AAT3681 Datasheet, PDF (12/15 Pages) Advanced Analogic Technologies – USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger

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 external LED. The status pin can indicate
the following conditions:
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
necessary to create reasonable illumination; there-
fore, a ballast 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 package, 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 LED. It is not recommended to exceed 8mA for
driving an individual status LED.
The required ballast resistor values can be esti-
mated 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 dissipa-
tion when it is properly bonded to a printed circuit
board and has a maximum thermal resistance of
160°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:
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
dissipation 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 following equation:
P_D = [(V_IN - V_BAT) · I_CH + (V_IN · I_OP)]
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3681.2006.05.1.1