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SC2446AEVB データシート(PDF) 13 Page - Semtech Corporation

部品番号 SC2446AEVB
部品情報  Dual-Phase, Single or Dual Output Synchronous Step-Down Controller
PDF  27 Pages
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メーカー  SEMTECH [Semtech Corporation]
ホームページ  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2446AEVB データシート(HTML) 13 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2446A
)
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 7)
C1a
R1a
C1b
R1b
Ceq
Req
Figure 7. Equivalent RC branch.
Req 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 8.
Figure 8. A simple model for the converter input
In Figure 8 the DC input voltage source has an internal
impedance R
in and the input capacitor Cin has an ESR of
R
esr. MOSFET and input capacitor current waveforms, ESR
voltage ripple and input voltage ripple are shown in Figure
9.
Figure 9. Typical waveforms at converter input.
It can be seen that the current in the input capacitor
pulses with high di/dt. 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
inductances around the pulse current loop.
The RMS value of the capacitor current is approximately
].
)
D
1
(
D
)
D
1
)(
12
1
[(
D
I
I
2
2
2
o
Cin
−
η
+
η
−
δ
+
=
Application Information (Cont.)



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