AAT3685 Datasheet, PDF (19/23 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3685
Lithium-Ion/Polymer Battery Charger
Where:
T_A = Ambient Temperature in Degrees C
T_J = Maximum Device Junction Temperature
Protected by the Thermal Limit Control
P_D = Total Power Dissipation by the Device
θ_JA = Package Thermal Resistance in °C/W
Example:
For an application where the fast charge current is
set to 500mA, V_USB = 5.0V and the worst case bat-
tery voltage at 3.0V, what is the maximum ambient
temperature at which the thermal limiting will
become active?
Given:
V_USB = 5.0V
V_BAT = 3.0V
I_CC = 500mA
I_OP = 0.75mA
T_J = 140°C
θ_JA = 50°C/W
Using Equation 4, calculate the device power dissi-
pation for the stated condition:
Eq. 6: P_D = (5.0V - 3.0V)(500mA) + (5.0V · 0.75mA)
= 1.00375W
The maximum ambient temperature before the
AAT3685 thermal limit protection will shut down
charging can now be calculated using Equation 5:
Eq. 7: T_A = 140°C - (50°C/W · 1.00375W)
= 89.81°C
Therefore, under the stated conditions for this
worst case power dissipation example, the
AAT3685 will suspend charging operations when
the ambient operating temperature rises above
89.81°C.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the ADP/USB 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 AAT3685 input is to be used in a system with
an external power supply source rather than a USB
port V_BUS, 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" in. 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 AAT3685 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 AAT3685 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 physi-
cally place the battery pack as close as possible
to the AAT3685 BAT pin. To minimize voltage
drops on the PCB, keep the high current carrying
traces adequately wide. For maximum power dis-
sipation of the AAT3685 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 AAT3685 evaluation board for a good
layout example (see Figures 5 and 6).
3685.2006.10.1.3
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