Datasheet

LM3420
SNVS116D –MAY 1998–REVISED MAY 2013 www.ti.com
Figure 33. High Efficiency Switching Regulator Constant Current/Constant Voltage 2-Cell Charger
Figure 34. Low Dropout Constant Current/Constant Voltage Li-Ion Battery Charger
A switching regulator, constant-current, constant-voltage two-cell Li-Ion battery charging circuit is shown in
Figure 33. This circuit provides much better efficiency, especially over a wide input voltage range than the linear
topologies. For a 1A charger an LM2575-ADJ. switching regulator IC is used in a standard buck topology. For
other currents, or other packages, other members of the SIMPLE SWITCHER™ buck regulator family may be
used.
Circuit operation is as follows. With a discharged battery connected to the charger, the circuit operates as a
constant current source. The constant-current portion of the charger is formed by the loop consisting of one half
of the LM358 op amp along with gain setting resistors R3 and R4, current sensing resistor R5, and the feedback
reference voltage of 1.23V. Initially the LM358's output is low causing the output of the LM2575-ADJ. to rise thus
causing some charging current to flow into the battery. When the current reaches 1A, it is sensed by resistor R5
(50 mΩ), and produces 50 mV. This 50 mV is amplified by the op-amps gain of 25 to produce 1.23V, which is
applied to the feedback pin of the LM2575-ADJ. to satisfy the feedback loop.
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