Datasheet

LM3411
www.ti.com SNVS113E –DECEMBER 1999–REVISED APRIL 2013
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 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 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 regulator circuit and observing the resultant output voltage response is a
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;
where
• R
f
≈ 52 kΩ for the 3.3V part
• R
f
≈ 94 kΩ for the 5V part (1)
The resistor (R
f
) in the formula is an internal resistor located on the die. Since this resistor value will affect the
phase margin, the worst case maximum and minimum values are important 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
Typical Performance Characteristics 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 21 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.
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