SC284PEVB datasheet(17/22 Pages) SEMTECH

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SC284P

Applications Information (continued)

In general, the inductance is chosen by making the

inductor ripple current to be less than 30% of maximum

load current. When choosing an inductor, it is important

to consider the change in inductance with DC bias

current. The inductor saturation current is specified as

the current at which the inductance drops a specific

percentage from the nominal value. This is approximately

30%. Except for short-circuit or other fault conditions,

the peak current must always be less than the saturation

current specified by the manufacturer. The peak current is

the maximum load current plus one half of the inductor

ripple current at the maximum input voltage. Load and/or

line transients can cause the peak current to exceed this

level for short durations. Maintaining the peak current

below the inductor saturation specification keeps the

inductor ripple current and the output voltage ripple at

acceptable levels. Manufacturers often provide graphs of

actual inductance and saturation characteristics versus

applied inductor current. The saturation characteristics of

the inductor can vary significantly with core temperature.

Core and ambient temperatures should be considered

when examining the core saturation characteristics.

When the inductance has been determined, the DC

resistance (DCR) must be examined. The efficiency that

can be achieved is dependent upon the DCR of the

inductor. Lower values give higher efficiency. The RMS DC

current rating of the inductor is associated with losses in

the copper windings and the resulting temperature rise of

the inductor. This is usually specified as the current which

produces a 40˚C temperature rise. Most copper windings

are rated to accommodate this temperature rise above

maximum ambient.

Magnetic fields associated with the output inductor can

interfere with nearby circuitry. This can be minimized by

the use of low noise shielded inductors which use the

minimum gap possible to limit the distance that magnetic

fields can radiate from the inductor. However shielded

inductors typically have a higher DCR and are thus less

efficient than a similarly sized non-shielded inductor.

Final inductor selection depends upon various design

considerations such as efficiency, EMI, size, and cost. Table

2 lists the manufacturers of recommended inductor

options. The saturation characteristics and DC current

ratings are also shown.

Manufacturer

Part Number

(μH)

DCR

Max

(Ω)

Rated

Current

(A)

L at

Rated

Current

(μH)

Dimen-

sions

LxWxH

(mm)

TOKO

1071AS-1R0N

.00±30% 0.040

2.70

0.70

2.8x3.0x.5

TOKO

1127AS-2R2M

2.20±20% 0.048

2.50

.54

3.5x3.7x.8

Panasonic

ELLVGG1R0N

.00±23% 0.062

2.20

0.70

3.2x3.2x.5

Table 2 – Recommended Inductors

OUT Selection

The internal voltage loop compensation in the SC284P

limits the minimum output capacitor value to 22µF if using

a 2.2µH inductor or 44µF if using a µH inductor.This is due

to its influence on the the loop crossover frequency, phase

margin, and gain margin. The total output capacitance

should not exceed 50µF to avoid any start-up problems.

For most typical applications it is recommended to use an

output capacitance of 22µF to 44µF. When choosing the

output capacitor’s capacitance, verify the voltage derating

effect from the capacitor vendor’s data sheet.

Capacitors with X7R or X5R ceramic dielectric are

recommended for their low ESR and superior temperature

and voltage characteristics. Y5V capacitors should not

be used as their temperature coefficients make them

unsuitable for this application.

The output voltage droop due to a load transient is

determined by the capacitance of the ceramic output

capacitor. The ceramic capacitor supplies the load current

initially until the loop responds. Within a few switching

cycles the loop will respond and the inductor current will

increase to match the required load. The output voltage

droop during the period prior to the loop responding

can be related to the choice of output capacitor by the

relationship from the following equation.

The outputcapacitorRMSripplecurrentmaybecalculated

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