SLUS249B − FEBRUARY 1997 − REVISED DECEMBER 2001
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
APPLICATION INFORMATION
trickle charge state
When the battery’s voltage is below a predetermined threshold, the battery is either deeply discharged or has shorted
cells. The trickle-charge state offers a low-charging current to bring the battery up above zero capacity. In the case
of shorted cells, the trickle-charge state prevents the charger from delivering high currents during this fault condition.
Stacking several cells makes the detection of a shorted cell more difficult.
For lithium-ion batteries, the trickle-charge threshold is typically set to a value around 2.5 V per cell (this corresponds
to near zero capacity). When the cell voltage is below the threshold, only a trickle current is applied to the battery.
The threshold is established by programming CHGENB to 2.05 V when the battery (or stack) voltage is at the
threshold. Referring to the application circuit of Figure 1, the trickle-charge voltage threshold is determined by:
V
TRICKLE_THRESHOLD
+
RS1
)
RS2
)
RS3
RS3
2.05
(6)
With a trickle threshold of 5 V (for two cells) and setting RS3 to 10 kΩ, (RS1+ RS2) should be approximately 14.4 kΩ.
The applications circuit’s trickle-charge current is set to about 7.5% of the bulk-charge current. The current value is
set by picking the appropriate value for RG1. Referring to the block diagram and Figure 1, during trickle charge a fixed
current (0.68/RSET) flows out of the current amplifier’s inverting input and into RG1. The voltage amplifier output is
disabled during trickle charge and acts as a high-impedance node. The resulting voltage at the output of the current
sense amplifier sets the trickle charge current.
I
TRICKLE
+
7.5
RG1
RSET RSENSE
(7)
In the application circuit the sense resistor is 0.18
Ω
and RSET is 162 kΩ, for a trickle current of about 90 mA a 20-kΩ
resistor is selected for RG1.
The converter is typically designed to run in discontinuous-conduction-mode during trickle charge. This allows a
reasonably small value of inductance to be used. The average current mode of the UCC3956 provides improved
discontinuous-duty-cycle control, when compared to peak-current mode implementations.
In Figure 2, the trickle-charge state corresponds to the time interval between t0 (when CHG is transitioned from low
to high) and t1. During the trickle-charge state, STAT0 and STAT1 are logic-level lows. At time t1 the trickle threshold
is met, and the charger transitions to the bulk-charge state. In many instances, the battery voltage is initially above
the trickle threshold. In this case, the trickle-charge state is not needed.
8
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