SC2446A Semtech Corporation, SC2446A Datasheet - Page 13

• Manufacturer Part Number: SC2446A
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

where R
electrolytic capacitors, and R
capacitance of the ceramic capacitors respectively.
(Figure 7)
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
= 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 8.
R
C
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Application Information (Cont.)
eq
eq
2005 Semtech Corp.
1
(
(
and C
: )
: )
1a
R
R (
1a
a 1
R (
a 1
and C
R
= C
2
a 1
b 1
C
R (
R (
a 1
Figure 7. Equivalent RC branch.
R
R1a
1b
C1a
eq
R
a 1
a 1
= C
1a
b 1
= 1/2 R
R
in
)
)
b 1
R
2
are the ESR and capacitance of
1
R
, then R
2
b 1
C
b 1
2
)
R1b
C
b 1
)
2
C1b
a 1
)
2
1
2
C
2
C
2
C
and C
C
a 1
1b
b 1
eq
a 1
a 1
2
2
C
2
and C
and C
C
C
b 1
b 1
b 1
(
2
C
eq
2
a 1
= 2C
(
R (
eq
C
1b
(
C
a 1
b 1
will be frequency-
C
are the ESR and
a 1
C
b 1
1
b 1
.
C
)
2
2
C
b 1
b 1
)
R
)
Req
2
Ceq
a 1
C
1a
a 1
= R
2
)
1b
13
In Figure 8 the DC input voltage source has an internal
impedance R
R
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
esr
. MOSFET and input capacitor current waveforms, ESR
Figure 8. A simple model for the converter input
I
Cin
in
and the input capacitor C
I
o
D
[(
1
12
2
)(
1
D
www.DataSheet4U.com
)
2
D
2
SC2446A
1 (
in
www.semtech.com
has an ESR of
D
]. )