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SC1404ITSTR Datasheet(PDF) 16 Page - Semtech Corporation

Part # SC1404ITSTR
Description  Mobile Multi-Output PWM Controller with Virtual Current Sense
PDF  27 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1404ITSTR Datasheet(HTML) 16 Page - Semtech Corporation

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 2003 Semtech Corp.
www.semtech.com
SC1404
PRELIMINARY
POWER MANAGEMENT
Applications Information
The SC1404 uses out-of-phase switching between the two
converters to reduce input ripple current, enabling the use of
smaller, cheaper input capacitors when compared to in-phase
switching. The two approaches are shown in the following figures.
The first figure 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, resulting 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.
In-phase Switching
The next figure shows out-of-phase switching. Since the 3.3V
and 5V converters are spaced apart, there is no resulting
overlap. This results in a two benefits; the peak current is
reduced and the frequency content is higher, both of which
make filtering easier. The third trace shows the total input
current, and the fourth trace shows the current in and out of the
input capacitors. The RMS value of this current is significantly
lower than the in-phase case and allows for smaller capacitors
due to reduced RMS current ratings.
Out-of-phase Switching
As the input voltage is reduced, the duty cycle of both converters
increases, as shown in the following figure. For inputs less than
8.3 volts it is impossible to prevent overlap when producing
3.3V and 5V outputs, regardless of the phase relationship
between the converters.
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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, otherwise
the two converters may affect each other due to switching
noise. The SC1404 implements this by changing the phase
relationship between the converter depending on the input
voltage.
Vin > 9.6V: 3.3V turn-on leads 5V turn-on by 41% of the
switching period. With Vin > 9.6V it is always possible to
achieve no overlap, which minimizes the input ripple current. At
Vin = 9.6V there is no overlap, but the 3.3V turn-on is nearing
the 5V turn-off.
6.7 < Vin < 9.6V: 3.3V turn-on leads 5V turn-on by 59% of the
period. To prevent the 3V turn-on from coinciding with the 5V
turn-off (which could adversely affect either output), the 5V
pulse is delayed in time slightly such that the 3V turn-on occurs
before the 5V turn-off. This creates a small overlap between the
3V turn-on and the 5V turn-off, with a resulting slight increase in
RMS input ripple, but this is preferred since it greatly reduces
noise problems caused by simultaneous transitions. Note that
at Vin = 6.7, the 3V turn-off is nearing the 5V turn-on.
Input Capacitor Selection/Out-of-phase Switching
I3in
I5in
Iin
0
average
Icap
0
period
phase lead
I3in
I5in
Iin
average
Icap
0
0
Icap
Iin
average
0
0
I3in
I5in



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