AAT3690 AAT, AAT3690 Datasheet - Page 16

• Manufacturer Part Number: AAT3690
• Description: 1.0A USB Port/Adapter Lithium-Ion/Polymer Battery Charger
• Manufacturer: AAT

Thermal Considerations
The AAT3690 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:
Where:
P
V
V
I
I
Next, the maximum operating ambient temperature
for a given application can be estimated based on
the thermal resistance of the 3x3 TDFN package
when sufficiently mounted to a PCB layout and the
internal thermal loop temperature threshold.
16
CC
OP
D
IN
BAT
Eq. 1:
= Total power dissipation by the device
= Either V
= Battery voltage as seen at the BAT pin
= Maximum constant fast charge current pro-
= Quiescent current consumed by the charg-
mode is selected
grammed for the application
er IC for normal operation
P
Eq. 2:
D
= [(V
ADP
T
IN
or V
A
- V
= T
BAT
USB
J
- (θ
) · I
, depending on which
JA
CC
· P
+ (V
D
)
IN
· I
OP
)]
Lithium-Ion/Polymer Battery Charger
Where:
T
T
P
θ
Example:
For an application where the fast charge current for
the adapter mode is set to 0.75A, V
the worst case battery voltage is 3.6V, what is the
maximum ambient temperature where the thermal
limiting will become active?
Given:
V
V
I
I
T
θ
Using Equation 3, calculate the device power dissi-
pation for the stated condition:
The maximum ambient temperature before the
AAT3690 thermal loop becomes active can now be
calculated using Equation 4:
Therefore, under the stated conditions for this
worst case power dissipation example, the
AAT3690 will enter the thermal loop and lower the
fast charge constant current when the ambient
operating temperature rises above 24.8°C.
Eq. 3:
CC
OP
JA
JA
A
J
J
D
ADP
BAT
Eq. 4:
= 5.0V
= 3.6V
= 0.75A
= 0.75mA
= 110°C
= 50°C/W
= Ambient temperature in degrees C
= Maximum device junction temperature
= Total power dissipation by the device
= Package thermal resistance in °C/W
P
below the thermal loop threshold
D
= 1.05375W
= (5.0V - 3.6V)(0.75A) + (5.0V · 0.75mA)
T
A
= 110°C - (50°C/W · 1.05375W)
= 57.3125°C
1.0A USB Port/Adapter
ADP
3690.2007.01.1.2
= 5.0V, and