device junction temperature
J
T = ambient temperature
A
P = device power dissipation
Factors that can greatly influence the measurement and calculation of θ include:
JA
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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.
It can be calculated from the following equation:
P + ǒV
Ǔ
* V
I
O(OUT)
IN
I(BAT)
(6)
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 it’s lowest. See Figure 2.
PCB LAYOUT CONSIDERATIONS
It is important to pay special attention to the PCB layout. The following provides some guidelines:
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To obtain optimal performance, the decoupling capacitor from V
OUT to VSS should be placed as close as possible to the bqTINY, with short trace runs to both signal and V
pins.
to V and the output filter capacitors from
CC SS
SS
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All low-current V connections should be kept separate from the high-current charge or discharge paths from
SS
the battery. Use a single-point ground technique incorporating both the small signal ground path and the power
ground path.
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The BAT pin is the voltage feedback to the device and should be connected with its trace as close to the battery
pack as possible.
The high current charge paths into IN and from the OUT pins must be sized appropriately for the maximum charge
current in order to avoid voltage drops in these traces.
The bqTINY-II is packaged in a thermally enhanced MLP package. The package includes a 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 Application
Note (TI Literature No. SLUA271).
16
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