ACT8740
Rev PrB, 25-Feb-08
SINGLE-CELL Li+ BATTERY CHARGER (CHGR)
FUNCTIONAL DESCRIPTION CONT’D
CHGR/ISET1[ ] to select the desired charge current,
then select this charge current by driving CHGLEV to
a logic low or clear CHGR/ICHGSET[ ] to [0].
Input Capacitor Selection
VIN is the power input pin for the ACT8740 battery
charger. The battery charger is automatically en-
abled whenever a valid voltage is present on VIN. In
most applications, VIN is connected to either a wall
adapter or a USB port. Under normal operation, the
input of the charger will often be “hot-plugged” di-
rectly to a powered USB or a wall adapter cable,
and supply voltage ringing and overshoot may ap-
pear at the VIN pin and can potentially be large
enough to damage the charger input. In most appli-
cations, a capacitor connected from VIN to GA,
placed as close as possible to the IC, is sufficient to
absorb the energy. The VIN pin is designed for en-
hanced robustness and has an absolute maximum
transient voltage rating of +7V, and attention must
be given to bypass techniques to ensure operation
within this limit.
Charge Safety Timer
The ACT8740 features a programmable charge
safety timer that is utilized during operation in the
PRECONDITION state. The safety timer has a de-
fault timeout period of 60 minutes, although it may
be programmed to either 90 minutes or 120 minutes
by writing to the CHGR/TIMOSET[ ] register. This
register also provides a timer-disable function, for
applications that do not require a charge safety
timer function.
Thermal Regulation
The ACT8740 features an internal thermal feedback
loop that reduces the charging current as necessary
to ensure that the die temperature does not exceed
the thermal regulation threshold of 110°C. This fea-
ture protects the ACT8740 against excessive junc-
tion temperature and makes the ACT8740 more
accommodating of aggressive thermal designs with-
out risk of damage. Note, however, that attention to
good thermal design is required to achieve the
shortest possible charge time.
Charge Current Programming
The charger was designed for maximum flexibility,
and charge current programming is performed using
the CHGR/ISET1[ ] and CHGR/ISET2[ ] registers
and (optionally) the multifunction CHGLEV input. For
applications that desire a combination of pin-control
and I2C control, the CHGLEV input provides charge
current selection between the current settings de-
fined by CHGR/ISET1[ ] and CHGR/ISET2[ ]. Alter-
natively, when complete I2C control is desired, sim-
Reverse Leakage Current
The ACT8740 includes internal circuitry that elimi-
nates the need for series blocking diodes, reducing
solution size and cost as well as dropout voltage
relative to conventional battery chargers. When the
input supply is removed, when VVIN goes below its
under-voltage-lockout (UVLO) voltage, or when VVIN
drops below VBAT, the ACT8740 automatically goes
into SUSPEND mode and reconfigures its power
switch to minimize current drain from the battery.
ply connect CHGLEV to
G
and utilize the
CHGR/ICHGSET[ ] bit. The two methods are func-
tionally equivalent, select the charge current pro-
grammed by CHGR/ISET1[ ] by driving CHGLEV to
a logic low or by clearing CHGR/ICHGSET[ ] to [0],
and select the charge current programmed by
CHGR/ISET2[ ] by driving CHGLEV to a logic high or
by setting CHGR/ICHGSET[ ] to [1].
The charger's default settings of CHGR/ISET1[ ]
=[0000] and CHGR/ISET2[ ]=[0000] ensure compati-
bility with lower-current input supplies, such as USB
ports. In the default configuration, drive CHGLEV to
a logic-low for a 90mA charge current, and drive
CHGLEV to a logic-high for 450mA charge current. If
different charge current settings are desired, the
charge current associated with either CHGLEV state
is easily modified via the I2C serial interface. For ex-
ample, in order to maintain compatibility with USB’s
high-current mode as well as charge at a higher cur-
rent if an AC adapter is available, simply reprogram
Charger State Machine
PRECONDITION State
A new charging cycle begins in the PRECONDI-
TION state. In this state, the cell is charged at a
reduced current of either 45mA or 10% of the se-
lected maximum fast-charge current, whichever is
greater. During a normal charge cycle, charging
continues at this rate until VBAT reaches the Precon-
dition Threshold Voltage, at which point the state
machine jumps to the FAST-CHARGE state. If VBAT
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