AOZ1014 데이터 시트보기 (PDF) - Alpha and Omega Semiconductor

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AOZ1014

5A Simple Buck Regulator

Alpha and Omega Semiconductor 

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and high ripple current rating. Depending on the

application circuits, other low ESR tantalum capacitor

may also be used. When selecting ceramic capacitors,

X5R or X7R type dielectric ceramic capacitors are

preferred for their better temperature and voltage

characteristics. Note that the ripple current rating from

capacitor manufactures are based on certain amount of

life time. Further de-rating may be necessary for

practical design requirement.

Inductor

The inductor is used to supply constant current to

output when it is driven by a switching voltage. For

given input and output voltage, inductance and

switching frequency together decide the inductor ripple

current, which is,

∆I L

=

VO × (1 − VO )

f ×L

VIN

The peak inductor current is:

I Lpeak

=

IO

+

∆I L

2

High inductance gives low inductor ripple current but

requires larger size inductor to avoid saturation. Low

ripple current reduces inductor core losses. It also

reduces RMS current through inductor and switches,

which results in less conduction loss. Usually, peak to

peak ripple current on inductor is designed to be 20%

to 30% of output current.

When selecting the inductor, make sure it is able to

handle the peak current without saturation even at the

highest operating temperature.

The inductor takes the highest current in a buck circuit.

The conduction loss on inductor needs to be checked

for thermal and efficiency requirements.

Surface mount inductors in different shape and styles

are available from Coilcraft, Elytone and Murata.

Shielded inductors are small and radiate less EMI noise.

But they cost more than unshielded inductors. The

choice depends on EMI requirement, price and size.

Table below lists some inductors for typical output

voltage design.

Table 2.

Vout L1

5.0 V Shielded, 4.7uH

MSS1278-472MLD

Shielded, 4.7uH

MSS1260-472MLD

3.3 V Un-shielded, 3.3uH

DO3316P-332MLD

Shielded, 3.3uH

DO1260-332NXD

Shield, 3.3uH

ET553-3R3

1.8 V Shield, 2.2uH

ET553-2R2

Un-shielded, 2.2uH

DO3316P-222MLD

Shielded, 2.2uH

MSS1260-222NXD

Manufacture

Coilcraft

Coilcraft

Coilcraft

Coilcraft

ELYTONE

ELYTONE

Coilcraft

Coilcraft

Output Capacitor

The output capacitor is selected based on the DC

output voltage rating, output ripple voltage specification

and ripple current rating.

The selected output capacitor must have a higher rated

voltage specification than the maximum desired output

voltage including ripple. De-rating needs to be

considered for long term reliability.

Output ripple voltage specification is another important

factor for selecting the output capacitor. In a buck

converter circuit, output ripple voltage is determined by

inductor value, switching frequency, output capacitor

value and ESR. It can be calculated by the equation

below:

∆ VO

=

∆I L

× (ESRCO

+

8×

1

f × CO

)

where CO is output capacitor value and ESRCO is the

Equivalent Series Resistor of output capacitor.

When low ESR ceramic capacitor is used as output

capacitor, the impedance of the capacitor at the

switching frequency dominates. Output ripple is mainly

caused by capacitor value and inductor ripple current.

The output ripple voltage calculation can be simplified

to:

∆VO

=

∆I L

×

8×

1

f × CO

February 2006

www.aosmd.com

Page 11 of 21

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