7.3.2 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 regulation, a few precautions are needed.

Figure 14 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.

Figure 14. Typical Application Using Fixed Output Regulator

When the adjustable regulator is used (Figure 15), 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 0.05 Ω × I_L. If R1 (=125 Ω) 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 improve load regulation.

Figure 15. Best Load Regulation Using Adjustable Output Regulator