UCC3956

SWITCHMODE LITHIUMION

BATTERY CHARGE CONTROLLER

SLUS249B − FEBRUARY 1997 − REVISED DECEMBER 2001

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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

STAT0

TEST CONDITION

Trickle Charge

0

0

CHGENB < 2.05 V

Bulk Charge

0

1

VA– < 95% VA+,

CHGENB > 2.05V

Overcharge

1

0

VA– > 95% VA+,

VIBAT < VIMIN

Overcharge (Top Off)

1

1

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.