SC2446AEVB datasheet(14/27 Pages) SEMTECH

14 / 27 page

 2005 Semtech Corp.

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POWER MANAGEMENT

SC2446A

The power dissipated in the input capacitors is then

Cin = ICin

esr.

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 high

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

C ≈

∆

From these two expressions, C

IN can be found to meet

the input voltage ripple specification. In a multi-phase

converter, channel interleaving can be used to reduce

ripple. The two step-down channels of the SC2446A

operate at 180 degrees from each other. If both step-

down channels in the SC2446A 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, smaller inductors

and capacitors can be used for each channel. The total

output ripple-voltage remains unchanged. Smaller

inductors speeds up output load transient.

When two channels with a common input are interleaved,

the total DC input current is simply the sum of the

individual DC input currents. The combined input current

waveform depends on duty ratio and the output current

waveform. Assuming that the output current ripple is

small, the following formula can be used to estimate the

RMS value of the 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

The power devices with integrated gate drivers such as

PIP212, R2J20601NP, PIP202, PIP201 and IP2001,

IP2002 are suitable for SC2446A 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 is used in the

SC2446A. This technique has the advantages of

1) lossless current sensing,

2) lower cost compared to resistive sense

3) more accurate compared to the R

DS(ON) sense

The basic arrangement of the inductive current sense is

shown in Figure 10.

Where, R

L is the equivalent series resistance of the output

inductor. The added R

s and Cs form a RC branch for

inductor current sensing.

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Figure 10. The basic structure of inductive current

sense.

In steady state, the DC value,

CS = RL IO

Application Information (Cont.)

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