SC5010 datasheet(24/36 Pages) SEMTECH

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SC5010

Components Selection

Inductor Selection

The choice of the inductor affects the converter’s steady

state operation, transient response, and its loop stability.

Special attention needs to be paid to three specifications

of the inductor, its value, its DC resistance and saturation

current. The inductor’s inductance value also determines

the inductor ripple current. The boost converter will

operate in either CCM (Continuous Conduction Mode) or

DCM (Discontinuous Conduction Mode) depending on its

operating conditions. The inductor DC current or input

current can be calculated using the following equation.

OUT

IN — Input current; IOUT — Output current;

OUT — Boost output voltage;

IN — Input voltage;

η — Efﬁciency of the boost converter.

Then the duty ratio under CCM is shown by the following

equation.

OUT

– Forward conduction drop of output rectifying diode

When the boost converter runs under DCM ( L < L

boundary

it takes the advantages of small inductance and quick

transient response; where as if the boost converter works

under CCM (L > L

boundary

), normally the converter has higher

efficiency.

When selecting an inductor, another factor to consider is

the peak-to-peak inductor current ripple, which is given

by the following equation.

Usually this peak-to-peak inductor current ripple can be

chosen between 30% to 50% of the maximum input DC

current. This gives the best compromise between the

inductor size and converter efficiency. The peak inductor

current can be calculated using the following equation.

L-peak

For most applications, an inductor with value of 2.2

μH to

μH should be acceptable, (refer to the Typical

Application Circuit on page 21). The inductor peak current

must be less than its saturation rating. When the inductor

current is close to the saturation level, its inductance can

decrease 20% to 35% from the 0A value depending on the

vendor specifications. Using a small value inductor forces

the converter in DCM, in which case the inductor current

ramps down to zero before the end of each switching

cycle. It reduces the boost converter’s maximum output

current and produces larger input voltage ripple. The DCR

of the inductor plays a significant role for the total system

efficiency and usually there is a trade-off between the DCR

and size of the inductor. Table 3 lists some recommended

inductors and their vendors.

Table 3

— Recommended Inductors

Inductor

Web site

XFL4020, 2.2μH ~ 4.7μH

www.coilcraft.com

DR73, 4.7μH ~ 22μH; DR74, 4.7μH ~ 22μH

www.cooperet.com

IHLP-2525CZ-01, 4.7μH ~ 10μH

www.vishay.com

DS84LC, 4.7μH ~ 10μH; D62LCB, 4.7μH ~ 22μH

www.toko.co.jp

Output Capacitor Selection

The next design task is targeting the proper amount of

output ripple voltage due to the constant-current LED

loads. Usually X5R or X7R ceramic capacitor is recom-

mended. The ceramic capacitor minimum capacitance

needed for a given ripple can be estimated using the fol-

lowing equation.

RIPPLE – Peak to peak output ripple.

The ripple voltage should be less than 200mV (pk-pk) to

ensure good LED current sink regulation. For example, a

typical application where 20mA/channel current is

needed, the total output current for 8 channels will be

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