AAT2552
Total Power Solution for Portable Applications
First, the maximum power dissipation for a given
Event Description
Status
OFF
situation should be calculated:
No battery charging activity
Battery charging via adapter
or USB port
ON
(TJ(MAX)
-
TA)
PD(MAX)
=
θJA
Charging completed
OFF
Where:
PD(MAX) = Maximum Power Dissipation (W)
θJA = Package Thermal Resistance (°C/W)
Table 2: LED Status Indicator.
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.
TJ(MAX) = Maximum Device Junction Temperature
(°C) [135°C]
TA
= Ambient Temperature (°C)
Figure 3 shows the relationship of maximum
power dissipation and ambient temperature of the
AAT2552.
The required ballast resistor values can be esti-
mated using the following formulas:
3.00
2.50
2.00
1.50
1.00
0.50
0.00
(VADP
- VF(LED)
ILED
)
R6 =
Example:
0
20
40
60
80
100
TA (°C)
(5.5V - 2.0V)
2mA
R6 =
= 1.75kΩ
Figure 3: Maximum Power Dissipation.
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.
Thermal Considerations
PD = [(VADP - VBAT) · ICH + (VADP · IOP)]
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 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 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 discussion.
Where:
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]
20
2552.2007.04.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
