PRODUCT DATASHEET
AAT2552178
SystemPowerTM
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.
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
The required ballast resistor values can be estimated
using the following formulas:
(VADP
- VF(LED)
ILED
)
R6 =
0
20
40
60
80
100
TA (°°C)
Example:
Figure 3: Maximum Power Dissipation.
(5.5V - 2.0V)
2mA
R6 =
= 1.75kΩ
Next, the power dissipation of the battery charger can be
calculated by the following equation:
Note: Red LED forward voltage (VF) is typically 2.0V @
2mA.
PD = [(VADP - VBAT) · ICH + (VADP · IOP)]
Thermal Considerations
Where:
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.
PD = Total Power Dissipation by the Device
VADP = ADP/USB Voltage
VBAT = Battery Voltage as Seen at the BAT Pin
ICH = Constant Charge Current Programmed for the
Application
IOP = 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.
(PD(MAX)
-
VIN
VIN - VBAT
· IOP)
First, the maximum power dissipation for a given situa-
tion should be calculated:
ICH(MAX)
=
(TJ(MAX) TA)
θJA
VIN - VBAT
-
-
VIN · IOP
(TJ(MAX) - TA)
θJA
PD(MAX)
=
ICH(MAX)
=
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.
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)
w w w . a n a l o g i c t e c h . c o m
2552.2008.02.1.2
19
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