LM3477AMM datasheet(19/23 Pages) NSC

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Compensation (Continued)

Figure 14 shows a bode plot of a typical current mode buck

regulator. It is an estimate of the actual plot using the as-

ymptotic approach. The three plots shown are of the com-

pensator, 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 selec-

tion 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 fre-

quency (crossover frequency), and improve the phase char-

acteristic.

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 =VIN(MIN) and R = RMIN (IOUT(MAX)) when calculat-

ing compensation components.

e =fS(VSL + 50x10

−6 R

SL)

With the power stage known, a compensator can be de-

signed to achieve improved performance and stability. The

LM3477/A will typically require only a single resistor and

capacitor for compensation, but depending on the power

stage it could require three or four external components.

It is a good idea to check that Q is between 0.15 and 2, if it

was not already done when selecting the inductor. If Q is less

than 0.15 or greater than 2, skip to the SAMPLING POLE

QUALITY FACTOR section below before continuing with the

compensator design.

First, a target crossover frequency (f

c) for the loop gain must

be selected. The crossover frequency is the bandwidth of the

converter. A higher bandwidth generally corresponds to

faster response times and lower overshoots to load tran-

sients. However, the bandwidth should not be much higher

than 1/10 the switching frequency. The LM3477/A operates

with a 500kHz switching frequency, so it is recommended to

choose a crossover frequency between 10kHz - 50kHz.

The schematic of the LM3477/A compensator is shown in

Figure 13. The default design uses R

c and CC1 to form a lag

(type 2) compensator. The C

C2 capacitor can be added to

form an additional pole that is typically used to cancel out the

esr zero of the output capacitor. Finally, if extra phase margin

is needed, the Cff capacitor can be added (this does not help

at low output voltages, see below).

20003390

FIGURE 13. LM3477 Compensation Components

200033J4

FIGURE 14. Typical Open Loop, Compensator, and

Power Stage Bode Plots for LM3477 Buck Circuits.

Poles, Zeros and Important Measurements are Labeled

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