AAT3686
USB Port/AC Adapter
Lithium-Ion/Polymer Battery Charger
Timing Diagram
SQPULSE
PDATA
SQ
System Reset
System Start
CK
TSYNC
TLAT
TOFF
Data
TDATA(RPT) = TSYNC + TLAT < 2.5 PDATA
TOFF > 2 PDATA
N=3
N=1
N=2
VBAT = Battery voltage as seen at the BAT pin
Thermal Considerations
The AAT3686 is offered in a 3x4mm TDFN pack-
age and a 4x4mm TDFN package, each of which
can provide up to 2.7W of power dissipation when
it is properly bonded to a printed circuit board and
has a maximum thermal resistance of 37°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 charger 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:
ICC = Maximum constant fast charge current pro-
grammed for the application
IOP = Quiescent current consumed by the charg-
er IC for normal operation
Next, the maximum operating ambient temperature
for a given application can be estimated based on
the thermal resistance of the 3x4mm and 4x4mm
TDFN packages when sufficiently mounted to a
PCB layout and the internal thermal loop tempera-
ture threshold.
Eq. 2:
TA = TJ - (θJA · PD)
First, the maximum power dissipation for a given
situation should be calculated:
Where:
TA
TJ
= Ambient temperature in degrees C
Eq. 1:
PD = [(VIN - VBAT) · ICC + (VIN · IOP)]
= Maximum device junction temperature
below the thermal loop threshold
PD = Total power dissipation by the device
Where:
θJA = Package thermal resistance in °C/W
PD = Total power dissipation by the device
VIN = Either VADP or VUSB, depending on which
mode is selected
3686.2006.10.1.9
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
