AAT3691 Datasheet, PDF (14/18 Pages) Skyworks Solutions Inc. – Dual Input 1.6A Linear Charger with 28V OVP in a 3x4 TDFN Package

DATA SHEET
AAT3691
Dual Input 1.6A Linear Charger with 28V OVP in a 3x4 TDFN Package
The LED anodes should be connected to either V_USBCH or
V_ADPCH, depending upon the system design requirements.
The LEDs should be biased with as little current as nec-
essary to create reasonable illumination; therefore, a
ballast resistor should be adopted to limit the current
flowing through the LED by connecting it with the LED in
series between STAT and V_USBCH or V_ADPCH. LED current
consumption will add to the overall thermal power bud-
get for the device package, so LED drive current should
be kept 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 value can be estimated using
the following formulas:
To connect to ADPCH:
R=
V_ADPCH - V_FLED
I_LED
To connect to USBCH:
R=
V_USBCH - V_FLED
I_LED
For example, using a red LED with 2.0V V_F @ 2mA, cal-
culate R under 5.5V V_ADPCH:
R=
5.5V - 2.0V
2mA
= 1.75kΩ
Capacitor Selection
Input Capacitor
An input capacitor is used to filter the input voltage by
placing a decoupling capacitor between the ADP, ADPCH,
USB and USBCH pins and ground. An input capacitor in
the range of 1μF to 10μF is recommended. If the source
supply is unregulated, it may be necessary to increase
the capacitance to keep the input voltage above the
under-voltage lockout threshold during device enable
and when battery charging is initiated. This input capac-
itor range is also suitable for a system with an external
power supply source, such as a typical AC-to-DC wall
adapter. It will minimize switching or power bounce
effects when the power supply is “hot plugged”. Likewise,
a 2.2μF or greater input capacitor is recommended for
the USB input to help buffer the effects of USB source
power switching, noise, and input cable impedance.
Output Capacitor
The AAT3691 requires a 1μF ceramic capacitor on the
BAT pin to maintain circuit stability. This value should be
increased to 10μF or more if the battery connection is
made any distance from the charger output. In a fast
charge application with current above 1A, a 22μF output
capacitor is required to obtain an accurate recharge volt-
age threshold. If the AAT3691 is used in applications
where the battery can be removed from the charger,
such as in the case of desktop charging cradles, an out-
put capacitor value greater than 10μF may be required
to prevent the device from cycling on and off when no
battery is present.
Thermal Considerations
The actual maximum charging current is a function of
the charge input voltage (USBCH and ADPCH), the bat-
tery voltage at the BAT pin, the ambient temperature,
the rising temperature when charge current passing
through the R_DS(ON) of the charging pass, and the thermal
impedance of the package. The maximum programma-
ble current may not be achievable under all operating
parameters.
The AAT3691 is offered in a 3x4mm TDFN package
which can provide up to 2.0W of power dissipation when
properly soldered to a printed circuit board and has a
maximum thermal resistance of 50°C/W. Many consider-
ations 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 ambi-
ent temperature around the charger IC will also have an
effect on the thermal limits of a battery charging appli-
cation. The maximum limits that can be expected for a
given ambient condition can be estimated by the follow-
ing discussion:
First, the maximum power dissipation for a given situa-
tion should be calculated:
Where:
P_D = [(V_IN - V_BAT) · I_CC + (V_IN · I_OP)]
P_D = Total power dissipation of the AAT3691
V_IN = V_ADP or V_USB, depending on which mode is selected
V_BAT = Battery voltage at the BAT pin
I_CC = Maximum constant fast charge current pro-
grammed for the application
I_OP = Quiescent current consumed by the charger IC
for normal operation.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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201894B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • April 27, 2012