AAT3686 Datasheet, PDF (20/24 Pages) Advanced Analogic Technologies – USB Port/AC Adapter Lithium-Ion/Polymer Battery Charger

AAT3686
USB Port/AC Adapter
Lithium-Ion/Polymer Battery Charger
Example:
For an application where the fast charge current for
the adapter mode is set to 1A, V_ADP = 5.0V, and the
worst case battery voltage is 3.0V, what is the max-
imum ambient temperature where the thermal lim-
iting will become active?
Given:
V_ADP = 5.0V
V_BAT = 3.0V
I_CC = 1A
I_OP = 0.75mA
T_J = 110°C
θ_JA = 37°C/W
Using Equation 3, calculate the device power dissi-
pation for the stated condition:
Eq. 3: P_D = (5.0V - 3.0V)(1A) + (5.0V · 0.75mA)
= 2.00375W
The maximum ambient temperature before the
AAT3686 thermal loop becomes active can now be
calculated using Equation 4:
Eq. 4: T_A = 110°C - (37°C/W · 2.00375W) = 35.86°C
= 35.86°C
Therefore, under the stated conditions for this
worst case power dissipation example, the
AAT3686 will enter the thermal loop and lower the
fast charge constant current when the ambient
operating temperature rises above 35.86°C.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the V_ADP and V_USB
pins 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 AAT3686 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 capaci-
tor in the range of 10µF should be used. A larger
input capacitor in this application will minimize
switching or power bounce effects when the power
supply is "hot plugged." Likewise, a 10µF or greater
input capacitor is recommended for the USB input
to help buffer the effects of USB source power
switching, noise, and input cable impedance.
Output Capacitor
The AAT3686 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 AAT3686 is to be used in
applications where the battery can be removed
from the charger, such as in the case of 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 physical-
ly place the battery pack as close to the AAT3686
BAT pin as possible. To minimize voltage drops on
the PCB, keep the high current carrying traces ade-
quately wide. For maximum power dissipation of the
AAT3686 TDFN packages, the metal substrate
should be solder bonded to the board. It is also rec-
ommended to maximize the substrate contact to the
PCB ground plane layer to further increase local
heat dissipation. Refer to the AAT3686 evaluation
board for a good layout example.
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3686.2006.10.1.9