AAT3685 Datasheet, PDF (11/23 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3685
Lithium-Ion/Polymer Battery Charger
Charge Complete Voltage
Regulated Current
Preconditioning
Trickle Charge
Phase
Constant Current
Charge Phase
I = Max CC
Constant Voltage
Charge Phase
Constant Current Mode
Voltage Threshold
Trickle Charge and I = CC / 10
Termination Threshold
Figure 1: Current vs. Voltage Profile During Charging Phases.
Charging Operation
The AAT3685 has four basic modes for the battery
charge cycle and is powered from the input: pre-con-
ditioning/trickle charge; constant current/fast charge;
constant voltage; and end of charge. For reference,
Figure 1 shows the current versus voltage profile
during charging phases.
Battery Preconditioning
Before the start of charging, the AAT3685 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 (V_UVLO), for the charging sequence to
begin. In addition, the cell temperature, as reported
by a thermistor connected to the TS pin from the bat-
tery, 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 AAT3685
checks the state of the battery. If the cell voltage is
below the Preconditioning Voltage Threshold (V_MIN),
the AAT3685 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
current is 500mA, then the preconditioning mode
(trickle charge) current will be 50mA. Cell precon-
ditioning is a safety precaution for a deeply dis-
charged battery and also aids in limiting power dis-
sipation in the pass transistor when the voltage
across the device is at the greatest potential.
Fast Charge / Constant Current Charging
Battery cell preconditioning continues until the volt-
age on the BAT pin exceeds the Preconditioning
Voltage Threshold (V_MIN). At this point, the AAT3685
begins the constant current fast charging phase.
The fast charge constant current (I_CC) amplitude is
determined by the selected charge mode SETH or
SETL and is programmed by the user via the R_SETH
and R_SETL resistors. The AAT3685 remains in con-
stant current charge mode until the battery reaches
the voltage regulation point, V_BAT.
Constant Voltage Charging
The system transitions to a constant voltage charg-
ing mode when the battery voltage reaches output
charge regulation threshold (V_BAT) during the con-
stant current, fast charge phase. The regulation
voltage level is factory programmed to 4.2V ( 1%).
The charge current in the constant voltage 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 7.5% of the pro-
grammed fast charge current level in the constant volt-
age 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 threshold (V_RCH).
When the input supply is disconnected, the charg-
er will also automatically enter power-saving sleep
mode. Only consuming an ultra-low 0.3μA in sleep
mode (1μA for AAT3685-1), the AAT3685 mini-
mizes 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 AAT3685 input voltage drops, the
device will enter the sleep mode and automatically
resume charging once the input supply has recov-
ered from its fault condition.
3685.2006.10.1.3
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