A7431
DC-DC CONVERTER BUCK (STEP-DOWN)
42V 3A CC/CV
AiT Semiconductor Inc.
www.ait-ic.com
The following steps should be used to compensate the IC:
STEP 1. Set the cross over frequency at 1/10 of the switching frequency via RCOMP
:
2πVOUT
C
OUT
f
SW
RCOMP
=
10GEA COMP x 0.808V
G
=5.12 x 107VOUTCOUT
(Ω)
STEP 2. Set the zero fZ1 at 1/4 of the cross over frequency. If RCOMP is less than 15kΩ, the equation for CCOMP
is:
2.83 x 105
RCOMP
CCOMP
=
(F)
If RCOMP is limited to 15kΩ, then the actual cross over frequency is 6.58 / (VOUTCOUT). Therefore:
CCOMP = 6.45 x 10-6VOUTCOUT
(F)
STEP 3. 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:
1.77x10−6
,0.006xVOUT
RESRCOUT
≥
Min
(Ω)
COUT
And the proper value for CCOMP2 is:
C
OUT
R
COMP
ESRCOUT
COMP2
=
R
Though CCOMP2 is unnecessary when the output capacitor has sufficiently low ESR, a small value CCOMP2 such
as 100pF may improve stability against PCB layout parasitic effects.
Table 1 shows some calculated results based on the compensation method above.
①
VOUT
COUT
RCOMP CCOMP CCOMP2
2.5V 47uF Ceramic CAP 5.6kΩ
3.3V 47uF Ceramic CAP 6.2kΩ
5.0V 47uF Ceramic CAP 12kΩ
2.2nF
2.2nF
2.2nF
2.2nF
2.2nF
2.2nF
None
None
None
47pF
47pF
47pF
2.5V 220uF/10V/30mΩ
3.3V 220uF/10V/30mΩ
5.0V 220uF/10V/30mΩ
20kΩ
20kΩ
20kΩ
CCOMP2 is needed for high ESR output capacitor.
COMP2 ≤ 47pF is recommended.
C
Table 1: Typical Compensation for Different Output Voltages and Output Capacitors
REV1.1
- JAN 2016 RELEASED, SEP 2016 UPDATED -
- 11 -
AITSEMI [ AiT Semiconductor ]
