SC183C

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15

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 induc-

tor 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 resis-

tance (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 similar 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

Applications Information (continued)

options. The saturation characteristics and DC current

ratings are also shown.

Manufacturer

Part Number

L

(μH)

DCR

Max

(Ω)

Rated

Current

(A)

L at

Rated

Current

(μH)

Dimen-

sions

LxWxH

(mm)

TOKO

1071AS-2R2M

2.20±20% 0.060

1.80

1.54

2.8x3.0x1.5

TOKO

1071AS-1R0N

1.00±30% 0.040

2.70

0.70

2.8x3.0x1.5

TOKO

1127AS-2R2M

2.20±20% 0.048

2.50

1.54

3.5x3.7x1.8

Panasonic

ELLVGG1R0N

1.00±23% 0.062

2.20

0.70

3.2x3.2x1.5

Table 2 – Recommended Inductors

C

The internal voltage loop compensation in the SC183C

limits the minimum output capacitor value to 22µF if

using an inductor value of 2.2µH or 44mF if using an induc-

tor of 1µH. 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

typical applications it is recommended to use an output

capacitor’s capacitance, verify the voltage derating effect

from the capacitor vendors data sheet.

Capacitors with X7R or X5R ceramic dielectric are recom-

mended 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 deter-

mined 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

Equation 4.