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SC2441AEVB Arkusz danych(PDF) 29 Page - Semtech Corporation

Numer części SC2441AEVB
Szczegółowy opis  1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter
PDF  37 Pages
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Producent  SEMTECH [Semtech Corporation]
Strona internetowa  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2441AEVB Arkusz danych(HTML) 29 Page - Semtech Corporation

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 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2441A
Applications Information
impedance is R
in and the input capacitor Cin has an ESR
denoted as R
esr. MOSFET and input capacitor current
waveforms, ESR voltage ripple and input voltage ripple
are shown in Figure 19.
Figure 19. Typical waveforms at the input of a
buck converter.
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
].
)
D
1
(
D
)
D
1
)(
12
1
[(
D
I
I
2
2
2
o
Cin
-
h
+
h
-
d
+
=
The power losses at 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. 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
.
I)
2
1
(
R
v
o
esr
ESR
d
+
=
D
The peak-to-peak input voltage ripple due to the capacitor
is
.
f
C
DI
v
s
in
o
C »
D
From these two expressions, C
in can be found to meet
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 SC2441A
operate at 180 degrees from each other. If both step-
down channels in the SC2441A 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.
When two channels with a common input are interleaved,
the total DC input current is simply the sum of the
individual DC input currents. The combined input current
waveform depends on duty ratio and the output current
waveform. Assuming that the output current ripple is
small, the following formula can be used to estimate the
RMS value of the ripple current in the input capacitor.
Let the duty ratios and output currents of Channel 1 and
Channel 2 be D
1, D2 and Io1, Io2 respectively.
If D
1<0.5 and D2<0.5, then
.
I
D
I
D
I
2
2
o
2
2
1
o
1
Cin
+
»
If D
1>0.5 and (D1-0.5) < D2<0.5, then
.
I)
5
.
0
D
D
(
)
I
I
)(
5
.
0
D
(
I
5
.
0
I
2
2
o
1
2
2
2
o
1
o
1
2
1
o
Cin
+
-
+
+
-
+
»
If D
1>0.5 and D2 < (D1-0.5) < 0.5, then
.
I)
5
.
0
D
D
(
)
I
I
(
D
I
5
.
0
I
2
2
o
2
1
2
2
o
1
o
2
2
1
o
Cin
-
-
+
+
+
»



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