Lithium-Ion/Polymer Linear Battery Charger
When these conditions have been met and a bat-
tery is connected to the BAT pin, the AAT3682
checks the state of the battery. If the cell voltage is
below V
MIN
, the AAT3682 begins preconditioning
the cell. This is performed by charging the cell with
10% of the programmed constant current amount.
For example, if the programmed charge current is
500mA, then the preconditioning mode (trickle
charge) current will be 50mA. Cell preconditioning
is a safety precaution for deeply discharged cells
and, furthermore, limits power dissipation in the
pass transistor when the voltage across the device
is largest. The AAT3682 features an optional T2X
mode, which allows faster trickle charging at
approximately two times the default rate. This
mode is selected by connecting the T2X pin to V
SS
.
If an over-temperature fault is triggered, the fast
trickle charge will be latched off, and the AAT3682
will continue at the default 10% charge current.
(recharge threshold voltage) or the AAT3682 is
reset by cycling the input supply through the
power-on sequence. Falling below V
RCH
signals the
IC that it is time to initiate a new charge cycle.
AAT3682
Sleep Mode
When the input supply is disconnected, the device
automatically enters power-saving sleep mode. Only
consuming an ultra-low 2µA current, the AAT3682
minimizes battery drain when it is not charging. This
feature is particularly useful in applications where
the input supply level may fall below the battery
charge or under-voltage lockout level. In such cases,
where the AAT3682 input voltage drops, the device
will enter the sleep mode and automatically resume
charging once the input supply has recovered from
its fault condition. This makes the AAT3682 well suit-
ed for USB battery charger applications.
Constant Current Charging
The cell preconditioning continues until the voltage
on the BAT pin reaches V
MIN
. At this point, the
AAT3682 begins constant current charging (fast
charging). Current level for this mode is pro-
grammed using a current sense resistor R
SENSE
between the VCC and CSI pins. The CSI pin mon-
itors the voltage across R
SENSE
to provide feedback
for the current control loop. The AAT3682 remains
in constant current charge mode until the battery
reaches the voltage regulation point, V
CH
.
Charge Inhibit
The AAT3682 charging cycle is fully automatic;
however, it is possible to stop the device from
charging even when all conditions are met for prop-
er charging. Switching the TS pin to either V
IN
or
GND will force the AAT3682 to turn off the pass
device and wait for a voltage between the low and
high temperature voltage thresholds.
Resuming Charge and the V
RCH
Threshold
The AAT3682 will automatically resume charging
under most conditions when a battery charge cycle
is interrupted. Events such as an input supply
interruption or under voltage, removal and replace-
ment of the battery under charge, or charging a
partially drained battery are all possible. The
AAT3682 will monitor the battery voltage and auto-
matically resume charging in the appropriate mode
based upon the measured battery cell voltage.
This feature is useful for systems with an unstable
input supply, which could be the case when power-
ing a charger from a USB bus supply. It is also
beneficial for charging or "topping off" partially dis-
charged batteries. The only restriction on resum-
ing charge of a battery is that the battery cell volt-
age must be below the battery recharge voltage
threshold (V
RCH
) specification. There is V
RCH
threshold hysteresis built into the charge control
Constant Voltage Charging
When the battery voltage reaches V
CH
during con-
stant current mode, the AAT3682 transitions to con-
stant voltage mode. The regulation voltage is facto-
ry programmed to 4.2V. In constant voltage opera-
tion, the AAT3682 monitors the cell voltage and ter-
minates the charging cycle when the voltage across
R
SENSE
decreases to approximately 10mV.
Charge Cycle Termination, Recharge
Sequence
After the charge cycle is complete, the AAT3682
shuts off the pass device and automatically enters
power-saving sleep mode. Either of two possible
conditions will bring the IC out of sleep mode: the
battery voltage at the BAT pin drops below V
RCH
10
3682.2006.12.1.3
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
