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SC2545 датащи(PDF) 12 Page - Semtech Corporation

номер детали SC2545
подробное описание детали  High Performance Wide Input Range Dual Synchronous Buck Controller
PDF  24 Pages
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производитель  SEMTECH [Semtech Corporation]
домашняя страница  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2545 датащи(HTML) 12 Page - Semtech Corporation

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© 2005 Semtech Corp.
www.semtech.com
SC2545
POWER MANAGEMENT
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.
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 5).
Figure 5. Equivalent RC branch.
In Figure 6 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
7.
Input Capacitor (Cin)
Input Capacitor (Cin)
Input Capacitor (Cin)
Input Capacitor (Cin)
Input Capacitor (Cin)
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 6.
C1a
C1b
Ceq
R1a
R1b
Req
Figure 6. A simple model for the converter input.
Figure 7. 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
+
=
The power dissipated in the input capacitors is then
P
Cin = ICin
2
R
esr.
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10oC of
temperature rise. Many manufacturers specify the
maximum allowable ripple current (ARMS) rating of the
capacitor at a given ripple frequency and ambient
temperature. The input capacitance should be high
enough to handle the ripple current. It is common pratice
that multiple capacitors are placed in parallel to increase
the ripple current handling capability.
1
2
Co
L1
D1
Cin
VDC
Ro
Q1
Resr
Rin
i Q1
i Cin
Vesr
VCin
i Q1
i Cin
Vesr
VCin
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 =
)
(
ω
eq
C
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 =
)
(
ω
eq
C
Applications Information (Cont.)



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