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SC1404 Datasheet(PDF) 15 Page - Semtech Corporation

Part # SC1404
Description  Mobile Multi-Output PWM Controller with Virtual Current SenseTM
PDF  25 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1404 Datasheet(HTML) 15 Page - Semtech Corporation

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 2004 Semtech Corp.
www.semtech.com
SC1404
POWER MANAGEMENT
Applications Information
Input Capacitor Selection and Out-of-phase Switching
The SC1404 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
for the 3V converter, I5in is the input current for the 5V converter.
The two converters start each switching cycle simultaneously,
causing a significant amount of overlap and a high peak current.
The total input current the third waveform, which shows how the
two currents add together. The fourth waveform is current 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 3V and 5V
pulses are spaced apart, so there is no overlap. This gives two
benefits; the peak current is reduced, and the effective switch
frequency is raised. Both of these make filtering easier. The third
waveform is the total input current, and the fourth waveform 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. At input voltages less than 8.3V, it is impossible to
prevent overlap regardless of the phase between the converters.
Overlap is 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 minimize
interaction between the two converters. The SC1404 keeps the
turn-on and turn-off transitions separated in time by changing the
phase between the converters 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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