LM3477AMM datasheet(20/23 Pages) NSC

20 / 23 page

Compensation (Continued)

The strategy taken here for choosing R

c and CC1 is to set the

crossover frequency with R

c, and set the compensator zero

with C

C1. Using the selected target crossover frequency, fC,

set R

C to:

C = Crossover frequency in Hertz (20kHz - 50kHz is recom-

mended)

GM = 50x10

Ω

GM = 1000x10

−6 A/V

The compensator zero, f

Z1, is set with CC1. When fast tran-

sient responses are desired, f

Z1 should be placed as high as

possible, however it should not be higher than the selected

crossover frequency f

C. The guideline proposed here is to

choose C

C1 such that fZ1 falls somewhere between the

power pole f

P1 and

⁄2 decade before the selected crossover

frequency fc:

In this compensation scheme, the pole created by C

C2 is

used to cancel out the zero created by the ESR of the output

capacitor. In other schemes such as the methods discussed

in the SAMPLING POLE QUALITY FACTOR subsection, the

ESR zero is used. For the typical case, use C

C2 if:

Plotting the Open Loop Response

The open loop response is expressed as:

T= A

DC xACM xHxFp(s)xFc(s)

Where A

DC and H are given above and

CM = GMxRGM

One can plot the magnitude and phase of the open loop

response to analyze the frequency response.

Example: Compensation Design

4.5V

≤ V

≤ 5.5V

OUT = 2.5V

OUT = 3A (R = 0.83

Ω)

SN = 0.02

Ω

L = 3.3µH

SL =0

Ω

OUT = 100µF

ESR = 0.01

Ω

First, calculate the power stage parameters using V

IN(MIN)

and R

(MAX):

In this example, a crossover frequency of 20kHz is chosen,

so: f

C = 20000. RC is now calculated using the power stage

information and the target crossover frequency f

This sets the high frequency gain of the compensator such

that a crossover frequency of f

C is obtained. The capacitor

C1 sets the compensator zero, fZ2. Set fZ2 between the

power pole f

P1 and the

⁄2 decade before the target cross-

over frequency f

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