AOZ1083 查看數據表（PDF） - Alpha and Omega Semiconductor

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AOZ1083

2A Simple Buck Regulator

Alpha and Omega Semiconductor 

AOZ1083

Application Information

The basic AOZ1083 application circuit is shown in

Figure 1. Component selection is explained below.

Input Capacitor

The input capacitor must be connected to the VIN pin

and PGND pin of the AOZ1083 to maintain steady input

voltage and filter out the pulsing input current. The

voltage rating of the input capacitor must be greater than

the maximum input voltage plus ripple voltage.

The input ripple voltage can be approximated by

equation below::

ΔVIN

=

-------I-O---------

×

⎛

⎜1

–

-V----O---

⎞

⎟

f × CIN ⎝ VIN⎠

× -V----O---

VIN

Since the input current is discontinuous in a buck

converter, the current stress on the input capacitor is

another concern when selecting the capacitor. For a buck

circuit, the RMS value of the input capacitor current can

be calculated by:

ICIN_RMS = IO ×

-V----O---

⎛

⎜1

–

-V----O---

⎞

⎟

VIN⎝ VIN⎠

if we let m equal the conversion ratio:

-V----O--- = m

VIN

The relationship between the input capacitor RMS

current and voltage conversion ratio is calculated and

shown in Figure 2. It can be seen that when VO is half of

VIN, CIN is under the worst current stress. The worst

current stress on CIN is at 0.5 x IO.

0.5

0.4

ICIN_RMS(m) 0.3

IO

0.2

0.1

0

0

0.5

1

m

Figure 2. ICIN vs. Voltage Conversion Ratio

For reliable operation and best performance, the input

capacitors must have current rating higher than ICIN_RMS

at the worst operating conditions. Ceramic capacitors are

preferred for input capacitors because of their low ESR

and high ripple current rating. Depending on the

application circuits, other low ESR tantalum capacitor or

aluminum electrolytic 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 manufacturers are

based on a certain life time. Further de-rating may need

to be considered for long term reliability.

Inductor

The inductor is used to supply constant current to output

when it is driven by a switching voltage. For a given input

and output voltage, inductance and switching frequency

together decide the inductor ripple current, which is:

ΔIL

=

--V----O----

×

⎛

⎜1

–

-V----O---

⎞

⎟

f × L ⎝ VIN⎠

The peak inductor current is:

ILpeak

=

IO

+

Δ-----I-L--

2

High inductance provides low inductor ripple current but

requires a 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.

When selecting the inductor, confirm 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 cost

more than unshielded inductors. The choice depends on

EMI requirement, price and size.

Output Capacitor

The output capacitor is selected based on the DC output

voltage rating, output ripple voltage specification and

ripple current rating.

Rev. 1.0 July 2011

www.aosmd.com

Page 7 of 12

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