SC195 datasheet(14/18 Pages) SEMTECH

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SC195

Inductor Selection

The SC195 is designed to operate with a 1μH inductor

between the LX pin and the OUT pin. Other values may

lead to instability, malfunction, or out-of-specification

performance. The specified current levels for PSAVE entry,

PSAVE exit, and current limit are dependent on the induc-

tor value.

The SC195 converter has internal loop compensation. The

compensation is designed to work with a specific single-

pole output filter corner frequency defined by the

equation

OUT

Where, L = 1μH and C

OUT

= 10μF.

When selecting output filter components, the LC product

should not vary over a wide range. Selection of smaller

inductor and capacitor values will move the corner fre-

quency, potentially impacting system stability.

It is also important to consider the change in inductance

with DC bias current when choosing an inductor. The

inductor saturation current is specified as the current at

which the inductance drops a specific percentage from

the nominal value (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 dura-

tions. 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 exam-

ining the core saturation characteristics.

Applications Information (continued)

When the inductor value has been determined, the DC

resistance (DCR) must be examined. Efficiency can be

optimized by lowering the inductor’s DCR as much as pos-

sible. Low DCR in an inductor requires either more surface

area for the increased wire diameter or fewer turns to

reduce the length of the copper winding. Fewer turns

requires an inductor core with a larger cross-sectional area

in order to maintain the same saturation characteristics.

The inductor size must always be considered when exam-

ining the inductor DCR to determine the best compromise

between DCR and component area on a PCB. Note that

the ripple component of the inductor is a small percent-

age of the DC load. AC losses in the inductor core and

winding do not contribute significantly to the total

losses.

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. Shielded inductors,

however, typically have a higher DCR and are, therefore,

less efficient than a similar sized non-shielded inductor.

Final inductor selection depends on various design con-

siderations such as efficiency, EMI, size, and cost. Table 2

lists the manufacturers of recommended inductor options.

The inductors with larger packages tend to provide better

overall efficiency, while the smaller package inductors

provide decent efficiency with reduced footprint or height.

The saturation current ratings and DC characteristics are

also shown.

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