+

IN

OUT

GND

COMP

31 kŸ

1.23 V

LM3420

SNVS116E – MAY 1998 – REVISED DECEMBER 2014

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8 Detailed Description

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

sourcing current. This current is then used to drive a feedback device (opto-coupler), or a power device (linear

In some applications, (even under normal operating conditions) the voltage on the IN pin can be forced above

addition, an external resistor may be required on the OUT pin to limit the maximum current to 20 mA.

8.2 Functional Block Diagram

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

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

10

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