AN017
5
Figure 2
An alternate method to look at phase margin is to step the output load and monitor the
response of the system to the transient. Filtering may be required to remove switching
frequency components of the signal to make the small transients more visible. Any filter
used for this measurement must be carefully designed such that it will not alter the signal of
interest. A well behaved loop will settle back quickly and smoothly (Figure 3-C) and is
termed critically damped. A loop with low phase margin will ring as it settles (Figure 3-B)
under damped. A loop with high phase margin will take longer 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
-80
-60
-40
-20
0
20
40
60
80
-200
-150
-100
-50
0
50
100
150
200
10
2
10
3
10
4
10
5
10
6
10
7
TR
1/
dB
TR2/
°
f/Hz
TR1: Mag(Gain)
TR2: Phase(Gain)
Gain Margin
Phase Margin
