AAT3686_08 Datasheet, PDF (19/23 Pages) Advanced Analogic Technologies – USB Port/AC Adapter Lithium-Ion/Polymer Battery Charger

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PRODUCT DATASHEET
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 maximum ambi-
ent temperature where the thermal limiting 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 dissipation
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 cur-
rent when the ambient operating temperature rises
above 35.86°C.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a decou-
pling capacitor between the VADP and VUSB 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 charg-
ing is initiated.
If the AAT3686 adapter input is to be used in a system
with an external power supply source, such as a typical
AC-to-DC wall adapter, then a C_IN capacitor 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 capacitor 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 physically
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 adequately wide. For
maximum power dissipation of the AAT3686 TDFN pack-
ages, the metal substrate should be solder bonded to
the board. It is also recommended to maximize the sub-
strate contact to the PCB ground plane layer to further
increase local heat dissipation. Refer to the AAT3686
evaluation board for a good layout example.
3686.2007.12.1.11
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