WIDE INPUT 2A STEP-DOWN CONVERTER
And finally, the third pole is due to R
COMP
and C
COMP2
(if C
COMP2
is used):
FSP3129
Follow the following steps to compensate the IC:
STEP1. Set the cross over frequency at 1/10 of the switching frequency via R
COMP
:
but limit R
COMP
to 15kΩ maximum.
STEP2. Set the zero f
Z1
at 1/4 of the cross over frequency. If R
COMP
is less than 15kΩ, the equation for CCOMP is:
If R
COMP
is limited to 15kΩ, then the actual cross over frequency is 3.4/(V
OUT
C
OUT
). 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 C
COMP2
is required. The condition for using C
COMP2
is:
And the proper value for C
COMP2
is:
A small value C
COMP2
such as 100pF may improve stability against PCB layout parasitic effects.
Table 2 shows some calculated results based on the compensation method above.
C
COMP2
100pF
100pF
100pF
100pF
100pF
100pF
1nF
1nF
1nF
V
OUT
2.5V
3.3V
5V
2.5V
3.3V
5V
2.5V
3.3V
5V
C
OUT
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP CAP
47µF SP CAP
47µF SP CAP
470µF/6.3V/30mΩ
470µF/6.3V/30mΩ
470µF/6.3V/30mΩ
R
COMP
8.2kΩ
12kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
C
COMP
2.2nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
Table 2: Typical Compensation for Different Output voltages and Output Capacitors
6/10
2007-1-4
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
