LM3420

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SNVS116E – MAY 1998 – REVISED DECEMBER 2014

Typical Application: Constant Current/Constant Voltage Li-Ion Battery Charger (continued)

9.2.1 Design Requirements

DESIGN PARAMETER

EXAMPLE VALUE

Input voltage

13 V - 20 V

Output voltage

8.4 V

Output current

1 A

9.2.2 Detailed Design Procedure

9.2.2.1 Compensation Capacitor

The capacitor between OUT pin and COMP pin can be increase or decreased depending on the desired loop

response. Functional Block Diagram can be referred as different capacitance selection. In this case, 0.01-µF

capacitor is used.

9.2.3 Application Curve

Figure 15. Regulation Voltage vs Output Voltage and Load Resistance

9.2.4 Other Application Circuits

NOTE

Although the application circuits shown here have been built and tested, they should be

thoroughly evaluated with the same type of battery the charger will eventually be used

with.

Different battery manufacturers may use a slightly different battery chemistry which may

require different charging characteristics. Always consult the battery manufacturer for

information on charging specifications and battery details, and always observe the

manufacturers precautions when using their batteries. Avoid overcharging or shorting

Lithium-Ion batteries.

9.2.4.1 Low Dropout Constant Current/Constant Voltage 2-Cell Charger

Figure 16 shows a Li-Ion battery charger that features a dropout voltage of less than one volt. This charger is a

constant-current, constant-voltage charger (it operates in constant-current mode at the beginning of the charge

cycle and switches over to a constant-voltage mode near the end of the charging cycle). The circuit consists of

two basic feedback loops. The first loop controls the constant charge current delivered to the battery, and the

second determines the final voltage across the battery.

With a discharged battery connected to the charger, (battery voltage is less than 8.4 V) the circuit begins the

charge cycle with a constant charge current. The value of this current is set by using the reference section of the

LM10C to force 200 mV across R7 thus causing approximately 100

μA of emitter current to flow through Q1, and

approximately 1 mA of emitter current to flow through Q2. The collector current of Q1 is also approximately 100

μA, and this current flows through R2 developing 50 mV across it. This 50 mV is used as a reference to develop

the constant charge current through the current sense resistor R1.

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