SC5014AEVB datasheet(17/27 Pages) SEMTECH

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SC5014A

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-Inputcurrent;IOUT–Outputcurrent;

OUT–Boostoutputvoltage;

IN–Inputvoltage;

η–Efficiencyoftheboostconverter

Then the duty ratio under CCM is shown by the following

equation.

OUT

−

D – Forward conduction drop of output rectifying diode

When the boost converter runs under DCM ( L < L

boundary),

it has the advantages of small inductance and quick tran-

sient 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:

ΔI

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:

peak

For most applications, an inductor with a value of 2.2

µH to

µH should be acceptable, (refer to the detailed applica-

tion circuit on page 20, Figure 2). The inductor peak

current must be less than its saturation rating. When the

inductor current is close to the saturation level, its induc-

tance can decrease 20% to 35% from the 0A value depend-

ing 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 a 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 2 lists some recommended inductors and their

vendors.

Table 2. Recommended Inductors

Inductor

Vendor Website

HCM0703, 2.2uH~0uH

www.cooperindustries.com

IHLP-2525CZ-0, 4.7uH~0uH

www.vishay.com

MLPC0730L, 2.2uH~4.7uH

www.nec-tokin.com/english

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

OUT

−

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