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SC2441ITSTRT Datasheet(PDF) 24 Page - Semtech Corporation

Part # SC2441ITSTRT
Description  Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter
PDF  35 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2441ITSTRT Datasheet(HTML) 24 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2441
P
2 is a pole for suppressing high-frequency switching
noise. So P
2 >> Z2. To simplify design, one usually assumes
that C
3<<C2, Roesr<<Ro, Sp1=Sz2 and specifies the loop
crossover frequency f
c. The loop crossover frequency
determines the converter dynamic response. With these
assumptions, the controller parameters are determined
as follows
,
f
2
hkR
g
C
c
o
m
2
π
=
2
o
o
2
C
C
R
R
=
and
.
R
C
R
C
2
o
oesr
3 =
For example, if V
in=3.3V, Vo=1.2V, Io=4A, fs=500kHz,
C
o=390µF, Roesr=16mΩ, one can calculate that
,
m
300
I
V
R
o
o
o
Ω
=
=
42
.
0
V
5
.
0
h
o
=
=
and
.
60
.
2
V
I
k
c
o
=
∆
∆
=
If the converter crossover frequency is set around 1/10
of the switching frequency, f
c=50kHz, the controller
parameters then can be calculated.
,
nF
497
.
0
f
2
hkR
g
C
c
o
m
2
≈
π
=
use C
2=0.47nF.
,
k
8
.
150
C
C
R
R
2
o
o
2
Ω
≈
=
use R
2=150kΩ.
It is further calculated that
,
pF
97
.
4
R
C
R
C
2
o
oesr
3
≈
=
use C
3=5pF. The Bode plots of the loop transfer function
(magnitude and phase) are shown in Figure 16.
Figure 16. Bode plots of the loop response.
The resulting crossover frequency is about 49.2kHz with
phase margin 90o.
If the circuit noise makes the converter jitter, a larger C
3
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 f
s3 of the step-up regulator is twice those of
the step-down controllers.
Applications Information (Cont.)



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