AAT3688
USB Port 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.
begins the constant current fast charging phase.
Charging Operation
The fast charge constant current (ICC) amplitude is
determined by the selected charge mode USBH or
USBL and is programmed by the user via the RSETH
and RSETL resistors. The AAT3688 remains in con-
stant current charge mode until the battery reaches
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 ref-
erence, Figure 1 shows the current versus voltage
profile during charging phases.
the voltage regulation point, VBAT
.
Constant Voltage Charging
Battery Preconditioning
The system transitions to a constant voltage charg-
ing mode when the battery voltage reaches output
charge regulation threshold (VBAT) 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.
Before the start of charging, the AAT3688 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. 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 AAT3688
checks the state of the battery. If the cell voltage is
below the Preconditioning Voltage Threshold (VMIN),
the AAT3688 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 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 (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 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.
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, the AAT3688 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 AAT3688 input volt-
age drops, the device will enter the sleep mode and
automatically resume charging once the input sup-
ply has recovered from its 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 AAT3688
3688.2006.10.1.5
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
