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LP3982ILDX-2.77 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LP3982ILDX-2.77 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 12 page Application Information (Continued) Similarity in the output capabilities exists between op amps and linear regulators. Just as rail-to-rail output op amps allow their output voltage to approach the supply voltage, low dropout regulators (LDOs) allow their output voltage to operate close to the input voltage. Both achieve this by the configuration of their output transistors. Standard op amps and regulator outputs are at the source (or emitter) of the output transistor. Rail-to-rail op amp and LDO regulator out- puts are at the drain (or collector) of the output transistor. This replaces the threshold (or diode drop) limitations on the output with the less restrictive source-to-drain (or V SAT) limi- tations. There is a trade-off, of course. The output imped- ance become significantly higher, thus providing a critically lower pole when combined with the capacitive load. That’s why rail-to-rail op amps are usually poor at driving capacitive loads and recommend a series output resistor when doing so. LDOs require the same series resistance except that the internal resistance of the output capacitor will usually suffice. Refer to the output capacitance section for more information. Output Capacitance The LP3982 is specifically designed to employ ceramic out- put capacitors as low as 2.2µF. Ceramic capacitors below 10µF offer significant cost and space savings, along with high frequency noise filtering. Higher values and other types and of capacitor may be used, but their equivalent series resistance (ESR) should be maintained below 0.5 Ω Ceramic capacitor of the value required by the LP3982 are available in the following dielectric types: Z5U, Y5V, X5R and X7R. The Z5U and Y5V types exhibit a 50% or more drop in capacitance value as their temperature increases from 25˚C, an important consideration. The X5R generally maintain their capacitance value within ±20%. The X7R type are desirable for their tighter tolerance of 10% over temperature. Ceramic capacitors pose a challenge because of their rela- tively low ESR. Like most other LDOs, the LP3982 relies on a zero in the frequency response to compensate against excessive phase shift in the regulator’s feedback loop. If the phase shift reaches 360˚ (i.e.; becomes positive), the regu- lator will oscillate. This compensation usually resides in the zero generated by the combination of the output capacitor with its equivalent series resistance (ESR). The zero is intended to cancel the effects of the pole generated by the load capacitance (C L) combined with the parallel combina- tion of the load resistance (R L) and the output resistance (R O) of the regulator. The challenge posed by low ESR capacitors is that the zero it generates can be too high in frequency for the pole that it’s intended to compensate. The LP3982 overcomes this challenge by internally generating a strategically placed zero. Figure 3 shows a basic model for the linear regulator that helps describe what happens to the output signal as it is processed through its feedback loop; that is, describe its loop gain (LG). The LG includes two main transfer functions: the error amplifier and the load. The error amplifier provides voltage gain and a dominant pole, while the load provides a zero and a pole. The LG of the model in Figure 3 is described by the following equation: The first term of the above equation expresses the voltage gain (numerator) and a single pole role-off (denominator) of the error amplifier. The second term expresses the zero (numerator) and pole (denominator) of the load in combina- tion with the R O of the regulator. Figure 4 shows a Bode plot that represents a case where the zero contributed by the load is too high to cancel the effect of the pole contributed by the load and R O. The solid line illustrates the loop gain while the dashed line illustrates the corresponding phase shift. Notice that the phase shift at unity gain is a total 360˚ -the criteria for oscillation. 20036916 FIGURE 2. Regulator Topology Simplified 20036917 FIGURE 3. Simplified Model of Regulator Loop Gain Components www.national.com 7 |
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