SC2616 datasheet(13/17 Pages) SEMTECH

13 / 17 page

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POWER MANAGEMENT

SC2616

Compensation Components

Once the filter components have been determined, the

compensation components can be calculated. The goal

of compensation is to modify the frequency response

characteristics of the error amplifier to ensure that the

closed loop feedback system has the highest gain and

bandwidth possible while maintaining stability.

A simplified stability criteria states that the open loop

gain of the converter should fall through 0dB at 20dB/

decade at a frequency no higher than 20-25% of the

switching frequency.

This objective is most simply met by generating asymp-

totic bode plots of the small signal response of the vari-

ous sections of the converter.

It is convenient to split the converter into two sections,

the Error amp and compensation components being one

section and the Modulator, output filter and divider be-

ing the other.

First calculate the DC Filter + Modulator + Divider gain.

The DC filter gain is always 0dB, the Modulator gain is

19dB at 5V in and is proportional to Vin, so modulator

gain at any input voltage is.













⋅

Log

MOD

the divider gain is given by









⋅

DIV

Log

So the total Filter + Modulator + Divider DC Gain is









⋅













⋅

FMD

Log

VOUT

SC2610 AND FETS

OUT

COMP

MODULATOR

REF

Resr

Applications Information (Cont.)

Calculate the filter double pole frequency (Fp(lc))

LCo

)

(

and calculate ESR Zero frequency (Fz(esr))

)

esr

(

⋅

Choose an open loop crossover frequency (Fco) no higher

than 20% of the switching frequency (Fs).

The proximity of Fz(esr) to the crossover frequency Fco

determines the type of compensation required, if

Fz(esr)>Fco/4, use type 3 compensation, otherwise use

type 2. Type 1 compensation is not appropriate and is

not discussed here.

Type 2 Example

As an example of type 2 compensation, we will use the

Evaluation board schematic.

The total Filter+Modulator+Divider DC Gain is:

Log

FMD













⋅













⋅

This is drawn as the line A-B in Figure 5.

kHz

LCo

)

(

3000

≈

⋅

−

3.3uH

VOUT

3000uF

SC2616 AND FETS

OUT

COMP

6.98k

MODULATOR

REF

8.06k

22mOhm

Vin=5V

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