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LM3411AM5X-3.3 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM3411AM5X-3.3 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page 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 www.national.com 7 |
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