SC284 datasheet(15/20 Pages) SEMTECH

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SC284

Applications Information (continued)

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 on the DCR of the inductor.

The 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 on 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-2R2M

2.20±20% 0.060

.80

.54

2.8x3.0x.5

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 SC284

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. Increasing

the output capacitor above this minimum value will

reduce the crossover frequency and provide greater

phase 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

from the following equation.

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