ACT8740 datasheet(38/40 Pages) ACTIVE-SEMI

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SINGLE-CELL Li+ BATTERY CHARGER (CHGR)

ACT8740

Rev PrB, 25-Feb-08

Innovative Products. Active Solutions.

- 38 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

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 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-

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

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].

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

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.

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.

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

FUNCTIONAL DESCRIPTION CONT’D

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