AAT3681_08 Datasheet, PDF (12/15 Pages) Advanced Analogic Technologies – USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger

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PRODUCT DATASHEET
AAT3681
USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger
Charge Status Output
The AAT3681 provides battery charge status via a status
pin. This pin is internally connected to an N-channel
open drain MOSFET, which can be used to drive an exter-
nal LED. The status pin can indicate the following condi-
tions:
Event Description
No battery charging activity
Battery charging via adapter or USB port
Charging completed
Status
OFF
ON
OFF
Table 2: LED Status Indicator.
The LED should be biased with as little current as neces-
sary to create reasonable illumination; therefore, a bal-
last resistor should be placed between the LED cathode
and the STAT pin. LED current consumption will add to
the overall thermal power budget for the device pack-
age, hence it is good to keep the LED drive current to a
minimum. 2mA should be sufficient to drive most low-
cost green or red LEDs. It is not recommended to exceed
8mA for driving an individual status LED.
The required ballast resistor values can be estimated
using the following formulas:
R₁=
(V_ADP - V_F(LED))
I_LED
Example:
R₁ =
(5.5V - 2.0V)
2mA
= 1.75kΩ
Note: Red LED forward voltage (V_F) is typically 2.0V @
2mA.
Thermal Considerations
The AAT3681 is offered in a SC70JW-8 package which
can provide up to 687mW of power dissipation when it
is properly bonded to a printed circuit board and has a
maximum thermal resistance of 160°C/W. Many consid-
erations 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:
P_D(MAX) =
(T_J(MAX) - T_A)
θ_JA
Where:
P_D(MAX) = Maximum Power Dissipation (W)
θ_JA = Package Thermal Resistance (°C/W)
T_J(MAX) = Maximum Device Junction Temperature (°C)
[135°C]
T_A
= Ambient Temperature (°C)
Figure 3 shows the relationship of maximum power dis-
sipation and ambient temperature of AAT3681.
1000
800
600
400
200
0
0
20
40
60
80
100
T_A (°C)
Figure 3: Maximum Power Dissipation.
Next, the power dissipation can be calculated by the fol-
lowing equation:
P_D = [(V_IN - V_BAT) · I_CH + (V_IN · I_OP)]
Where:
P_D = Total Power Dissipation by the Device
V_IN = Input Voltage
V_BAT = Battery Voltage as Seen at the BAT Pin
I_CH = Constant Charge Current Programmed for the
Application
I_OP = Quiescent Current Consumed by the Charger IC
for Normal Operation [0.5mA]
By substitution, we can derive the maximum charge cur-
rent before reaching the thermal limit condition (thermal
cycling). The maximum charge current is the key factor
when designing battery charger applications.
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