AAT3687_08 Datasheet, PDF (16/20 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

BatteryManager^TM
Thermal Considerations
The AAT3687 is offered in a 3x3mm TDFN package
which can provide up to 2.0W of power dissipation when
it is properly bonded to a printed circuit board and has
a maximum thermal resistance of 50°C/W. Many con-
siderations should be taken into account when designing
the printed circuit board layout, as well as the placement
of the charger IC package in proximity to other heat
generating devices in a given application design. The
ambient temperature around the charger IC will also
have an effect on the thermal limits of a battery charg-
ing application. The maximum limits that can be expect-
ed for a given ambient condition can be estimated by the
following discussion.
First, the maximum power dissipation for a given situa-
tion should be calculated:
Eq. 5: P_D = [(V_IN - V_BAT) · I_CC + (V_IN · I_OP)]
Where:
P_D = Total Power Dissipation by the Device
V_IN = Input Voltage Amplitude, V_ADP
V_BAT = Battery Voltage as Seen at the BAT Pin
I_CC = Maximum Constant Fast Charge Current
Programmed for the Application
I_OP = Quiescent Current Consumed by the Charger IC
for Normal Operation
Next, the maximum operating ambient temperature for
a given application can be estimated based on the ther-
mal resistance of the 3x3mm TDFN package when suf-
ficiently mounted to a PCB layout and the internal ther-
mal loop temperature threshold.
Where:
Eq. 6: T_A = T_J - (θ_JA · P_D)
T_A = Ambient Temperature in °C
T_J = Maximum Device Junction Temperature Below the
Thermal Loop Threshold
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 for the
adapter mode is set to 1A, V_ADP = 5.0V and the battery
voltage at 3.6V, what is the maximum ambient tempera-
ture at which the thermal loop will become active?
PRODUCT DATASHEET
AAT3687
Lithium-Ion/Polymer Battery Charger
Given:
V_ADP = 5.0V
V_BAT = 3.6V
I_CC = 1A
I_OP = 0.75mA
T_J = 110°C
θ_JA = 50°C/W
Using Equation 5, calculate the device power dissipation
for the stated condition:
Eq. 7: _{PD = (5.0V - 3.6V)(1A) + (5.0V · 0.75mA)}
= 1.40375W
The maximum ambient temperature before the AAT3687
thermal loop becomes active can now be calculated
using Equation 6:
Eq. 8: T_A = 110°C - (50°C/W · 1.40375W)
= 39.8125°C
Therefore, under the stated conditions for this worst
case power dissipation example, the AAT3687 will enter
the thermal loop and lower the fast charge constant cur-
rent when the ambient operating temperature rises
above 39.8°C.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a decou-
pling capacitor between the ADP pin and ground. An
input capacitor in the range of 1µF to 22µF is recom-
mended. 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 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 transient
effects when the power supply is “hot plugged” in.
16
www.analogictech.com
3687.2008.04.1.12