SC189AEVB datasheet(19/23 Pages) SEMTECH

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SC189

Applications Information (continued)

compensation is designed to work with a output filter

corner frequency of less than 100kHz over any operating

condition, tolerance and bias effect. The corner frequency

of output filter can be defined by the equation

OUT

Values outside this range may lead to instability,

malfunction, or out-of-specification performance.

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 his 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

varysignificantlywithcoretemperature.Coreandambient

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 similar sized non-shielded inductor.

TheSC189iscompatiblewithsmallshieldedchipinductors

for low cost, low profile applications. The inductance roll

off characteristic of chip inductor is worse resulting in

high ripple current and increased output voltage ripple

at heavy load operation. SC189 has OCP peak inductor

current threshold of 2.0A minimum, to support 1.5A DC

load current, the inductor ripple current at 1.5A DC load

current needs to be less than 1A.

Final inductor selection depends on various design

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

2aand2blistthemanufacturersofrecommendedinductor

and output capacitors. Chip inductors provide smaller

footprint and height with lower efficiency and increased

output voltage ripple. Transient load performance is

equivalent to wire wound inductors. Figure 6 shows the

typical efficiency curves for different inductors.

Efficiency

60%

65%

70%

75%

80%

85%

90%

95%

100%

0.0

0.3

0.6

0.9

1.2

1.5

Output Current (A)

VIN= 5.0V

VOUT= 3.3V

TA=25°C

L=1071AS-1R0N (33m

_typ)

L=1071AS-2R2N (50m

_typ)

L=LQM2HP1R0MG0 (55m

_typ)

L=MDT2520-CR1R0M (60m

_typ)

Figure 6 — Typical efficiency curves

IN=5.0V, VOUT=3.3V)

OUT Selection

TheinternalvoltageloopcompensationintheSC189limits

the minimum output capacitor value to 10μF if using the

inductor of 2.2μH. 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. A total output capacintance should

not exceed 30uF to avoid any start-up problems. For most

typical applications, it is recommended to use output

capacitance of 10uF to 22uF. When choosing output

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