SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441ITSTRT

Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441

POWER MANAGEMENT

Applications Information (Cont.)

∆v ESR

= Resr (1+

δ

2

)Io

.

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

is

∆v C

≈

DIo

Cinfs

.

If D1>0.5 and D2 > 0.5, then

ICin ≈ (D1 + D2 − 1)(Io1 + Io2 )2 + (1− D2 )Io12 + (1− D1)Io22 .

Power MOSFET Selection and Gate Drive

From these two expressions, CIN 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 SC2441 operate at

180 degrees from each other. If both step-down channels

in the SC2441 are connected in parallel, both the input

and the output RMS currents will be reduced.

Main considerations in selecting the MOSFET’s are power

dissipation, cost and packaging. Switching losses and

conduction losses of the MOSFET’s are directly related to

the total gate charge (Cg) and channel on-resistance

(Rds(on)). In order to judge the performance of MOSFET’s,

the product of the total gate charge and on-resistance is

used as a figure of merit (FOM). Transistors with the same

FOM follow the same curve in Figure 10.

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.

50

40

Cg(100, Rds)

Cg(200, Rds)

Cg(500, Rds) 20

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 ratios and output currents of Channel 1 and

Channel 2 be D1, D2 and Io1, Io2 respectively.

If D1<0.5 and D2<0.5, then

ICin ≈ D1Io12 + D2Io22 .

If D1>0.5 and (D1-0.5) < D2<0.5, then

ICin ≈ 0.5Io12 + (D1 − 0.5)(Io1 + Io2 )2 + (D2 − D1 + 0.5)Io22 .

If D1>0.5 and D2 < (D1-0.5) < 0.5, then

ICin ≈ 0.5Io12 + D2 (Io1 + Io2 )2 + (D1 − D2 − 0.5)Io22 .

1 00

5

10

15

20

1

Rds

20

On-resistance (mOhm)

FOM:100*10^{-12}

FOM:200*10^{-12}

FOM:500*10^{-12}

Figure 10. Figure of merit curves.

The closer the curve is to the origin, the lower is the FOM.

This means lower switching loss or lower conduction loss

or both. It may be difficult to find MOSFET’s with both low

Cg and low Rds(on. Usually a trade-off between Rds(on and Cg

has to be made.

MOSFET selection also depends on applications. In many

applications, either switching loss or conduction loss

dominates for a particular MOSFET. For synchronous buck

converters with high input to output voltage ratios, the top

MOSFET is hard switched but conducts with very low duty

cycle. The bottom switch conducts at high duty cycle but

switches at near zero voltage. For such applications,

MOSFET’s with low Cg are used for the top switch and

MOSFET’s with low Rds(on) are used for the bottom switch.

 2005 Semtech Corp.

17

www.semtech.com

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