UCC2956 Datasheet, PDF (4/10 Pages) Texas Instruments – Switch Mode Lithium-Ion Battery Charger Controller

UCC2956
UCC3956
PIN DESCRIPTIONS (cont.)
Frequency =
3.475
(COSC + 20pF) • RSET
A rising edge on CHG initiates the oscillator.
CTO: The slow oscillator ramp pin which is used to gen-
erate a clock signal for the 14 bit timer to program the
over charge time. A capacitor to ground is charged and
discharged with equal currents at a frequency pro-
grammed between 0.75Hz to 5Hz. The ramp oscillates
between 1.0V to 3.0V and the frequency is determined
by:
Frequency =
0.06
CTO • RSET
The oscillator operates only while in overcharge.
GND: The reference point for the internal reference, all
thresholds, and the return for the remainder of the de-
vice.
IBAT: The output of the current sense amplifier.
IMIN: The minimum charge current programming pin is
provided to program an optional charge termination in
addition to the programmable timer.
OUT: The output of the PWM driver.
REF: The 4.1V precision reference which should be by-
passed with a 0.1µF capacitor.
RSET: This pin programs the charge current for the oscil-
lator ramp. The oscillator charge current is determined
by:
1.37V _.
RSET
The trickle control current (Itrck_control) is determined
by:
0.68V _.
RSET
STAT0, STAT1: CMOS open drain binary output decode
pins indicating the four different charge states. The maxi-
mum high voltage sense comparator.
VA–: The inverting input to the voltage error amplifier that
is used as a battery sense input. It is also the input to the
voltage sense comparator. The bulk charge state is com-
pleted and over charge state is initiated when VA–
reaches 95% of VA+.
VA+: The non-inverting input to the voltage error ampli-
fier that is used as the battery charge reference voltage.
VAO: The output of the voltage error amplifier. The upper
output clamp of this amplifier is 4.1V.
VDD: The input voltage of the chip. This chip is opera-
tional between 6V and 18V and should be bypassed with
a 0.1µF capacitor.
CHARGE STATE DECODE CHART
Trickle Charge
Bulk Charge
Over Charge
Over Charge (Top Off)
STAT1
0
0
1
1
STAT0
0
1
0
1
CHGENB < 2.05V
VA– < 95% VA+ and CHGENB > 2.05V
VA– > 95% VA+ and VIBAT< VIMIN
VIBAT > VIMIN
APPLICATION INFORMATION
The UCC3956 contains all the necessary control func-
tions for implementing an efficient switch mode Lithium-
Ion battery charger. Lithium-Ion batteries are rapidly be-
coming the battery of choice for rechargeable portable
and lap top products. When compared to NiCd, NiMH,
and Lead Acid batteries, Lithium-Ion offer less weight
and volume for the same energy. Lithium-Ion batteries do
not suffer from the memory effect found in NiCd batteries.
This effect, caused by not completely discharging and
charging a battery, will reduce battery capacity over sev-
eral charge cycles. Because Lithium-Ion batteries have a
high average cell voltage of around 3.6V, they can often
replace 2 to 3 Nickel based cells.
The advantages that Lithium-Ion batteries offer come at
the cost of a wide operating voltage. Near zero capacity,
the cell will typically have a voltage of 2.5V. A fully
charged cell will typically have a voltage of 4.1V. Unlike
many so called “smart” or “universal” chargers, the
UCC3956 is optimized for Lithium-Ion characteristics. In
order to restore capacity quickly, the chip features both
constant current and constant voltage modes of opera-
tion. A programmable over charge time, provided by the
UCC3956 timer, allows the charger to predictably restore
100% capacity to the battery.
Charger Operation
When CHG is transitioned from a low to high logic level,
the chip will cycle through several charge states. If the
battery voltage is severely depleted, the charger will be-
gin in a low current trickle charge state. When the bat-
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