SC2544 datasheet(13/24 Pages) SEMTECH

13 / 24 page

 2005 Semtech Corp.

www.semtech.com

SC2544

POWER MANAGEMENT

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

∆

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 SC2544

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

down channels in the SC2544 are connected to the

same input rail, the input RMS currents will be reduced.

Ripple cancellation effect of interleaving allows the

use of smaller input capacitors.

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

≈

If D

1>0.5 and (D1-0.5) < D2<0.5, then

(

)

)(

(

Cin

−

≈

If D

1>0.5 and D2 < (D1-0.5) < 0.5, then

(

)

(

Cin

−

≈

Choosing P

Choosing Pooooow

wer MOSFET

er MOSFET

er MOSFETsssss

Main considerations in selecting the MOSFET’s are

power dissipation, MOSFETs cost, and packaging.

Switching losses and conduction losses of the MOSFET’s

are directly related to the total gate charge (C

g) and

channel on-resistance (R

ds(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 8.

FOM:100*10^{-12}

FOM:200*10^{-12}

FOM:500*10^{-12}

On-resistance (mOhm)

Cg 100 Rds

()

Cg 200 Rds

()

Cg 500 Rds

()

Rds

Figure 8. 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 C

g and low Rds(on. Usually a

trade-off between R

ds(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 C

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

ds(on) are used for the bottom switch.

If D

1>0.5 and D2 > 0.5, then

(

)

)(

(

Cin

−

≈

MOSFET power dissipation consists of

a) conduction loss due to the channel resistance R

ds(on);

b) switching loss due to the switch rise time t

r and

fall time t

f; and

c) the gate loss due to the gate resistance R

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

capacitor is

Applications Information (Cont.)

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