AAT2556 Datasheet, PDF (17/29 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
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.
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 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 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 generating devices in a
given application design. The ambient temperature
around the IC will also have an effect on the ther-
mal limits of a battery charging application. 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
situation should be calculated:
P_D(MAX) =
(T_J(MAX) - T_A)
θ_JA
2556.2006.09.1.2
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 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 current 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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