NX2124ACSTR datasheet(9/17 Pages) MICROSEMI

9 / 17 page

NX2124/2124A

Rev.1.8

02/28/08

Case 1:

LC<FO<FESR

40dB/decade

20dB/decade

loop gain

ESR

compensator

power stage

FZ1 Z2

FP2

Figure 4 - Bode plot of Type III compensator

Design example for type III compensator are in

order. The crossover frequency has to be selected as

LC<FO<FESR and FO<=1/10~1/5Fs. Here two POSCAP

2R5TPE220MC(220uF,12 m

Ω) are chosen as output

capacitor.

1.Calculate the location of LC double pole F

and ESR zero F

ESR.

OUT

1.5uH 440uF

6.2kHz

×π ××

ESR

OUT

ESRC

440uF

60.3kHz

×π ××

×π ×

Ω×

2. Set R

2 equal to 10kΩ.

Ω×

=Ω

REF

OUT

REF

10k

0.8V

R =

-V

1.8V-0.8V

Choose R

1=8kΩ.

3. Set zero F

Z2 = FLC and Fp1 =FESR .

4. Calculate R

4 and C3 with the crossover

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

O=30kHz.

C =

(-)

(

10k

6.2kHz 60.3kHz

=2.3nF

×π×

×π ×Ω

OSCO

out

in3

R =C

1.5V

30kHz 1.5uH

440uF

2.2nF

=16.9k

×π ××

××

×π ××

××

Ω

Choose C

3=2.2nF, R4=16.9kΩ.

5. Calculate C

2 with zero Fz1 at 75% of the LC

double pole by equation (11).

Z14

0.75 6.2kHz 16.9k

2nF

×π ××

×π ×

×Ω

Choose C

2=2.2nF.

6. Calculate C

1 by equation (14) with pole Fp2 at

half the switching frequency.

16.9k

150kHz

63pF

×π ××

×π ×

Ω×

Choose C

1=68pF.

7. Calculate R

3 by equation (13).

P13

60.3kHz 2.2nF

1.2k

×π××

=Ω

Choose R

3=1.2kΩ.

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