App Note 83

"Performance verification of low noise, low dropout regulators: Silence of the amps." 20 pages.

This app note discusses testing LDO regulators, primarily the measurement of output noise. In effect, this app note is the linear-regulator version of App Note 70 (which discussed the measurement of output noise for low-noise switching regulators).

The schematic of the measurement chain is shown in Figure 2. Note that single-pole highpass filters on use on the input and output to remove any DC components from the signals. This arrangement leads to a non-optimal low-frequency cutoff. (See the frequency response in Figure 3, which shows a nice high-frequency roll-off, but a considerably "rounder" low-frequency roll-off... Butterworth is better. He could have done better by moving the second-order Butterworth poles to the third-order locations, since he already had a pole (or two) on the axis.)

Nevertheless, the plots on pages 8 and 9 are very nice, and are clearly the product of painstaking work.

The best quote (from page 3, with footnote): "The metal can encloses the regulator under test and its internal battery power supply (the cookies were excellent, particularly the thin ones with sugar on top)."

Appendix A discusses the internal architecture of low-dropout regulators. This verbiage is an updated and expanded version of Appendix A from App Note 32. Note that Figures A3 and A4 show the transient response for a load step (from 10mA to 100mA); he will return to this topic in a later app note.

Appendix B discusses capacitor selection, and warns of the voltage-dependent and temperature-dependent horrors of some ceramic types.

Appendix C is an in-depth discussion of RMS voltmeters. He has discusses RMS voltmeters before (most notably in Appendix C of App Note 65), but this discussion is the most detailed, and includes comparisons of voltmeter types (Figures C1, C2, and C3), as well as comparisons of specific models (Figure C5). Of course, the HP3400A and HP3403C work very well, as does his special baby, the LT1088-based circuit in Figure C6. Thermal great, logarithmic good, rectify and average bad.

The app note ends with a cartoon. "Speak softly and carry a big PNP."