UCC3956

SWITCHMODE LITHIUMION

BATTERY CHARGE CONTROLLER

SLUS249B − FEBRUARY 1997 − REVISED DECEMBER 2001

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APPLICATION INFORMATION

bulk-charge state

As the name implies, the bulk-charge state is responsible for restoring a majority of the charge back into the battery.

The bulk-charge current is determined by the C-rate and the capacity of the battery. In the application circuit, two

stacked 1200-mAh batteries are charged at a 1C rate. This requires 1.2 A of current during bulk charge. In this case,

a fully-discharged battery takes about 60 minutes to reach approximately 80% capacity. Battery packs with a high

ESR typically have a shorter bulk period, due to the voltage drop generated by the bulk current and the ESR of the

battery.

Both the voltage-sense amplifier and current-sense amplifier are enabled during bulk charge. The voltage amplifier

is saturated in this state as the battery voltage is slowly rising, but is not yet high enough to drive the voltage amplifier

into regulation. The voltage amplifier output is clamped at a nominal voltage of 4.1 V. The current-sense amplifier

is configured so that its output voltage increases with decreasing RSENSE current. RSENSE should be sized such

V

IBAT BULK +

2.05

* 5

IBULK

RSENSE

With 1.2 A of bulk current and the current sense amplifier output set at 1 V, a sense resistor of 0.18

Ω is required. As

always, power dissipation and converter efficiency must be considered when choosing RSENSE.

Referring to the feedback diagram of Figure 3, the output of the voltage-sense amplifier and current-sense amplifier

are summed at the inverting input of the current amplifier. Assuming that the current-sense amplifier is within

regulation, the required value of RG2 can be calculated. The application circuit uses a value of 38.3 k

Ω for RG2,

setting the bulk current to 1.2 A.

RG2

+

2.05

RG1

5

IBULK

RSENSE

Referring to Figure 2, the bulk-charge state corresponds to the interval between t1 and t2. The step-in voltage at time

t1 is caused by bulk current flowing into the battery ESR and sense resistor. In the bulk-charge state, STAT0 is a

logic-level high and STAT1 is a logic-level low.

UDG−96263−1

Figure 3. Typical Charge Cycle Levels

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