SC2447TSTRT datasheet(19/26 Pages) SEMTECH

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POWER MANAGEMENT

SC2447

For reliable operation, the maximum power dissipation

in the capacitors should not result in more than 10oC of

temperature rise. Many manufacturers specify the

maximum allowable ripple current (ARMS), rating of the

capacitor at a given ripple frequency and ambient

temperature. The input capacitance should be large

enough to handle the ripple current. For higher power

applications, multiple capacitors are placed in parallel to

increase the ripple current handling capability.

Sometimes meeting tight input voltage ripple

specifications may require the use of larger input

capacitance. At full load, the peak-to-peak input voltage

ripple due to the ESR is

)

(

esr

ESR

∆

The peak-to-peak input voltage ripple due to the capacitor

≈

∆

From these two expressions, C

IN can be found to meet

the input voltage ripple specification. In a multi-phase

converter, interleaved switching reduces ripple. The two

step-down channels of the SC2447 operate at 180 degrees

from each other. If both step-down channels in the SC2447

are connected in parallel, both the input and the output

RMS currents will be reduced.

Ripple cancellation effect of interleaving allows the use

of smaller input capacitors. When converter outputs are

connected in parallel and interleaved, a smaller inductor

and capacitor can be used for each channel. The total

output ripple-voltage remains unchanged. The use of a

smaller inductor helps speed up the output load

transient.

When two channels with a common input are interleaved,

the combined input current waveform depends on the duty

ratios and the output currents of both channels. Assuming

that the output current ripple is small, the following formula

can be used to estimate the RMS ripple current in the

input capacitor.

Let the duty ratio and output current of Channel 1 and

Channel 2 be D

1, D2 and Io1, Io2, respectively.

If D

1<0.5 and D2<0.5, then

Cin

≈

Choosing Power MOSFETs

Power MOSFETs with integrated gate drivers such as

PIP212, R2J20601NP, PIP202, PIP201 and IP2001,

IP2002 are suitable for SC2447 application.

Current Sensing

Inductor current sensing is required for the current-mode

control. Although the inductor current can be sensed

with a precision resistor in series with the inductor, the

lossless inductive current sense technique can be used

in the SC2447. This technique has the advantages of,

1) lossless current sensing

2) lower cost compared to resistive sensing

3) more accurate compared to R

DS(ON) sensing

The basic arrangement of the inductive current sense is

shown in Figure 13.

L is the equivalent series resistance of the output inductor.

s and Cs form a RC network for inductor current sensing.

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Figure 13. The Basic Structure of Inductive Current

Sense

In steady state, the DC voltage across R

L is

CS = RL IO .

Notice that the DC value of V

Cs is independent of the

values of L, R

s and Cs.

This means that, if only the

average load current information is needed (such as in

Application Information (Cont.)

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