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LM1086IS-2.85 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM1086IS-2.85 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 15 page Application Note (Continued) Capacitors other than tantalum or aluminum can be used at the adjust pin and the input pin. A 10uF capacitor is a reasonable value at the input. See Ripple Rejection section regarding the value for the adjust pin capacitor. It is desirable to have large output capacitance for applica- tions that entail large changes in load current (microproces- sors for example). The higher the capacitance, the larger the available charge per demand. It is also desirable to provide low ESR to reduce the change in output voltage: ∆V= ∆I x ESR It is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the output voltage by reducing the overall ESR. Output capacitance can be increased indefinitely to improve transient response and stability. RIPPLE REJECTION Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 1 and Figure 2). The LM1086 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than V REF, the ripple rejection decreases as function of adjustment gain: (1+R1/R2) or V O/VREF. There- fore a 5V adjustment decreases ripple rejection by a factor of four (−12dB); Output ripple increases as adjustment voltage increases. However, the adjustable version allows this degradation of ripple rejection to be compensated. The adjust terminal can be bypassed to ground with a capacitor (C ADJ). The imped- ance of the C ADJ should be equal to or less than R1 at the desired ripple frequency. This bypass capacitor prevents ripple from being amplified as the output voltage is in- creased. 1/(2 π*f RIPPLE*CADJ) ≤ R 1 LOAD REGULATION The LM1086 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 3 shows a typical application using a fixed output regulator. Rt1 and Rt2 are the line resistances. V LOAD is less than the V OUT 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 4), 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 regu- lation. For example, a 5V regulator with 0.05 Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05 Ω xI L. If R1 (=125 Ω) is con- nected 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 sens- ing and improve load regulation. PROTECTION DIODES Under normal operation, the LM1086 regulator does not need any protection diode. With the adjustable device, the internal resistance between the adjustment and output ter- minals limits the current. No diode is needed to divert the current around the regulator even with a capacitor on the adjustment terminal. The adjust pin can take a transient signal of ±25V with respect to the output voltage without damaging the device. When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and rate of decrease of V IN. In the LM1086 regu- lator, the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor ( ≥1000 µf), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an external diode is recom- mended between the output and input pins to protect the regulator, shown in Figure 5. 10094818 FIGURE 3. Typical Application using Fixed Output Regulator 10094819 FIGURE 4. Best Load Regulation using Adjustable Output Regulator www.national.com 9 |
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