AAT3687 Datasheet, PDF (17/22 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3687
Lithium-Ion/Polymer Battery Charger
Thermal Considerations
The AAT3687 is offered in a 3x3mm TDFN pack-
age which can provide up to 2.0W of power dissi-
pation when it is properly bonded to a printed cir-
cuit board and has a maximum thermal resistance
of 50°C/W. Many considerations 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 gen-
erating devices in a given application design. The
ambient temperature around the charger IC will
also have an effect on the thermal limits of a bat-
tery charging application. The maximum limits that
can be expected for a given ambient condition can
be estimated by the following discussion.
First, the maximum power dissipation for a given
situation should be calculated:
Eq. 5: ^PD ^{= [(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 thermal resistance of the 3x3mm TDFN pack-
age when sufficiently mounted to a PCB layout and
the internal thermal loop temperature threshold.
Eq. 6: ^TA ^{= T}J ^{- (θ}JA ^{· P}D⁾
Where:
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 ambi-
ent temperature at which the thermal loop will
become active?
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 dissi-
pation for the stated condition:
Eq. 7: P_D = (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 current when the ambient
operating temperature rises above 39.8°C.
3687.2006.10.1.7
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