Datasheet

LM3420

SNVS116D –MAY 1998–REVISED MAY 2013

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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.

Application Circuits

The circuit shown in Figure 31 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 8.4V, 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

LIM

= 1.25V/R

LIM

(12)

Figure 31. Constant Current/Constant Voltage Li-Ion Battery Charger

Figure 32. 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 removed.

Product Folder Links: LM3420