AAT3681A
500mA USB Port or AC Adapter
Lithium-Ion/Polymer Battery Charger
Preconditioning
Trickle Charge
Phase
Constant Current
Charge Phase
Constant Voltage
Charge Phase
Charge Complete Voltage
Regulated Current
I = Max CC
Constant Current Mode
Voltage Threshold
Trickle Charge and
Termination Threshold
I = CC / 10
Figure 1: Current vs. Voltage Profile During Charging Phases.
Battery Preconditioning
Constant Voltage Charging
Before the start of charging, the AAT3681A checks
several conditions in order to assure a safe charging
environment. The input supply must be above the
minimum operating voltage, or under-voltage lock-
out threshold (VUVLO), for the charging sequence to
begin. When these conditions have been met and a
battery is connected to the BAT pin, the AAT3681A
checks the state of the battery. If the cell voltage is
below the preconditioning voltage threshold (VMIN),
the charge control begins preconditioning the cell.
The battery preconditioning trickle charge current is
equal to the fast charge constant current divided by
10. For example, if the programmed fast charge cur-
rent is 300mA, then the preconditioning mode (trick-
le charge) current will be 30mA. Cell preconditioning
is a safety precaution for deeply discharged battery
cells and also aids in limiting power dissipation in the
pass transistor when the voltage across the device
is at the greatest potential.
The system transitions to a constant voltage charg-
ing mode when the battery voltage reaches the
output charge regulation threshold (VBAT_EOC) dur-
ing the constant current fast charge phase. The
regulation voltage level is factory programmed to
4.2V ( 0.5%). Charge current in the constant volt-
age mode drops as the battery cell under charge
reaches its maximum capacity.
End of Charge Cycle Termination and Recharge
Sequence
When the charge current drops to 10% of the pro-
grammed fast charge current level in the constant
voltage mode, the device terminates charging and
goes into a sleep state. The charger will remain in
a sleep state until the battery voltage decreases to
a level below the battery recharge voltage thresh-
old (VRCH). (AAT3681A-3 does not terminate
charging and thus does not go into the sleep state.
AAT3681A-3 continues to source current to the
battery and allows the charge current to asymptot-
ically approach zero while in constant voltage
charging mode.)
Constant Current Charging
Battery cell preconditioning continues until the volt-
age on the BAT pin exceeds the preconditioning
voltage threshold (VMIN). At this point, the
AAT3681A begins the constant current charging
phase. The charge constant current (ICH) amplitude
is programmed by the user via the RSET resistor.
The AAT3681A remains in the constant current
charge mode until the battery reaches the voltage
regulation point, VBAT_EOC.
Consuming very low current in sleep state, the
AAT3681A minimizes battery drain when it is not
charging. This feature is particularly useful in appli-
cations where the input supply level may fall below
the battery charge or under-voltage lockout level.
In such cases where the AAT3681A input voltage
drops, the device will enter sleep state and auto-
matically resume charging once the input supply
has recovered from the fault condition.
3681A.2007.10.1.2
9
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