SC2542 datasheet(18/27 Pages) SEMTECH

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SC2542

POWER MANAGEMENT

the ESR is:

ESR

)

(

∆

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

tor is:

≈

∆

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 SC2542 op-

erate at 180 degrees from each other. If both step-down

channels in the SC2542 are connected to the same in-

put 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 inter-

leaved, the total DC input current is simply the sum of

the individual DC input currents. The combined input cur-

rent 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 esti-

mate 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 D1, D2 and Io1, Io2, respectively.

If D1<0.5 and D2<0.5, then:

CIN

≈

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

0.5

(

0.5)(

) (

0.5)

CIN

D D

≈+

−

− +

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

2 1)(

) (1

)

CIN

ID D

D I

≈

+ −

+−

Choosing Power MOSFETs

Main considerations in selecting the MOSFETs are power

dissipation, MOSFETs cost, and packaging. Switching

losses and conduction losses of the MOSFET are directly

related to the total gate charge (Cg) and channel on-re-

sistance (Rds(on)). In order to judge the performance of

MOSFET, 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 16

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 16. 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 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 domi-

nates 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 with low Cg are used for the top switch and

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

MOSFET power dissipation consists of:

a) conduction loss due to the channel resistance Rds(on);

b) switching loss due to the switch rise time tr and fall

time tf; and

c) the gate loss due to the gate resistance RG.

Applications Information (Cont.)

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