SC2463 Semtech, SC2463 Datasheet - Page 14

• Manufacturer Part Number: SC2463
• Description: High Performance Quad Output Switching Regulator
• Manufacturer: Semtech

Input Capacitor (C
Input Capacitor (C
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
Input Capacitor (C
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 3.
In Figure 3 the DC input voltage source has an internal
impedance R
R
voltage ripple and input voltage ripple are shown in Figure
4.
POWER MANAGEMENT
Applications Information (Cont.)
esr
2007 Semtech Corp.
. MOSFET and input capacitor current waveforms, ESR
1
Figure 3. A simple model for the converter input
and C
Figure 4. Typical waveforms at converter input
1a
= C
in
and the input capacitor C
R
1b
eq
= C
= 1/2 R
1
in in in in in
, then R
) ) ) ) )
1
and C
eq
and C
eq
= 2C
eq
will be frequency-
in
1
.
has an ESR of
1a
= R
1b
14
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.
Sometimes meeting tight input voltage ripple
specifications may require the use of larger input
capacitance. At full load, the peak-to-peak input voltage
ripple due to the ESR is
The peak-to-peak input voltage ripple due to the capacitor
is
From these two expressions, C
the input voltage ripple specification. In a multi-phase
converter, channel interleaving can be used to reduce
ripple. The two step-down channels of the SC2463
operate at 180 degrees from each other. If both step-
down channels in the SC2463 are connected in parallel,
both the input and the output RMS currents will be
reduced.
Ripple cancellation effect of interleaving allows the use
of smaller input capacitors. When converter outputs are
connected in parallel and interleaved, smaller inductors
and capacitors can be used for each channel. The total
output ripple-voltage remains unchanged. Smaller
inductors speeds up output load transient.
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