SC2545TSTRT datasheet(10/24 Pages) SEMTECH

10 / 24 page

ã 2005 Semtech Corp.

www.semtech.com

SC2545

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

HVU

HVO

This basic equation illustrates the effects of ESR, ESL,

and C

o on the output voltage.

/HVO

5HVU

General Design Procedure for a Step-down Power

Converter

Selection criterias and design procedures for the follow-

ing 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: K

Inductor (L) and Ripple Current

Both step-down controllers in the SC2545 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 be

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 Capacitor (C

o) and Vout Ripple

UPV

Applications Information (Cont.)

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