SLUS249B − FEBRUARY 1997 − REVISED DECEMBER 2001
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
APPLICATION INFORMATION
overcharge state
The overcharge state of the converter starts when the battery reaches 95% of its final voltage (time t2 of Figure 2).
The overcharge state is initiated when the voltage at the inverting input of the voltage amplifier is 95% of the
non-inverting-input voltage. Using 95% rather than 100% of the final battery voltage assures that the overcharge timer
is always set before the battery current tapers off. At the beginning of overcharge state, STAT0 indicates a logic-level
low and STAT1 indicates a logic-level high.
In the application circuit of Figure 1, the voltage at which overcharge is initiated is set by resistors RS1, RS2 and RS3.
These resistors are also used to set the trickle-charge threshold. A 0.1-µF decoupling capacitor is added to this node
as a filter. The battery (or stack) voltage that initiates the overcharge state is:
VOC_THRESHOLD
+
0.95
RS1
)
RS2
)
RS3
RS2
)
RS3
4.1
(10)
For a single-cell stack, RS1 should be 0
Ω.
This results in a final battery voltage of 4.1 V. It is important not to charge
a lithium-ion battery above 4.2 V. When charging a battery stack, RS1 should be selected to properly set the
final-stack voltage. In the application circuit, RS1 is selected to be 12.21 kΩ and RS2 is selected to be 2.21 kΩ. This
sets the overcharge level at 8.2 V, while setting the trickle-charge threshold to about 5 V.
The battery voltage at the beginning of the overcharge state may not correspond to the voltage amplifier coming out
of saturation. Therefore, bulk current may continue in the battery during the initial portion of the overcharge state (see
Figure 2). When the voltage amplifier comes into regulation, the amplifier’s output voltage begins to decrease. The
current-sense amplifier’s output voltage needs to increase in order for the current amplifier’s inverting input to remain
at 2.05 V. This translates into a decreasing battery current. The battery current continues to decrease as the battery
approaches 100% capacity.
Although the bulk-charge state restores a majority of the capacity to the battery, the overcharge state typically takes
a majority of the charge-cycle time. The bulk-charge state usually takes one-third of the total charge time, while the
overcharge state takes the remaining two-thirds. Different methods are used to terminate the charge of lithium-ion
batteries. Many chargers use a current threshold to terminate charge. While this method is simple to implement, the
current tail near the end of charge is often quite flat (see Figure 2). To make matters worse, the current level versus
battery capacity may differ from cell to cell. This makes it difficult to accurately terminate at 100% capacity. In order
to avoid the possibility of overcharging the battery, the design may require termination at a higher current level (before
100% capacity is reached). A more predictable method of charge termination is to use a fixed overcharge time.
The UCC3956 provides current-level detection as well as a timer. In a typical design, the current-level detection is
used to give an indication of near-full charge. As shown in Figure 2 this occurs at time t4. This indication is useful since
the time to charge from t4 to t5 may be quite long. Since lithium-ion batteries have no memory effect, there is little
reason to have the user wait for the battery to be 100% charged. If the battery is not taken from the charger at time
t4, the charger continues charging. The timer expires and the charge cycle terminates at time t5.
A typical value used to indicate near-full charge is one−tenth of the bulk current value. This current level is established
by setting the appropriate voltage on IMIN. IMIN is tied to an internal comparator along with the output of the current
sense amplifier. When the current sense amplifier voltage becomes greater than the voltage on IMIN, the internal
state machine indicates near full charge by setting STAT0 and STAT1 to logic level highs. In the application circuit
of Figure 1, resistors RS4 and RS5 determine the voltage at IMIN. With RS4 at 11 kΩ and RS5 at 10 kΩ, near full
charge is indicated at 120 mA.
V
IMIN
+
4.1
RS5
(RS4
)
RS5)
+
2.05
*
V
5
R
IMIN
(12)
(11)
I
NEAR_FULL
SENSE
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