LM3420 Datasheet, PDF (10/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
8 Detailed Description
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8.1 Overview
The LM3420 is a shunt regulator specifically designed to be the reference and control section in an overall
feedback loop of a lithium-ion battery charger. The regulated output voltage is sensed between the IN pin and
GROUND pin of the LM3420. If the voltage at the IN pin is less than the LM3420 regulating voltage (V_REG), the
OUT pin sources no current. As the voltage at the IN pin approaches the V_REG voltage, the OUT pin begins
sourcing current. This current is then used to drive a feedback device (opto-coupler), or a power device (linear
regulator, switching regulator, etc.), which servos the output voltage to be the same value as V_REG.
In some applications, (even under normal operating conditions) the voltage on the IN pin can be forced above
the V_REG voltage. In these instances, the maximum voltage applied to the IN pin should not exceed 20 V. In
addition, an external resistor may be required on the OUT pin to limit the maximum current to 20 mA.
8.2 Functional Block Diagram
IN
OUT
1.23 V
+
31 kŸ
COMP
R_f
GND
8.3 Feature Description
8.3.1 Compensation
The inverting input of the error amplifier is brought out to allow overall closed-loop compensation. In many of the
applications circuits shown here, compensation is provided by a single capacitor (C_C) connected from the
compensation pin to the out pin of the LM3420. The capacitor values shown in the schematics are adequate
under most conditions, but they can be increased or decreased depending on the desired loop response.
Applying a load pulse to the output of a regulator circuit and observing the resultant output voltage response is
an easy method of determining the stability of the control loop.
Analyzing more complex feedback loops requires additional information.
The formula for AC gain at a frequency (f) is shown in Equation 1:
and where
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