Datasheet
H = feedback gain =
R
FB2
R
FB1
R
FB2
+
-60
-40
60
MAGNITUDE (dB)
10
2
10
3
10
4
10
5
10
6
FREQUENCY (Hz)
-20
0
20
40
10
7
f
P1
m
f
P2
f
Z1
f
C
f
n
-180
-135
-90
-45
0
-225
f
ESR
Open Loop (T)
Power Stage
Compensator
(º)
PHASE
+
-
BG
VC
R
FB1
R
FB2
C
FF
C
C2
C
C1
R
C
LM3477
R
GM
G
M
V
OUT
FB
LM3477
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SNVS141K –OCTOBER 2000–REVISED MARCH 2013
The compensation network is designed around the power components, or the power stage. An isolated
schematic of the error amplifier and the various compensation components is shown in Figure 32. The error
amplifier in conjunction with the compensation network makes up the compensator block in Figure 31. The
purpose of the compensator block is to stabilize the control loop and achieve high performance in terms of the
transient response, audio susceptibility and output impedance.
Figure 32. LM3477 Compensation Components
Figure 33 shows a bode plot of a typical current mode buck regulator. It is an estimate of the actual plot using the
asymptotic approach. The three plots shown are of the compensator, powerstage, and loop gain, which is the
product of the power stage, compensator, and feedback gain. The loop gain determines both static and dynamic
performance of the converter. The power stage response is fixed by the selection of the power components,
therefore the compensator is designed around the powerstage response to achieve a good loop response.
Specifically, the compensator is added to increase low frequency magnitude, extend the 0dB frequency
(crossover frequency), and improve the phase characteristic.
Poles, Zeros and Important Measurements are Labeled
Figure 33. Typical Open Loop, Compensator, and Power Stage Bode Plots for LM3477 Buck Circuits
There are several different types of compensation that can be used to improve the frequency response of the
control loop. To determine which compensation scheme to use, some information about the power stage is
needed.
Use V
IN
= V
IN(MIN)
and R = R
MIN
(I
OUT(MAX)
) when calculating compensation components.
(36)
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