AAT2556 Datasheet, PDF (16/27 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

DATA SHEET
AAT2556
Battery Charger and Step-Down Converter for Portable Applications
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:
Example:
R₁ =
V_ADP - V_F(LED)
I_LED
R₁ =
5.5V - 2.0V
2mA
= 1.75kΩ
Note: Red LED forward voltage (V_F) is typically 2.0V @
2mA.
Thermal Considerations
The AAT2556 is offered in a TDFN33-12 package which
can provide up to 2W of power dissipation when it is
properly bonded 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 gener-
ating devices in a given application design. The ambient
temperature around the IC will also have an effect on the
thermal limits of a battery charging application. The
maximum limits that can be expected for a given ambi-
ent condition can be estimated by the following discus-
sion.
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)
_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 the AAT2556.
3000
2500
2000
1500
1000
500
0
0
20
40
60
80
100
120
T_A (°C)
Figure 3: Maximum Power Dissipation.
Next, the power dissipation of the battery charger can
be calculated by the following equation:
P_D = [(V_ADP - V_BAT) · I_CH + (V_ADP · I_OP)]
Where:
P_D
= Total Power Dissipation by the Device
V_ADP = ADP/USB 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.
I_CH(MAX) =
(P_D(MAX) - V_IN · I_OP)
V_IN - V_BAT
I_CH(MAX) =
(T_J(MAX) -
θ_JA
T_A)
-
V_IN
·
I_OP
V_IN - V_BAT
In general, the worst condition is the greatest voltage
drop across the IC, when battery voltage is charged up
to the preconditioning voltage threshold. Figure 4 shows
the maximum charge current in different ambient tem-
peratures.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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202177B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 19, 2013