V

R

R

For LM3420-8.4

R

R

For LM3420-12.6

R

R

For LM3420-16.8

R

R

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

current of

I

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

DS012359-1

FIGURE 5. Constant Current/Constant Voltage Li-Ion Battery Charger

DS012359-11

FIGURE 6. Low Drop-Out Constant Current/Constant Voltage 2-Cell Charger

www.national.com

12