SC2616 datasheet(14/17 Pages) SEMTECH

14 / 17 page

 2003 Semtech Corp.

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POWER MANAGEMENT

SC2616

This is point B in Figure 5.

kHz

3000

)

esr

(

⋅

−

This is point C in Figure 5., the line joining B-C slopes at -

40dB/decade, the line joining C-D slopes at -20dB/de-

cade.

For 600kHz switching frequency, crossover is designed

for 100kHz.

Since Fz(esr)<<Fco/4 Type 2 compensation is appropri-

ate. Having plotted the line ABCD, and confirmed the

type of compensation necessary, compensation compo-

nent values can be determined.

At Fco, the line ABCD shows a gain of -27.5dB and a

slope of -20dB/decade. In order for the total open loop

gain to be 0dB with a -20dB/decade slope at this fre-

quency, the compensated error amp gain at Fco must be

+27.5dB with a 0dB slope. This is the line FG on the plot

below.

Since open loop DC gain should be as high as possible to

minimize errors, a zero is placed at F and to minimize

high frequency gain and switching interference a pole is

placed at G.

The zero at F should be no higher than Fco/4 and the

pole at G no lower than 4*Fco. The equations to set the

gain and the pole and zero locations allow Shutdown:

dB)

(in

Fco

gain

where

⋅

For this example, this results in the following values.

Ω

≈

Ω

⋅

≈

)

rolloff

due

(unecessar

400

⋅

≈

Fco

Fz(esr)

Fp(lc)

-60

-40

-20

100

100.0E+0

1.0E+3

10.0E+3

100.0E+3

1.0E+6

Frequency (Hz)

Fp1

Fz1

Filter+modulator

+divider gain

Compensated

Error Amp gain

Total open

loop gain

Figure 5: Type 2 Error Amplifier Compensation

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