PRODUCT DATASHEET
AAT3688
BatteryManager
TM
Preconditioning
Trickle Charge
Phase
Charge Complete Voltage
Regulated Current
I = Max CC
USB Port Lithium-Ion/Polymer Battery Charger
Constant Current
Charge Phase
Constant Voltage
Charge Phase
Constant Current Mode
Voltage Threshold
Trickle Charge and
Termination Threshold
I = CC / 10
Figure 1: Current vs. Voltage Profile During Charging Phases.
Charging Operation
The AAT3688 has four basic modes for the battery
charge cycle and is powered from the USB input: pre-
conditioning/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.
constant current fast charging phase. The fast charge
constant current (I
CC
) amplitude is determined by the
selected charge mode USBH or USBL and is programmed
by the user via the R
SETH
and R
SETL
resistors. The
AAT3688 remains in 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 output charge
regulation threshold (V
BAT
) during the constant current,
fast charge phase. The regulation voltage level is fac-
tory programmed to 4.2V ( 1%). The charge current in
the constant voltage mode drops as the battery cell
under charge reaches its maximum capacity.
Battery Preconditioning
Before the start of charging, the AAT3688 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. In addition,
the cell temperature, as reported by a thermistor con-
nected 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 AAT3688 checks the state of the battery. If the
cell voltage is below the Preconditioning Voltage Threshold
(V
MIN
), the AAT3688 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 preconditioning is a safety pre-
caution 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% 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
also automatically enter power-saving sleep mode. Only
consuming an ultra-low 0.3μA in sleep mode, the
AAT3688 minimizes battery drain when it is not charg-
ing. 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 AAT3688 input voltage drops, the device will
enter the sleep mode and automatically resume charg-
ing once the input supply has recovered from its fault
condition.
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 AAT3688 begins the
3688.2007.12.1.6
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