AAT3687_08 Datasheet, PDF (10/20 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

BatteryManager^TM
PRODUCT DATASHEET
AAT3687
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 AAT3687 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.) (AAT3687-2 does not have trickle charge.)
Battery Preconditioning
Before the start of charging, the AAT3687 checks sev-
eral conditions in order to assure a safe charging envi-
ronment. The input supply must be above the minimum
operating voltage, or under-voltage lockout threshold
(V_UVLO), 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 volt-
age is below the preconditioning voltage threshold (V_MIN),
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 current is
1.5A, then the preconditioning mode (trickle charge)
current will be 150mA. 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.
Fast Charge / Constant Current Charging
Battery cell preconditioning continues until the voltage
on the BAT pin exceeds the preconditioning voltage
threshold (V_MIN). At this point, the AAT3687 begins the
constant current fast charging phase. The fast charge
constant current (I_CC) amplitude is programmed by the
user via the R_SET resistor. The AAT3687 remains in the
constant current charge mode until the battery reaches
the voltage regulation point, V_BAT.
Constant Voltage Charging
The system transitions to a constant voltage charging
mode when the battery voltage reaches the output
charge regulation threshold (V_BAT) during the constant
current fast charge phase. The regulation voltage level
is factory programmed to 4.2V (±1%). 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 charger 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 applications 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
automatically resume charging once the input supply
has recovered from the fault condition.
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