AAT3687
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
I = CC / 10
Termination Threshold
Figure 1: Current vs. Voltage Profile During Charging Phases.
The fast charge constant current (ICC) amplitude is
Charging Operation
programmed by the user via the RSET resistor. The
AAT3687 remains in the constant current charge
mode until the battery reaches the voltage regula-
The AAT3687 has four basic modes for the battery
charge cycle: pre-conditioning / trickle charge; con-
stant current / fast charge; constant voltage; and
end of charge (see Figure 1.) (AAT3687-2 does not
have trickle charge.)
tion 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.
Battery Preconditioning
Before the start of charging, the AAT3687 checks
several conditions in order to assure a safe charging
environment. The input supply must be above the
minimum operating voltage, or under-voltage lockout
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 AAT3687 checks the
state of the battery. If the cell voltage is below the
preconditioning voltage threshold (VMIN), the charge
control begins preconditioning the cell.
End of Charge Cycle Termination and Recharge
Sequence
When the charge current drops to 7.5% 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).
The battery preconditioning trickle charge current is
equal to the fast charge constant current divided by
10. For example, if the programmed fast charge
current is 1.5A, then the preconditioning mode
(trickle charge) current will be 150mA. Cell precon-
ditioning is a safety precaution for deeply dis-
charged battery cells and also aids in limiting power
dissipation in the pass transistor when the voltage
across the device is at the greatest potential.
When the input supply is disconnected, the charg-
er will automatically transition into a power-saving
sleep mode. Only consuming an ultra-low 0.3μA in
sleep mode (1μA for AAT3687-1 and AAT3687-2),
the AAT3687 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 AAT3687 input voltage
drops, the device will enter sleep mode and auto-
matically resume charging once the input supply
has recovered from the fault condition.
Fast Charge / 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 AAT3687
begins the constant current fast charging phase.
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3687.2006.10.1.7
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
