SC2545MLTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2545MLTRT

High Performance Wide Input Range Dual Synchronous Buck Controller

Semtech Corporation 

SC2545

POWER MANAGEMENT

Applications Information (Cont.)

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

'Y (65

5 HVU

 

G



,R



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

is

'Y &

|

',R

&LQIV



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 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 D1, D2 and Io1, Io2, respectively.

If D1<0.5 and D2<0.5, then

,&LQ | ',R  ',R 

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

,&LQ | ,R  '  ,R  ,R   '  '  ,R 

Choosing Power MOSFETs

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 (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 8.

$

Bh‡rÃ8uh…trÁ8 888ttt



!

$







Sq†

Sq†

Sq†







#

!





$



$

!

Sq†

!

P…r†v†‡hprÀPu€

)20A^`

)20A^`

)20A^`

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 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.

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

,&LQ | ,R  ' ,R  ,R   '  '  ,R 

If D1>0.5 and D2 > 0.5, then

,&LQ | '  '  ,R  ,R    ' ,R   ',R 

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.

ã 2005 Semtech Corp.

13

www.semtech.com

Share Link: