NX2142ACUTR datasheet(13/18 Pages) MICROSEMI

13 / 18 page

NX2142/2142A

Rev. 1.1

10/28/07

Case 2:

LC<FESR<FO(for electrolytic capacitors)

Figure 11 - Bode plot of Type III compensator

LC<FESR<FO)

40dB/decade

20dB/decade

loop gain

ESR

compensator

power stage

FZ1 Z2

FP2

If electrolytic capacitors are used as output

capacitors, typical design example of type III

compensator in which the crossover frequency is

selected as F

LC<FESR<FO and FO<=1/10~1/5Fs is shown

as the following steps. Here one SANYO 6ME-WG1000

with 30 m

Ω is chosen as output capacitor, output

inductor is 6.8uH.

1. Calculate the location of LC double pole F

and ESR zero F

ESR.

OUT

6.8uH 1000uF

1.93kHz

×π ××

ESR

OUT

ESRC

30m

1000uF

5.3kHz

×π ××

×π ×

Ω×

2. Set R

4 equal to 10kΩ.

3. Calculate C2 with zero Fz1 at 75% of the LC

double pole by equation (11).

Z14

0.75 1.93kHz 10k

10nF

×π ××

×π ×

×Ω

Choose C

2=10nF.

4. Calculate C

1 by equation (14) with pole Fp2 at

half the switching frequency.

10k

300kHz

53pF

×π ××

×π ×

Ω×

Choose C

1=52pF.

5. Calculate C

3 with the crossover frequency at

1/10~ 1/5 of the switching frequency. Set F

O=60kHz.

OS CO

ES R

60kH z

6 .8uH

1 0

30 m

10 k

5 .3kHz

=25nF

××

Ω ×

Ω×

Choose C

3=25nF.

6. Set zero F

Z2 = FLC and Fp1 =FESR, calculate R2.

R =

(-)

(

25nF

1.93kHz 5.3kHz

=2k

×π×

Ω

Choose R

2=20kΩ.

7. Calculate R

3 by equation (13) with Fp1 =FESR.

8. Calculate R

REF

OUT

REF

20k

0.8V

R =

5.7k

-V

5V-0.8V

Ω×

=Ω

Choose R

1=5.7kΩ.

NX2142ACUTR Datasheet(PDF) 13 Page - Microsemi Corporation

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