Datasheet
LM3420
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SNVS116D –MAY 1998–REVISED MAY 2013
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Regulation Voltage vs
Output Voltage and Normalized
Load Resistance Temperature Drift
Figure 23. Figure 24.
Regulation Voltage vs
Output Saturation Output Voltage and
Voltage (V
SAT
) Load Resistance
Figure 25. Figure 26.
PRODUCT DESCRIPTION
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 20V. In
addition, an external resistor may be required on the OUT pin to limit the maximum current to 20 mA.
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 as follows;
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