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SC2447TSTRT 데이터시트(PDF) 18 Page - Semtech Corporation

부품명 SC2447TSTRT
상세설명  Dual-Phase, Single or Dual Output Synchronous Step-Down Controller
PDF  26 Pages
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제조업체  SEMTECH [Semtech Corporation]
홈페이지  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2447TSTRT 데이터시트(HTML) 18 Page - Semtech Corporation

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© 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2447
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
+
+
ω
+
+
+
ω
+
=
ω
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
+
+
ω
+
+
+
ω
+
=
ω
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 10)
C1a
R1a
C1b
R1b
Ceq
Req
Figure 10. 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)
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 11.
In Figure 11 the DC input power supply has an internal
impedance R
in and the input capacitor Cin has an ESR of
R
esr. The MOSFET and the input capacitor current
waveforms, the ESR voltage ripple and the input voltage
ripple are shown in Figure 12.
Figure 11. A Simple Model for the Converter Input
Figure 12. Typical Waveforms at Converter Input
It can be seen that high di/dt pulse current flows in the
input capacitor. Capacitors with low ESL should be used.
It is also important to place the input capacitor close to
the MOSFETs on the PC board to reduce trace inductance
around the pulse current loop.
The RMS value of the capacitor current is approximately
].
)
1
(
)
1
)(
12
1
[(
2
2
2
D
D
D
D
I
I
o
Cin
−
+
−
+
=
η
η
δ
The power dissipated in the input capacitor is then
P
Cin = ICin
2
R
esr.
Application Information (Cont.)



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