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LM1117-5.0-220 Folha de dados(PDF) 8 Page - First Components International |
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LM1117-5.0-220 Folha de dados(HTML) 8 Page - First Components International |
8 / 14 page APPLICATION NOTE 1.0 External Capacitors/Stability 1.1 Input Bypass Capacitor An input capacitor is recommended. A 10µF tantalum on the input is a suitable input bypassing for almost all applications. 1.2 Adjust Terminal Bypass Capacitor The adjust terminal can be bypassed to ground with a by- pass capacitor (C ADJ) to improve ripple rejection. This by- pass capacitor prevents ripple from being amplified as the output voltage is increased. At any ripple frequency, the im- pedance of the C ADJ should be less than R1 to prevent the ripple from being amplified: (2 π*f RIPPLE*CADJ) < R1 The R1 is the resistor between the output and the adjust pin. Its value is normally in the range of 100-200 Ω. For example, with R1=124 Ω and f RIPPLE=120Hz, the CADJ should be > 11µF. 1.3 Output Capacitor The output capacitor is critical in maintaining regulator stabil- ity, and must meet the required conditions for both minimum amount of capacitance and ESR (Equivalent Series Resis- tance). The minimum output capacitance required by the LM1117 is 10µF, if a tantalum capacitor is used. Any increase of the output capacitance will merely improve the loop stabil- ity and transient response. The ESR of the output capacitor should be less than 0.5 Ω. In the case of the adjustable regu- lator, when the C ADJ is used, a larger output capacitance (22µf tantalum) is required. 2.0 Output Voltage The LM1117 adjustable version develops a 1.25V reference voltage, V REF, between the output and the adjust terminal. As shown in Figure 1, this voltage is applied across resistor R1 to generate a constant current I1. The current I ADJ from the adjust terminal could introduce error to the output. But since it is very small (60µA) compared with the I1 and very constant with line and load changes, the error can be ig- nored. The constant current I1 then flows through the output set resistor R2 and sets the output voltage to the desired level. For fixed voltage devices, R1 and R2 are integrated inside the devices. 3.0 Load Regulation The LM1117 regulates the voltage that appears between its output and ground pins, or between its output and adjust pins. In some cases, line resistances can introduce errors to the voltage across the load. To obtain the best load regula- tion, a few precautions are needed. Figure 2, shows a typical application using a fixed output regulator. The Rt1 and Rt2 are the line resistances. It is ob- vious that the V LOAD is less than the VOUT by the sum of the voltage drops along the line resistances. In this case, the load regulation seen at the R LOAD would be degraded from the data sheet specification. To improve this, the load should be tied directly to the output terminal on the positive side and directly tied to the ground terminal on the negative side. When the adjustable regulator is used ( Figure 3), the best performance is obtained with the positive side of the resistor R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regula- tion. For example, a 5V regulator with 0.05 Ω resistance be- tween the regulator and load will have a load regulation due to line resistance of 0.05 Ω xI L. If R1 (=125Ω) is connected near the load, the effective line resistance will be 0.05 Ω (1+R2/R1) or in this case, it is 4 times worse. In addition, the ground side of the resistor R2 can be returned near the ground of the load to provide remote ground sensing and im- prove load regulation. 4.0 Protection Diodes Under normal operation, the LM1117 regulators do not need any protection diode. With the adjustable device, the internal resistance between the adjust and output terminals limits the current. No diode is needed to divert the current around the regulator even with capacitor on the adjust terminal. The ad- just pin can take a transient signal of ±25V with respect to the output voltage without damaging the device. When a output capacitor is connected to a regulator and the input is shorted to ground, the output capacitor will discharge FIGURE 1. Basic Adjustable Regulator FIGURE 2. Typical Application using Fixed Output Regulator FIGURE 3. Best Load Regulation using Adjustable Output Regulator 8 LM1117 800mA Low-dropout Linear Regulator |
Nº de peça semelhante - LM1117-5.0-220 |
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Descrição semelhante - LM1117-5.0-220 |
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