LM3420 Datasheet, PDF (13/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

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LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
Typical Application: Constant Current/Constant Voltage Li-Ion Battery Charger (continued)
9.2.1 Design Requirements
DESIGN PARAMETER
Input voltage
Output voltage
Output current
EXAMPLE VALUE
13 V - 20 V
8.4 V
1A
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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