LP2975 Datasheet, PDF (23/37 Pages) National Semiconductor (TI) – MOSFET LDO Driver/Controller

LP2975
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SNVS006F – SEPTEMBER 1997 – REVISED APRIL 2013
Summary of Stability Information
This section will present an explanation of theory and terminology used to analyze loop stability, along with
specific information related to stabilizing LP2975 applications.
Bode Plots and Phase Shift
Loop gain information is most often presented in the form of a Bode Plot, which plots Gain (in dB) versus
Frequency (in Hertz).
A Bode Plot also conveys phase shift information, which can be derived from the locations of the poles and
zeroes.
POLE: A pole causes the slope of the gain curve to decrease by an additional −20 dB/decade, and it also
causes phase lag (defined as negative phase shift) to occur.
A single pole will cause a maximum −90° of phase lag (see Figure 25). It should be noted that when the total
phase shift at 0 dB reaches (or gets close to) −180°, oscillations will result. Therefore, it can be seen that at least
two poles in the gain curve are required to cause instability.
ZERO: A zero has an effect that is exactly opposite to a pole. A zero will add a maximum +90° of phase lead
(defined as positive phase shift). Also, a zero causes the slope of the gain curve to increase by an additional
+20 dB/decade (see Figure 26).
Figure 25. Effects of a Single Pole
Total phase shift
The actual test of whether or not a regulator is stable is the amount of phase shift that is present when the gain
curve crosses the 0 dB axis (the frequency where this occurs was previously defined as f_c).
The phase shift at f_c can be estimated by looking at all of the poles and zeroes on the Bode plot and adding up
the contributions of phase lag and lead from each one. As shown in the graphs, most of the phase lag (or lead)
contributed by a pole (or zero) occurs within one decade of the frequency of the pole (or zero).
In general, a phase margin (defined as the difference between the total phase shift and −180°) of at least 20° to
30° is required for a stable loop.
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