LM3420-42 Datasheet, PDF (12/17 Pages) National Semiconductor (TI) – Lithium-Ion Battery Charge Controller

V_REG External Voltage Trim (Continued)
R_increase = 26x10⁵/%incr
R_decrease = (150x10⁵/%decr) − 176x10³
For LM3420-8.4
R_increase = 26x10⁵/%incr
R_decrease = (154x10⁵/%decr) − 181x10³
For LM3420-12.6
R_increase = 28x10⁵/%incr
R_decrease = (259x10⁵/%decr) − 287x10³
For LM3420-16.8
R_increase = 29x10⁵/%incr
R_decrease = (364x10⁵/%decr) − 392x10³
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 effi-
ciently. Output voltage tolerances better than 0.5% are pos-
sible without using trim pots or precision resistors. The cir-
cuits shown are designed for 2 cell operation, but they can
readily be changed for either 1, 3 or 4 cell charging applica-
tions.
One item to keep in mind when designing with the LM3420 is
that there are parasitic diodes present. In some designs, un-
der 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.
Application Circuits
The circuit shown in Figure 5 performs constant-current,
constant-voltage charging of two Li-Ion cells. At the begin-
ning of the charge cycle, when the battery voltage is less
than 8.4V, 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 1.5A, and the LM350 and
LM338 can be used for higher currents.) The LM317 forces
a constant 1.25V across R_LIM, thus generating a constant
current of
I_LIM = 1.25V/R_LIM
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FIGURE 5. Constant Current/Constant Voltage Li-Ion Battery Charger
DS012359-11
FIGURE 6. Low Drop-Out Constant Current/Constant Voltage 2-Cell Charger
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 re-
moved.
As the battery charges, its voltage begins to rise, and is
sensed at the IN pin of the LM3420. Once the battery voltage
reaches 8.4V, the LM3420 begins to regulate and starts
sourcing current to the base of Q2. Transistor Q2 begins
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