SC2446 Semtech Corporation, SC2446 Datasheet - Page 14

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

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.
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.
POWER MANAGEMENT
Application Information (Cont.)
1
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2004 Semtech Corp.
and C
Figure 8. A simple model for the converter input
. MOSFET and input capacitor current waveforms, ESR
1a
= C
in
1b
and the input capacitor C
Figure 7. Equivalent RC branch.
R
R1a
= C
C1a
eq
in
= 1/2 R
1
)
, then R
R1b
C1b
1
and C
eq
and C
eq
= 2C
eq
will be frequency-
in
1
.
has an ESR of
Req
Ceq
1a
= R
1b
=
14
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
MOSFET’s on the PC board to reduce trace inductances
around the pulse current loop.
The RMS value of the capacitor current is approximately
The power dissipated in the input capacitors is then
For reliable operation, the maximum power dissipation in
the capacitors should not result in more than 10
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. For higher power applications,
multiple capacitors are placed in parallel to increase the
ripple current handling capability.
I
Cin
I
o
D
[(
1
P
Cin
12
= I
2
)(
1
Cin
2
R
D
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