NX2124 datasheet(10/17 Pages) MICROSEMI

10 / 17 page

NX2124/2124A

Rev.1.8

02/28/08

Case 2:

LC<FESR<FO

40dB/decade

20dB/decade

loop gain

ESR

compensator

power stage

FZ1 Z2

FP2

Figure 5 - Bode plot of Type III compensator

LC<FESR<FO)

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 two SANYO 16MV-WG1500

with 13 m

Ω is chosen as output capacitor.

1. Calculate the location of LC double pole F

and ESR zero F

ESR.

OUT

1.5uH 3000uF

2.3kHz

×π ××

ESR

OUT

ESRC

6.5m

3000uF

8.2kHz

×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=8.06kΩ.

3. Set zero F

Z2 = FLC and Fp1 =FESR .

4. Calculate C

3 .

C =

(-)

10k

2.3kHz 8.2kHz

=4.76nF

×π×

×π ×Ω

Choose C

3=4.7nF.

5. Calculate R

3 .

P13

8.2kHz 4.7nF

4.1k

×π ××

=Ω

Choose R

3 =4kΩ.

6. Calculate R

4 with FO=30kHz.

OSC

ESR

1.5V

30kHz 1.5uH 10k

6.5m

10k

=37.3k

×π × ××

××

×π ×

Ω ×Ω

××

Ω

Ω +Ω

Ω

Choose R

4=37.4kΩ.

7. Calculate C

2 with zero Fz1 at 75% of the LC

double pole by equation (11).

Z14

0.75 2.3kHz 37.4k

2.4nF

×π ××

×π ×

×Ω

Choose C

2=2.2nF.

8. Calculate C

1 by equation (14) with pole Fp2 at

half the switching frequency.

37.4k

150kHz

28pF

×π ××

×π ×

Ω×

Choose C

1=27pF.

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