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SC1403ITSTRT Datasheet(PDF) 20 Page - Semtech Corporation

Part # SC1403ITSTRT
Description  Mobile Multi-Output PWM Controller with Lossless Current Sense
PDF  31 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1403ITSTRT Datasheet(HTML) 20 Page - Semtech Corporation

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 2004 Semtech Corp.
SC1403
PRELIMINARY
POWER MANAGEMENT
United States Patent No. 6,377,032
www.semtech.com
Applications Information
Input Capacitor Selection and Out-of-phase Switching
The SC1403 uses out-of-phase switching between the two
converters to reduce input ripple current, allowing smaller, cheaper
input capacitors compared to in-phase switching.
The figure below shows in-phase switching. I3in is the input current
drawn by the 3.3V converter, I5in is the input current drawn by the
5V converter. The two converters start each switching cycle
simultaneously, causing in a significant amount of overlap. This
overlap increases the peak current. The total input current to the
converter is the third trace, Iin, which shows how the two currents
add together. The fourth trace shows the current flowing in and
out of the input capacitors.
I3in
I5in
Iin
average
Icap
0
0
D3
D5
The next figure shows out-of-phase switching. The 3.3V and 5V
converters are spaced apart, thus there is no overlap. This gives
two benefits. The peak current is reduced, and the effective switch
frequency is raised; both of which make filtering easier. The third
trace shows the total input current, and the fourth trace shows the
current flowing in and out of the input capacitors. The RMS value
of the capacitor current is significantly lower than the in-phase
case, which allows for smaller capacitors.
I3in
I5in
Icap
Iin
average
0
0
D3
D5
As the input voltage is reduced, the duty cycle of both converters
increases. For all input voltages less than 8.3V it is impossible to
prevent overlap when producing 3.3V and 5V outputs, regardless
of the phase relationship between the two converters. Overlap
can be seen in the following figure.
I5in
Iin
0
average
Icap
0
period
phase lead
D3
D5
From an input filter standpoint it is desirable to minimize the
overlap; but it is also desirable to keep the turn-on and turn-off
transitions of the two converters separated in time, to prevent the
two converters from affecting each other due to switching noise.
The SC1403 keeps the turn-on and turn-off transitions separated
in time by changing the phase relationship between the converter
depending on the input voltage. The following table shows the
phase relationship between 3V and 5V turn-on, based on input
voltage.
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