SC2447 Semtech Corporation, SC2447 Datasheet - Page 18

• Manufacturer Part Number: SC2447
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

they should be calculated using the following formulae.
where R
electrolytic capacitors, and R
capacitance of the ceramic capacitors respectively.
(Figure 10)
R
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
R
independent and
Input Capacitor (C
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
R
waveforms, the ESR voltage ripple and the input voltage
ripple are shown in Figure 12.
R
C
POWER MANAGEMENT
Application Information (Cont.)
eq
1
esr
eq
eq
2006 Semtech Corp.
and C
(
(
and C
. The MOSFET and the input capacitor current
ω
ω
: )
: )
=
=
1a
1a
R
R (
eq
a 1
= C
R (
Figure 10. Equivalent RC Branch
are both functions of frequency. For rigorous
a 1
and C
R
2
in
a 1
b 1
C
1b
R (
and the input capacitor C
R (
+
a 1
R
R1a
= C
C1a
R
eq
a 1
+
a 1
1a
b 1
R
= 1/2 R
in
+
+
1
)
)
b 1
, then R
R
2
are the ESR and capacitance of
R
ω
2
b 1
C
b 1
2
)
R1b
C
b 1
)
2
C1b
ω
a 1
ω
)
ω
2
2
C
2
1
C
2
C
eq
C
a 1
and C
1b
b 1
a 1
a 1
2
and C
2
C
2
and C
C
C
+
b 1
b 1
b 1
(
2
C
2
eq
+
+
+
a 1
(
eq
= 2C
R (
C
1b
(
+
C
will be frequency-
a 1
b 1
C
are the ESR and
a 1
C
in
+
b 1
1
b 1
+
has an ESR of
.
C
)
2
2
C
b 1
+
b 1
)
R
)
Req
2
Ceq
a 1
1a
C
= R
a 1
2
1b
)
=
18
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
The power dissipated in the input capacitor is then
Figure 12. Typical Waveforms at Converter Input
Figure 11. A Simple Model for the Converter Input
I
Cin
=
I
o
D
1 [(
P
+
Cin
δ
12
= I
2
1 )(
Cin
−
2
R
η
D
esr
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)
.
2
+
η
D
2
1 (
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−
SC2447
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]. )