SC2545 datasheet(13/24 Pages) SEMTECH

13 / 24 page

ã 2005 Semtech Corp.

www.semtech.com

SC2545

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

HVU

(65

& |

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 SC2545

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

down channels in the SC2545 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

&LQ

If D

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

&LQ

If D

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

&LQ

Choosing Power MOSFETs

Main considerations in selecting the MOSFETs are power

dissipation, MOSFETs cost, and packaging. Switching

losses and conduction losses of the MOSFETs are

directly related to the total gate charge (C

g) and channel

on-resistance (R

ds(on)). In order to judge the performance

of MOSFETs, 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.

)20A^`

PrvhprÃPu

rÃ

8t Sq

8t ! Sq

8t $ Sq

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 MOSFETs with both

low C

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

and C

g 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,

MOSFETs with low C

g are used for the top switch and

MOSFETs with low R

ds(on) are used for the bottom switch.

If D

1>0.5 and D2 > 0.5, then

&LQ

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

Applications Information (Cont.)

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