SC1403(2002) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC1403 Datasheet PDF : 30 Pages
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SC1403
POWER MANAGEMENT
Applications Information
Input Capacitor Selection
Input bulk capacitor is selected based on the input RMS current
requirement of the converter.
The input RMS ripple current can be calculated as follows:
IRMS =
VOUT
⋅ (VIN
−
VOUT
)
⋅
Iout
VIN
The worst case input RMS current occurs at 50% duty cycle and
therefore under this condition the Irms current can be approxi-
mated by
IRMS
=
ILOAD
2
Therefore, for a maximum load current of 6A, the input capacitor
should be able to handle 3A of ripple current. For the reference
circuit design, there are two such regulators that operate out-of-
phase. Therefore, 3A ripple current is the most these two convert-
ers will see under the normal steady state operating condition. For
the combined two regulators, one SMT OS-CON 47uF, 25V is used.
The maximum allowable ripple current for the cap is rated 3.5A
rms @ 100KHz, 45 °C . Considering the derating at higher ambi-
ent temperature and higher operating frequency, two additional
MLC caps are also used (Vishay MLC, 12uF, 25V, Y5V, size 2225).
PRELIMINARY
External Feedback Design
In order to optimize the ripple voltage during Power Save mode, it
is strongly recommended to use external voltage dividers (R10
and R9 for 5V power train; R8 and R11 for 3.3V power train) to
achieve the required output voltages. In addition, a 56pF (C22 for
5V and C21 for 3.3V) cap is recommended connecting from the
output to both feedback pins (pin # 3 and #12). The signal to
noise ratio is therefore increased due to the added zeroes.
Input Capacitor Selection/Out-of-phase Switching
The SC1403 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
Choosing Synchronous MOSFET and Schottky diode
Since this is a buck topology, the voltage and current ratings of the
synchronous MOSFET is the same as the main switching MOSFET.
It makes sense cost-volume-wise to use the same MOSFET for the
main switch as for the synchronous MOSFET. Therefore,
STS12NF30L is used again in the design for synchronous MOSFET.
To improve overall efficiency, an external schottky diode is used in
parallel to the synchronous MOSFET. The freewheeling current is
going into the schottky diode instead of the body diode of the
synchronous MOSFET, which usually has very high forward drop
and slow transient behavior. It is really important when laying out
the board, to place both the synchronous MOSFET and Schottky
diode close to each other to reduce the current ramp-up and ramp-
down time due to parasitic inductance between the channel of
the MOSFET and the Schottky diode. The current rating of the
Schottky diode can be determined by the following equation,
IF
_
AVG
=
ILOAD
⋅
100n
TS
=
0.2A
where 100nsec is the estimated time between the MOSFET turn-
ing off and the Schottky diode taking over and Ts = 3.33uS. There-
fore a Schottky diode with a forward current of 0.5A is sufficient
for this design.
I3 in
I5 in
Iin
a ve ra g e
0
0
Icap
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.
2002 Semtech Corp.
19
www.semtech.com
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