FSP3129SD datasheet(6/10 Pages) FOSLINK

6 / 10 page

WIDE INPUT 2A STEP-DOWN CONVERTER

2007-1-4

6/10

FSP3129

And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used):

Follow the following steps to compensate the IC:

STEP1. Set the cross over frequency at 1/10 of the switching frequency via RCOMP:

but limit RCOMP to 15kΩ maximum.

STEP2. Set the zero fZ1 at 1/4 of the cross over frequency. If RCOMP is less than 15kΩ, the equation for CCOMP is:

If RCOMP is limited to 15kΩ, then the actual cross over frequency is 3.4/(VOUTCOUT). Therefore:

STEP3. If the output capacitor’s ESR is high enough to cause a zero at lower than 4 times the cross over

frequency, an additional compensation capacitor CCOMP2 is required. The condition for using CCOMP2 is:

And the proper value for CCOMP2 is:

A small value CCOMP2 such as 100pF may improve stability against PCB layout parasitic effects.

Table 2 shows some calculated results based on the compensation method above.

VOUT

COUT

RCOMP

CCOMP

CCOMP2

2.5V

22µF Ceramic

8.2kΩ

2.2nF

100pF

3.3V

22µF Ceramic

12kΩ

1.5nF

100pF

22µF Ceramic

15kΩ

1.5nF

100pF

2.5V

47µF SP CAP

15kΩ

1.5nF

100pF

3.3V

47µF SP CAP

15kΩ

1.8nF

100pF

47µF SP CAP

15kΩ

2.7nF

100pF

2.5V

470µF/6.3V/30mΩ

15kΩ

15nF

1nF

3.3V

470µF/6.3V/30mΩ

15kΩ

22nF

1nF

470µF/6.3V/30mΩ

15kΩ

27nF

1nF

Table 2: Typical Compensation for Different Output voltages and Output Capacitors

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