LM3420 Datasheet, PDF (12/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The LM3420 regulator/driver provides the reference and feedback drive functions for a Lithium-Ion battery
charger. It can be used in many different charger configurations using both linear and switching topologies to
provide the precision needed for charging lithium-ion batteries safely and efficiently. Output voltage tolerances
better than 0.5% are possible without using trim pots or precision resistors. The circuits shown are designed for
2-cell operation, but they can readily be changed for either 1-, 3-, or 4-cell charging applications.
One item to keep in mind when designing with the LM3420 is that there are parasitic diodes present. In some
designs, under special electrical conditions, unwanted currents may flow. Parasitic diodes exist from OUT to IN,
as well as from GROUND to IN. In both instances the diode arrow is pointed toward the IN pin.
9.2 Typical Application: Constant Current/Constant Voltage Li-Ion Battery Charger
The circuit shown in Figure 14 performs constant-current, constant-voltage charging of two Li-Ion cells. At the
beginning of the charge cycle, when the battery voltage is less than 8.4 V, the LM3420 sources no current from
the OUT pin, keeping Q2 off, thus allowing the LM317 Adjustable voltage regulator to operate as a constant-
current source. (The LM317 is rated for currents up to 1.5 A, and the LM350 and LM338 can be used for higher
currents.) The LM317 forces a constant 1.25 V across R_LIM, thus generating a constant current of
I_LIM = 1.25V/R_LIM
(4)
Transistor Q1 provides a disconnect between the battery and the LM3420 when the input voltage is removed.
This prevents the 85-μA quiescent current of the LM3420 from eventually discharging the battery. In this
application Q1 is used as a low offset saturated switch, with the majority of the base drive current flowing through
the collector and crossing over to the emitter as the battery becomes fully charged. It provides a very low
collector to emitter saturation voltage (approximately 5 mV). Diode D1 is also used to prevent the battery current
from flowing through the LM317 regulator from the output to the input when the DC input voltage is removed.
As the battery charges, its voltage begins to rise, and is sensed at the IN pin of the LM3420. Once the battery
voltage reaches 8.4 V, the LM3420 begins to regulate and starts sourcing current to the base of Q2. Transistor
Q2 begins controlling the ADJ pin of the LM317 which begins to regulate the voltage across the battery and the
constant voltage portion of the charging cycle starts. Once the charger is in the constant voltage mode, the
charger maintains a regulated 8.4 V across the battery and the charging current is dependent on the state of
charge of the battery. As the cells approach a fully charged condition, the charge current falls to a very low value.
Figure 14. Constant Current/Constant Voltage Li-Ion Battery Charger
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