LM3411 Datasheet, PDF (7/13 Pages) National Semiconductor (TI) – Precision Secondary Regulator/Driver

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 R_f ≈ 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 I_OUT as specified in
the Electrical Characteristics section. Use a DVM at the
“measure” test points to read the data.
FIGURE 2. LM3411 Test Circuit
DS011987-14
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