SC2441ITSTRT datasheet(14/35 Pages) SEMTECH

14 / 35 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2441

conversion ratio at a given switching frequency. If the

desired output voltage requires high operating duty-cycle,

then operating frequency will have to be lowered to allow

modulating headroom.

Setting the Step-down Channel Frequency

The switching frequency of both step-down controllers is

set with an external resistor from Pin 10 to the ground.

The set frequency is inversely proportional to the resistor

value (Figure 5).

100

200

300

400

500

600

700

800

100

150

200

250

Rosc (k Ohm)

Figure 5. Step-down Channel Free-running frequency vs.

OSC.

Inductor (L) and Ripple Current

Both step-down controllers in the SC2441 operate in

synchronous continuous-conduction mode (CCM) regardless

of the output load. The output inductor selection/design

is based on the output DC and transient requirements.

Both output current and voltage ripples are reduced with

larger inductors but it takes longer to change the inductor

current during load transients. Conversely smaller inductors

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)

δ*I

o, the inductance will then be

)

(

−

The peak current in the inductor becomes (1+

δ/2)*Io

and the RMS current is

rms

The followings are to be considered when choosing

inductors.

a) Inductor core material: For high efficiency applications

above 350KHz, ferrite, Kool-Mu and polypermalloy

materials should be used. Low-cost powdered iron cores

can be used for cost sensitive-applications below 350KHz

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 Capacitor (C

o) and Vout Ripple

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 (R

esr) and ESL (Lesl) (Figure

6).

Lesl

Resr

Figure 6. C

o equivalent circuit

If the current through the branch is i

b(t), the voltage across

the terminals will then be

Applications Information (Cont.)

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