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and plot the loop’s gain and phase response between 1 kHz and 1 MHz. This provides a
full picture of the situation on both sides of the unity gain frequency (20 kHz in this
case). Figure 2 illustrates typical results for the default configuration. The phase margin
is the phase value at the unity gain frequency, or about 68 Deg. The gain margin is the
gain at the 0° phase frequency, or approximately 32.5dB.
-80
-60
-40
-20
0
20
40
60
80
-200
-150
-100
-50
0
50
100
150
200
10
3
10
4
10
5
10
6
TR
1/
dB
TR
2
/°
f/Hz
TR1: Mag(Gain)
TR2: Phase(Gain)
Figure 2
An alternate method is to step the output load and monitor the response of the system to
the transient. Low pass filtering may be required to remove switching frequency
components of the signal to make the small transients more visible. A well behaved loop
will settle back quickly and smoothly (Figure 3-C) and is termed critically damped,
whereas a loop with poor phase or gain margin will either ring as it settles (Figure 3-B)
under damped, or take too long to achieve the setpoint (Figure 3-A) over damped. The
number of rings indicates the degree of stability, and the frequency of the ringing shows
the approximate unity-gain frequency of the loop. The amplitude of the signal is not
particularly important, as long as the amplitude is not so high that the loop behaves
nonlinearly. This method is easy to implement in labs not equipped with network
analyzers, but it does not indicate gain margin or evidence of conditional stability. In
these situations, a small shift in gain or phase caused by production tolerances or
temperature could cause instability even though the circuit functioned properly in
development.
Figure 3-A
Figure 3-B
Figure 3-C
Gain Margin
Phase Margin