SC1403_04 Datasheet, PDF (20/31 Pages) Semtech Corporation – Mobile Multi-Output PWM Controller with Lossless Current Sense

SC1403
POWER MANAGEMENT
Applications Information
PRELIMINARY
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.
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.
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.
period
phase lead
D3
I5in
D5
Iin
average
D3
I3in
0
D5
I5in
0
Icap
Iin
average
0
0
Icap
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.
D3
I3in
D5 ^I5in
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.
Input Phase lead from 3V to 5V rising edge
voltage
Vin > 9.6 V
41% of switching period.
No switching overlap between 3V and 5V.
9.6V > Vin
> 6.7V
59% of switching period.
Small overlap to prevent simultaneous
3V/5V switching.
Iin
6.7 > Vin
64% of switching period.
average
Small overlap to prevent simultaneous
0
3V/5V switching.
0
Icap
 2004 Semtech Corp.
20
United States Patent No. 6,377,032
www.semtech.com