LM3411AM5-3.3 datasheet(7/13 Pages) NSC

No de pièce	LM3411AM5-3.3

Description	Precision Secondary Regulator/Driver

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Fabricant	NSC [National Semiconductor (TI)]

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Product Description

The LM3411 is a shunt regulator specifically designed to be

the reference and control section in an overall feedback loop

of a regulated power supply. The regulated output voltage is

sensed between the IN pin and GROUND pin of the LM3411.

If the voltage at the IN pin is less than the LM3411 regulating

voltage (V

REG), the OUT pin sources no current. As the volt-

age 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 condi-

tions) the voltage on the IN pin can be forced above the

V

REG voltage. In these instances, the maximum voltage ap-

plied to the IN pin should not exceed 20V. In addition, an ex-

ternal 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 al-

low overall closed-loop compensation. In many of the appli-

cations circuits shown here, compensation is provided by a

single capacitor connected from the compensation pin to the

out pin of the LM3411. 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 regu-

lator circuit and observing the resultant output voltage re-

sponse is a easy method of determining the stability of the

control loop. Analyzing more complex feedback loops re-

quires additional information.

The formula for AC gain at a frequency (f) is as follows;

where R

f ≈ 52 kΩ for the 3.3V part, and Rf ≈ 94 kΩ for the 5V

part.

The resistor (R

f) in the formula is an internal resistor located

on the die. Since this resistor value will affect the phase mar-

gin, the worst case maximum and minimum values are im-

portant when analyzing closed loop stability. The minimum

and maximum room temperature values of this resistor are

specified in the Electrical Characteristics section of this data

sheet, and a curve showing the temperature coefficient is

shown in the curves section. In the applications shown here,

the worst case phase margin occurs with minimum values of

R

f.

Test Circuit

The test circuit shown in

Figure 2 can be used to measure

and verify various LM3411 parameters. Test conditions are

set by forcing the appropriate voltage at the V

OUT Set test

point and selecting the appropriate R

L or IOUT as specified in

the Electrical Characteristics section. Use a DVM at the

“measure” test points to read the data.

DS011987-14

FIGURE 2. LM3411 Test Circuit

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