H = feedback gain =

RFB2

RFB1 RFB2

+

-60

-40

60

10

2

10

3

10

4

10

5

10

6

FREQUENCY (Hz)

-20

0

20

40

10

7

fP1

m

fP2

fZ1

fC

fn

-180

-135

-90

-45

0

-225

fESR

Open Loop (T)

Power Stage

Compensator

+

-

BG

VC

RFB1

RFB2

CFF

CC2

CC1

RC

LM3477

RGM

GM

VOUT

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 VIN = VIN(MIN) and R = RMIN (IOUT(MAX)) when calculating compensation components.

(36)

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