ACT3704 datasheet(8/15 Pages) ACTIVE-SEMI

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ACT3704

Rev2, 26-Jul-07

Innovative Products. Active Solutions.

- 8 -

www.active-semi.com

FUNCTIONAL DESCRIPTION

The ACT3704 is an intelligent, stand-alone Con-

stant-Current, Constant-Voltage control (CC/CV),

linear-mode, single-cell charger for Lithium-Based

cell chemistries. The device incorporates current

and voltage sense circuitry, an internal 12V power

MOSFET, a 120°C thermal-regulation loop that mini-

mizes total charge time, a complete state-machine

that implements charge safety features, and circuitry

that eliminates the reverse-blocking diode required

by conventional charger designs.

The ACT3704 features an accurate charge termina-

tion voltage, programmable fast-charge constant

current, and a programmable charge safety timeout

period. Other features include current-limited nSTAT

and nEOC outputs that can directly drive LED indi-

cators without external resistors or provide a logic-

level status signal to the host microprocessor.

CC/CV Regulation Loop

At the core of the ACT3704 is a CC/CV regulation

loop, which regulates either current or voltage as

necessary to ensure fast and safe charging of the

battery.

In a normal charge cycle, this loop regulates the cur-

rent to the value set by RISET. Charging continues at

this current until the battery voltage reaches the

charge termination voltage. At this point the CV loop

takes over, and charge current is allowed to de-

crease as necessary to maintain charging at the

charge termination voltage.

Setting The Charge Termination Voltage

The ACT3704 offers two pin-programmable battery

termination voltages; connect ADJ to G to select a

4.10V termination voltage, connect ADJ to IN (or to

a voltage greater than 1.4V) to select a 4.20V termi-

nation voltage.

Charge Current Programming

The maximum charging current is programmed by

an external resistor (RISET) connected from ISET to

Calculate RISET as follows:

Where IBAT is Amps.

The voltage at ISET is fixed at 1.20V, and the maxi-

mum charge current at BAT is set by:

The RISET values in Table 1 are standard 1%. Note

that the actual charging current may be limited to a

current that is lower than the programmed fast-

charge current due to the ACT3704’s internal ther-

mal-regulation loop. See the Thermal Regulation

Loop section for more information.

Thermal Regulation Loop

The ACT3704 features an internal thermal regula-

tion loop that reduces the charging current as nec-

essary to ensure that the die temperature does not

rise beyond the thermal regulation threshold of

120°C. This feature protects the ACT3704 against

excessive junction temperature and makes the

ACT3704 more accommodating to aggressive ther-

mal designs. Note, however, that attention to good

thermal designs is required to achieve the fastest

possible charge time by maximizing charge current.

In order to account for the extended total charge

time resulting from operation in thermal regulation

mode, the charge timeout periods are extended

proportionally to the reduction in charge current. In

order to ensure a safe charge, the maximum time-

out periods are limited to 2x the room temperature

values.

The conditions that cause the ACT3704 to reduce

charge current in accordance to the internal thermal

regulation loop can be approximated by calculating

the power dissipated in the part. Most of the power

dissipation is generated from the internal charge

MOSFET (Q1 in the Functional Block Diagram).

The power dissipation is calculated to be approxi-

mately:

Table 1:

Charge Current Programming

RISET(kΩ)

Charge Current (mA)

297

413

470

562

800

989

()

ISET

BAT

Ω

(2)

()

BAT

ISET

Ω

(1)

(

)

BAT

(3)

PD is the power dissipated, VIN is the input supply

voltage, VBAT is the battery voltage and IBAT is the

charge current. The approximate ambient tempera-

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