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SC2441AEVB Arkusz danych(PDF) 28 Page - Semtech Corporation

Numer części SC2441AEVB
Szczegółowy opis  1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter
PDF  37 Pages
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Producent  SEMTECH [Semtech Corporation]
Strona internetowa  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2441AEVB Arkusz danych(HTML) 28 Page - Semtech Corporation

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 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2441A
2
b
1
a
1
2
b
1
2
a
1
2
2
b
1
a
1
2
a
1
a
1
2
b
1
b
1
2
b
1
2
a
1
2
b
1
a
1
b
1
a
1
eq
)
C
C
(
C
C
)
R
R
(
)
C
R
C
R
(
C
C
)
R
R
(
R
R
:
)
(
R
+
+
w
+
+
+
w
+
=
w
where R
1a and C1a are the ESR and capacitance of
electrolytic capacitors, and R
1b and C1b are the ESR and
capacitance of the ceramic capacitors respectively
(Figure 17).
C1a
R1a
C1b
R1b
Ceq
Req
Figure 17. Equivalent RC branch.
R
eq and Ceq are both functions of frequency. For rigorous
design, the equivalent ESR should be evaluated at the
ripple frequency for voltage ripple calculation when both
ceramic and electrolytic capacitors are used. If R
1a = R1b
= R
1 and C1a = C1b = C1, then Req and Ceq will be frequency-
independent and
R
eq = 1/2 R1 and Ceq = 2C1.
Input Capacitor (C
in) in Step-down Sections
The input supply to the converter usually comes from a
pre-regulator. Since the input supply is not ideal, input
capacitors are needed to filter the current pulses at the
switching frequency. A simple buck converter is shown in
Figure 18.
Figure 18. Buck converter input model
As shown in Fig. 18, the internal DC input voltage source
Applications Information
.
3
2
I
o
d
Usually it is necessary to have several capacitors of the
same type in parallel to satisfy the ESR requirement.
The voltage ripple cause by the capacitor charge/
discharge should be an order of magnitude smaller than
the voltage ripple caused by the ESR. To guarantee this,
the capacitance should satisfy
.
R
f
2
10
C
esr
s
o
p
>
In many application circuits, several low ESR ceramic
capacitors are added in parallel with the aluminum
capacitors to further reduce ESR and improve high
frequency decoupling. Since the capacitances and the
ESR’s of ceramic and aluminum capacitors are different,
the following remarks are made to clarify some practical
issues.
Remark 1: High frequency ceramic capacitors may not
carry most of the ripple current. It also depends on the
capacitor value. Only when the capacitor value is set
properly, the effect of ceramic capacitor low ESR starts
to be significant.
For example, if a 10
µF, 4mΩ ceramic capacitor is
connected in parallel with 2x1500
µF, 90mΩ electrolytic
capacitors, the ripple current in the ceramic capacitor is
only about 42% of the current in the electrolytic
capacitors at the ripple frequency. If a 100
µF, 2mΩ
ceramic capacitor is used, the ripple current in the
ceramic capacitor will be about 4.2 times of that in the
electrolytic capacitors. When two 100
µF, 2mΩ ceramic
capacitors are used, the current ratio increases to 8.3.
In this case most of the ripple current flows in the
ceramic decoupling capacitor. The ESR of the ceramic
capacitors will then determine the output ripple-voltage.
Remark 2: The total equivalent capacitance of the filter
bank is not simply the sum of all the paralleled capacitors.
The total equivalent ESR is not simply the parallel
combination of all the individual ESR’s either. Instead
they should be calculated using the following formulae.
)
C
C
(
C
C
)
C
R
C
R
(
)
C
C
(
C
C
)
R
R
(
:
)
(
C
b
1
a
1
b
1
a
1
2
b
1
2
b
1
a
1
2
a
1
2
b
1
a
1
2
b
1
2
a
1
2
2
b
1
a
1
eq
+
+
w
+
+
+
w
+
=
w



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