SC2544 datasheet(10/24 Pages) SEMTECH

10 / 24 page

 2005 Semtech Corp.

www.semtech.com

SC2544

POWER MANAGEMENT

The output capacitor provides output current filtering

in steady state and serves as a reservoir during load

transient. The output capacitor can be modeled as

an ideal capacitor in series with its parasitic ESR and

ESL as shown in Figure 4.

Figure 4. An equivalent circuit of output.

If the current through the branch is i

b(t), the voltage

across the terminals will then be

(

)

(

)

(

)

(

esr

esl

∫

This basic equation illustrates the effects of ESR,

ESL, and C

o on the output voltage.

Lesl

Resr

General Design Pr

General Design Procedure f

ocedure f

ocedure for a St

or a St

or a Step-do

ep-do

ep-down P

wn P

wn Pooooow

wer

Converter

Selection criterias and design procedures for the fol-

lowing parameters are described:

1) Output inductor (L) type and value

2) Output capacitor (C

o) type and value

3) Input capacitor (C

in) type and value

4) Power MOSFETs

5) Current sensing and limiting circuit

6) Voltage sensing circuit

7) Loop compensation network

The following step-down converter specifications are

needed:

Input voltage range: V

in,min and Vin,max

Input voltage ripple (peak-to-peak): DV

Output voltage: V

Output voltage accuracy: e

Output voltage ripple (peak-to-peak): DV

Nominal output (load) current: I

Maximum output current limit: I

o,max

Output (load) current transient slew rate: dI

o (A/ s)

Circuit efficiency: η

Inductor (L) and Ripple Current

Both step-down controllers in the SC2544 operate in

synchronous continuous-conduction mode (CCM)

regardless of the output load level. The output

inductor selection/design is based on the output DC

and transient requirements. Both output current and

voltage ripples are reduced with larger inductance

but it takes longer to change the inductor current

during load transients. Conversely smaller inductance

results in lower DC copper losses but the AC core

losses (flux swing) and the winding AC resistance losses

are higher. A compromise is to choose the inductance

such that peak-to-peak inductor ripple-current is 20%

to 30% of the rated output load current.

Assuming that the inductor current ripple (peak-to-

peak) value is

o, the inductance value will then

The peak current in the inductor becomes

(1+

/2)*Io

and the RMS current is

The followings are to be considered when choosing

inductors.

a) Inductor core material: For higher efficiency

applications above 300 KHz, ferrite, Kool-Mu and

polypermalloy materials should be used. Low-cost

powdered iron cores can be used for cost sensitive-

applications below 300 KHz but with attendant higher

core losses.

b) Select inductance value: Sometimes the calculated

inductance value is not available off-the-shelf. The

designer can choose the adjacent (larger) standard

inductance value. The inductance varies with

temperature and DC current. It is a good engineering

practice to re-evaluate the resultant current ripple

at the rated DC output current.

c) Current rating: The saturation current of the

inductor should be at least 1.5 times of the peak

inductor current under all conditions.

Output Capacit

Output Capacitor (C

or (C

ooooo) and V

) and V

out

out Ripple

Ripple

rms

)

(

−

Applications Information (Cont.)

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