SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441ITSTRT
Semtech Corporation
SC2441ITSTRT Datasheet PDF : 35 Pages
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SC2441
POWER MANAGEMENT
Applications Information (Cont.)
P2 is a pole for suppressing high-frequency switching
noise. So P2 >> Z2. To simplify design, one usually assumes
that C3<<C2, Roesr<<Ro, Sp1=Sz2 and specifies the loop
crossover frequency fc. The loop crossover frequency
determines the converter dynamic response. With these
assumptions, the controller parameters are determined
as follows
C2
=
gmhkR o
2πfc
,
and
R2
=
RoCo
C2
C3
=
R oesr C o
R2
.
For example, if Vin=3.3V, Vo=1.2V, Io=4A, fs=500kHz,
Co=390µF, Roesr=16mΩ, one can calculate that
Ro
=
Vo
Io
= 300mΩ,
h
=
0.5
Vo
=
0.42
and
k=
∆Io
∆Vc
= 2.60.
Figure 16. Bode plots of the loop response.
If the converter crossover frequency is set around 1/10
of the switching frequency, fc=50kHz, the controller
parameters then can be calculated.
C2
=
gmhkRo
2πfc
≈ 0.497nF,
use C2=0.47nF.
R2
=
RoCo
C2
≈ 150.8kΩ,
use R2=150kΩ.
It is further calculated that
C3
=
Roesr Co
R2
≈ 4.97pF,
use C3=5pF. The Bode plots of the loop transfer function
(magnitude and phase) are shown in Figure 16.
The resulting crossover frequency is about 49.2kHz with
phase margin 90o.
If the circuit noise makes the converter jitter, a larger C3
than the calculated value can be used. Effectively the
converter bandwidth is reduced to reject high frequency
noises. The final circuit should be checked for stability
under load transients at different line voltages. The load
transient also needs to be measured to ensure that the
output voltage is within the specification window.
Step-up Converter
The step-up regulator in the SC2441 is integrated with a
1.7A power switch. Switch current sensing and ramp
compensation are implemented internally. The switching
frequency fs3 of the step-up regulator is twice those of
the step-down controllers.
2005 Semtech Corp.
24
www.semtech.com
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