ACT3704
Rev2, 26-Jul-07
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.
Table 1:
Charge Current Programming
R
ISET
(kΩ)
89
64
56
47
33
27
Charge Current (mA)
297
413
470
562
800
989
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 R
ISET.
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.
The R
ISET
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:
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 (R
ISET
) connected from ISET to
G.
Calculate R
ISET
as follows:
R
ISET
=
22k
Ω
×
(
1.20V / I
BAT
)
Where I
BAT
is Amps.
(1)
P
D
=
(
V
IN
- V
BAT
)
×
I
BAT
(3)
The voltage at ISET is fixed at 1.20V, and the maxi-
mum charge current at BAT is set by:
I
BAT
=
22k
Ω
×
(
1.20V / R
ISET
)
Innovative Products.
Active Solutions.
(2)
P
D
is the power dissipated, V
IN
is the input supply
voltage, V
BAT
is the battery voltage and I
BAT
is the
charge current. The approximate ambient tempera-
-8-
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