Some low-dropout regulators (LDOs) use multiple feedback loops to improve output voltage transient response. In many of these cases, some or all of the loops are inside the IC. Designers can assess the stability of these regulators by using the non-invasive stability measurement capability of the OMICRON Lab Bode 100 vector network analyzer (VNA).
The simplified schematic in Figure 1 shows traditional error amplifier feedback along with a second loop that connects some function of the output voltage directly to the pass transistor drive. In fact, three loops are present: the “fast” loop, the feedback loop, and the combined loop. The second and combined loops often are not accessible for external measurement.
Each feedback loop contributes to the overall regulator stability, so they all should be included in the stability margin assessment. This would be the case if the combined loop could be measured. Since there is no external place to break the combined loop for a Bode plot measurement, which is the only method available to us, we need to use the non-invasive stability measurement to get an exact assessment.2