SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441AEVB
(Rev.:2006)

1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441A

POWER MANAGEMENT

Applications Information

impedance is Rin and the input capacitor Cin has an ESR

denoted as R . MOSFET and input capacitor current

esr

waveforms, ESR voltage ripple and input voltage ripple

are shown in Figure 19.

Dv ESR

= Resr (1+

d

2

)Io

.

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

is

Dv C

»

DIo

Cinfs

.

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 SC2441A

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

down channels in the SC2441A are connected in parallel,

both the input and the output RMS currents will be

reduced.

Figure 19. Typical waveforms at the input of a

buck converter.

It can be seen that the current in the input capacitor

pulses with high di/dt. Capacitors with low ESL should be

used. It is also important to place the input capacitor

close to the MOSFET’s on the PC board to reduce trace

inductances around the pulse current loop.

The RMS value of the capacitor current is approximately

ICin = Io

D[(1+

d2 )(1-

12

D )2

h

+

D

h2

(1-

D) ].

The power losses at the input capacitors is then

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.

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.

PCin = ICin2Resr.

Let the duty ratios and output currents of Channel 1 and

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

For reliable operation, the maximum power dissipation

in the capacitors should not result in more than 10oC of

temperature rise. Many manufacturers specify the

maximum allowable ripple current (ARMS) rating of the

capacitor at a given ripple frequency and ambient

temperature. The input capacitance should be high

enough to handle the ripple current. For higher power

applications, multiple capacitors are placed in parallel to

increase the ripple current handling capability.

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 .

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

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

ICin » 0.5Io12 + D2 (Io1 + Io2 )2 + (D1 - D2 - 0.5)Io22 .

 2006 Semtech Corp.

29

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