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LM1085ISX-ADJ Folha de dados(PDF) 10 Page - Texas Instruments |
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LM1085ISX-ADJ Folha de dados(HTML) 10 Page - Texas Instruments |
10 / 29 page LM1085 SNVS038H – JULY 1999 – REVISED JANUARY 2015 www.ti.com 7.3 Feature Description 7.3.1 Ripple Rejection Ripple rejection is a function of the open loop gain within the feed-back loop (refer to and Figure 13). The LM1085 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than VREF, the ripple rejection decreases as function of adjustment gain: (1+R1/R2) or VO/VREF. Therefore 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 (CADJ). The impedance of the CADJ 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 increased. 1/(2 π*fRIPPLE*CADJ) ≤ R1 (1) 7.3.2 Load Regulation The LM1085 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 regulation, a few precautions are needed. Figure 11 shows a typical application using a fixed output regulator. Rt1 and Rt2 are the line resistances. VLOAD 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 RLOAD 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. Figure 11. Typical Application Using Fixed Output Regulator When the adjustable regulator is used (Figure 12), 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 regulation. 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 0.05Ω x IL. If R1 (= 125 Ω) is connected near the load the effective line resistance will be 0.05Ω (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 improve load regulation. Figure 12. Best Load Regulation Using Adjustable Output Regulator 10 Submit Documentation Feedback Copyright © 1999–2015, Texas Instruments Incorporated Product Folder Links: LM1085 |
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