AAT3687 Datasheet, PDF (18/22 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3687
Lithium-Ion/Polymer Battery Charger
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the ADP pin and
ground. An input capacitor in the range of 1μF to
22μF is recommended. If the source supply is
unregulated, it may be necessary to increase the
capacitance to keep the input voltage above the
under-voltage lockout threshold during device
enable and when battery charging is initiated.
If the AAT3687 adapter input is to be used in a sys-
tem with an external power supply source, such as
a typical AC-to-DC wall adapter, then a C_IN capac-
itor in the range of 10μF should be used. A larger
input capacitor in this application will minimize
switching or power transient effects when the
power supply is "hot plugged" in.
Output Capacitor
The AAT3687 only requires a 1μF ceramic capaci-
tor on the BAT pin to maintain circuit stability. This
value should be increased to 10μF or more if the
battery connection is made any distance from the
charger output. If the AAT3687 is to be used in
applications where the battery can be removed
from the charger, such as with desktop charging
cradles, an output capacitor greater than 10μF may
be required to prevent the device from cycling on
and off when no battery is present.
Printed Circuit Board Layout
Considerations
For the best results, it is recommended to physi-
cally place the battery pack as close as possible to
the AAT3687 BAT pin. To minimize voltage drops
on the PCB, keep the high current carrying traces
adequately wide. For maximum power dissipation
of the AAT3687 3x3mm TDFN package, the metal
substrate should be solder bonded to the board. It
is also recommended to maximize the substrate
contact to the PCB ground plane layer to further
increase local heat dissipation. Refer to the
AAT3687 evaluation board for a good layout exam-
ple (see Figures 4 and 5).
AAT3687 Evaluation Board Layout
Figure 4: AAT3687 Evaluation Board
Component Side Layout.
18
Figure 5: AAT3687 Evaluation Board
Solder Side Layout.
3687.2006.06.1.5