NX2154CSTR datasheet(11/17 Pages) MICROSEMI

11 / 17 page

NX2154/2154A

Rev.1.2

02/26/07

B. Type II compensator design

If the electrolytic capacitors are chosen as power

stage output capacitors, usually the Type II compensa-

tor can be used to compensate the system.

Type II compensator can be realized by simple RC

circuit without feedback as shown in figure 6. R

3 and C1

introduce a zero to cancel the double pole effect. C

introduces a pole to suppress the switching noise. The

following equations show the compensator pole zero lo-

cation and constant gain.

Gain=g

... (15)

R +R

F =

... (16)

... (17)

××

×π ××

≈

×π ××

compensator

loop gain

power stage

40dB/decade

20dB/decade

Gain

ESR

Figure 6 - Bode plot of Type II compensator

Vout

Vref

Figure 7 - Type II compensator with

transconductance amplifier

For this type of compensator, F

O has to satisfy

LC<FESR<<FO<=1/10~1/5Fs.

The following is parameters for type II compensa-

tor design. Input voltage is 40V, output voltage is 5V,

output inductor is 6uH, output capacitor is one 1000uF

with 30m

Ω electrolytic capacitors.

1.Calculate the location of LC double pole F

and ESR zero F

ESR.

OUT

15uH 1000uF

1.3kHz

×π ××

ESR

OUT

ESRC

30m

1000uF

5.3kHz

×π ××

×π ×

Ω×

2.Set R

2 equal to 1kΩ.

REF

OUT

REF

0.8V

R =

191

-V

5V-0.8V

Ω×

=Ω

Choose R

1=191Ω.

3. Set crossover frequency at 1/10~ 1/5 of the

swithing frequency, here FO=30kHz.

4.Calculate R

3 value by the following equation.

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