AAT2552_08 Datasheet, PDF (19/31 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2552178
Total Power Solution for Portable Applications
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:
R₆ =
(V_ADP - V_F(LED))
I_LED
Example:
R₆ =
(5.5V - 2.0V)
2mA
= 1.75kΩ
Note: Red LED forward voltage (V_F) is typically 2.0V @
2mA.
Thermal Considerations
The AAT2552 is offered in a TDFN34-16 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 gen-
erating devices in a given application design. The ambi-
ent 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
ambient condition can be estimated by the following dis-
cussion.
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)
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 dis-
sipation and ambient temperature of the AAT2552.
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0
20
40
60
80
100
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.
2552.2008.02.1.2
www.analogictech.com
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