AAT2557
500mA Battery Charger and 300mA
LDO Regulator for Portable Systems
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.
Event Description
Status
OFF
No battery charging activity
Battery charging via adapter
or USB port
ON
Charging completed
OFF
First, the maximum power dissipation for a given
situation should be calculated:
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) - TA)
θJA
PD(MAX)
=
Where:
PD(MAX) = Maximum Power Dissipation (W)
θJA = Package Thermal Resistance (°C/W)
TJ(MAX) = Maximum Device Junction Temperature
(°C) [135°C]
The required ballast resistor values can be esti-
mated using the following formulas:
TA
= Ambient Temperature (°C)
PD = [(VADP - VBAT) · ICH + (VADP · IOP)] + (VINLDO - VOUTLDO) ILDOLOAD + VINLDO · IQLDO
(VADP
- VF(LED)
ILED
)
R1=
Where:
Example:
PD
= Total Power Dissipation by the Device
= ADP/USB Voltage
VADP
VBAT
ICH
(5.5V - 2.0V)
2mA
R1 =
= 1.75kΩ
= Battery Voltage as Seen at the BAT Pin
= Constant Charge Current Programmed
for the Application
Note: Red LED forward voltage (VF) is typically
2.0V @ 2mA.
IOP
= Quiescent Current Consumed by the
Charger IC for Normal Operation [0.5mA]
Thermal Considerations
VINLDO = Input Voltage as Seen at the INLDO Pin
The AAT2557 is offered in a TSOPJW-14 package
which can provide up to 625mW of power dissipation
when it is properly bonded to a printed circuit board
and has a maximum thermal resistance of 160°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
VOUTLDO = Output Voltage as Seen at the OUTLDO
Pin
ILDOLOAD = LDO Load Current
IQLDO
= LDO Quiescent Current
2557.2007.06.1.0
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