SC1403ITSTR datasheet(19/30 Pages) SEMTECH

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 2002 Semtech Corp.

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SC1403

POWER MANAGEMENT

PRELIMINARY

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:

The worst case input RMS current occurs at 50% duty cycle and

therefore under this condition the Irms current can be approxi-

mated by

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).

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,

out

OUT

RMS

)

⋅

−

⋅

LOAD

RMS =

100

LOAD

AVG

⋅

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.

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

I3 in

I5 in

Iin

averag e

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.

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