AAT3681A
500mA USB Port or AC Adapter
Lithium-Ion/Polymer Battery Charger
The required ballast resistor values can be esti-
mated using the following formulas:
Protection Circuitry
Over-Voltage Protection
An over-voltage event is defined as a condition
where the voltage on the BAT pin exceeds the
maximum battery charge voltage and is set by the
over-voltage protection threshold (VOVP). If an over-
voltage condition occurs, the AAT3681A charge
control will shut down the device until voltage on
the BAT pin drops below VOVP. The AAT3681A will
resume normal charging operation after the over-
voltage condition is removed.
(VADP
-
ILED
VF(LED)
)
R1=
Example:
(5.5V - 2.0V)
2mA
R1 =
= 1.75kΩ
Over-Temperature Shutdown
Note: Red LED forward voltage (VF) is typically
2.0V @ 2mA.
The AAT3681A has a thermal protection control cir-
cuit which will shut down charging functions should
the internal die temperature exceed the preset ther-
mal limit threshold. Once the internal die tempera-
ture falls below the thermal limit, normal operation
will resume the previous charging state.
Thermal Considerations
The AAT3681A 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.
Charge Status Output
The AAT3681A 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
Status
OFF
No battery charging activity
Battery charging via adapter
or USB port
ON
First, the maximum power dissipation for a given
situation should be calculated:
1
Charging completed
OFF
Table 2: LED Status Indicator.
(TJ(MAX) - TA)
θJA
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 LEDs. It is not recommended to exceed 8mA
for driving an individual status LED.
PD(MAX)
=
Where:
PD(MAX) = Maximum Power Dissipation (W)
θJA = Package Thermal Resistance (°C/W)
TJ(MAX) = Maximum Device Junction Temperature
(°C) [135°C]
TA
= Ambient Temperature (°C)
1. AAT3681A-3 does not terminate charging. However, the status pin turns off when IBAT ≤ ITERM and turns back on when VBAT
BAT_EOC - 0.1V.
≤
V
12
3681A.2007.10.1.2
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