SLUS249B − FEBRUARY 1997 − REVISED DECEMBER 2001
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
pin descriptions (continued)
STAT0, STAT1:
CMOS open-drain binary-output decode pins indicating the four different charge states. The
maximum 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 completed and overcharge state is initiated when VA– reaches
95% of VA+.
VA+:
The non-inverting input to the voltage-error amplifier 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 4.1 V.
VDD:
The input voltage of the chip. This chip is operational between 6 V and 18 V and should be bypassed with a
0.1-µF capacitor.
Table 1. Charge State Decode Chart
STAT1
Trickle Charge
Bulk Charge
Overcharge
Overcharge (Top Off)
0
0
1
1
STAT0
0
1
0
1
TEST CONDITION
CHGENB < 2.05 V
VA– < 95% VA+,
VA– > 95% VA+,
VIBAT > VIMIN
CHGENB > 2.05V
VIBAT < VIMIN
APPLICATION INFORMATION
The UCC3956 contains all the necessary control functions for implementing an efficient-switch-mode lithium-ion
battery charger. lithium-ion batteries are rapidly becoming the battery of choice for rechargeable portable and laptop
products. When compared to NiCd, NiMH, and lead-acid batteries, lithium-ion offers 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, reduces battery capacity over several charge cycles. Because
lithium-ion batteries have a high average cell voltage of around 3.6 V, they can often replace two to three nickel-based
cells.
The advantages that lithium-ion batteries offer, come at the cost of a wide operating voltage. Near zero capacity, the
cell has a typical voltage of 2.5 V. A fully-charged cell has a typical voltage of 4.1 V. 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 operation. A programmable overcharge 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 cycles through several charge states. If the battery
voltage is severely depleted, the charger begins in a low-current trickle-charge state. When the battery voltage is
above a user-set threshold, the charger initiates a constant-current bulk-charge state. Once the battery reaches 95%
of it’s final voltage, the charger enters an overcharge state. During the overcharge state, the converter transitions
from a constant-current to a constant-voltage mode of operation. Figure 2 shows typical current, voltage, and
capacity levels of a lithium-ion battery during a complete charge cycle.
6
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