AAT3686
USB Port/AC Adapter
Lithium-Ion/Polymer Battery Charger
Charging Operation
Regardless of which charge input function is selected (i.e., either the adapter input or USB input), the AAT3686
has four basic modes for the battery charge cycle: pre-conditioning/trickle charge; constant current/fast charge;
constant voltage; and end of charge (see Figure 1).
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
I = CC / 10
Termination Threshold
Figure 1: Current vs. Voltage Profile During Charging Phases.
threshold (VMIN). At this point, the AAT3686 begins
Battery Preconditioning
the constant current fast charging phase. The fast
charge constant current (ICC) amplitude is deter-
mined by the charge mode, ADP, USBH or USBL,
Before the start of charging, the AAT3686 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. Also, the cell temperature, as reported by a
thermistor connected to the TS pin from the battery,
must be within the proper window for safe charging.
When these conditions have been met and a battery
is connected to the BAT pin, the AAT3686 checks
the state of the battery. If the cell voltage is below the
preconditioning voltage threshold (VMIN), the
AAT3686 begins preconditioning the cell.
and is programmed by the user via the RSET, RSETH
,
and RSETL resistors. The AAT3686 remains in con-
stant current charge mode until the battery reaches
the voltage regulation point, VBAT
.
Constant Voltage Charging
The system transitions to a constant voltage charg-
ing mode when the battery voltage reaches the
output charge regulation threshold (VBAT) during
the constant current fast charge phase. The regu-
lation voltage level is factory programmed to 4.2V
(±1%). Charge current in the constant voltage
mode drops as the battery cell under charge reach-
es its maximum capacity.
The battery preconditioning trickle charge current is
equal to the fast charge constant current divided by
10 (USBL divided by 2). For example, if the pro-
grammed fast charge current is 500mA, then the
preconditioning mode (trickle charge) current will be
50mA. Cell preconditioning is a safety precaution for
a deeply discharged battery and also aids in limiting
power dissipation in the pass transistor when the
voltage across the device is at the greatest potential.
End of Charge Cycle Termination and
Recharge Sequence
When the charge current drops to 7.5% (35% for
USBL) of the programmed fast charge current level
in the constant voltage mode, the device termi-
nates charging and goes into a standby state. The
charger will remain in a standby state until the bat-
tery voltage decreases to a level below the battery
recharge voltage threshold (VRCH).
Fast Charge / Constant Current Charging
Cell preconditioning continues until the voltage on
the BAT pin exceeds the preconditioning voltage
3686.2006.10.1.9
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
