NX2113 datasheet(13/19 Pages) MICROSEMI

13 / 19 page

NX2113/2113A

Rev. 2.0

11/18/05

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.

Vout

Vref

Figure 11 - Type II compensator with

transconductance amplifier

compensator

loop gain

power stage

40dB/decade

20dB/decade

Gain

ESR

F FP

Figure 12 - Bode plot of Type II compensator

Type II compensator can be realized by simple

RC circuit without feedback as shown in figure 11. R3

and C1 introduce a zero to cancel the double pole

effect. C2 introduces a pole to suppress the switching

noise. The following equations show the compensator

pole zero location and constant gain.

Gain=g

... (15)

R +R

F =

... (16)

... (17)

××

×π ××

≈

×π ××

For this type of compensator, F

O has to satisfy

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

The following uses typical design in figure 18 as

an example for type II compensator design, two 680uF

with 36m

Ω electrolytic capacitors are used.

1.Calculate the location of LC double pole F

and ESR zero F

ESR.

OUT

4.7uH 1360uF

2.0kHz

×π ××

ESR

OUT

ESRC

18m

1360uF

6.5kHz

×π ××

×π ×

Ω×

2.Set R

2 equal to10kΩ. Using equation 18.

10k

0.8V

4.7k

2.5V-0.8V

Ω×

=Ω

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.

OSCO

ESR

R +R

30kHz 4.7uH1

12V

18m

2.5mA/V

10k +4.7k

4.7k

=10.3k

×π ××

××

×π ××

××

Ω

ΩΩ

Ω

Choose R

3 =10kΩ.

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