In reference applications it may be advantageous to reduce the AC noise present at the output. One method is to reduce the regulator bandwidth by increasing the size of the output capacitor. This is the only way noise can be reduced on the 3-lead LP2950-N but is relatively inefficient, as increasing the capacitor from 1 µF to 220 µF only decreases the noise from 430 µVRMS to 160 µVRMS for a 100-kHz bandwidth at 5-V output.
Noise can be reduced fourfold by a bypass capacitor across R1, because it reduces the high frequency gain from 4 to unity. Pick
or about 0.01 µF. When doing this, the output capacitor must be increased to 3.3 µF to maintain stability. These changes reduce the output noise from 430 µV to 100 µV rms for a 100-kHz bandwidth at 5-V output. With the bypass capacitor added, noise no longer scales with output voltage so that improvements are more dramatic at higher output voltages.