AOZ1052PI datasheet(8/14 Pages) AOSMD

8 / 14 page

AOZ1052PI

Rev. 0.5 September 2012

www.aosmd.com

Page 8 of 14

Application Information

The basic AOZ1052PI application circuit is show in

Figure 1. Component selection is explained below.

Input Capacitor

The input capacitor must be connected to the VIN pin and

PGND pin of AOZ1052PI to maintain steady input

voltage and filter out the pulsing input current. The

voltage rating of input capacitor must be greater than

maximum input voltage plus ripple voltage.

The input ripple voltage can be approximated by

equation below:

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 input capacitor current can be

calculated by:

if we let m equal the conversion ratio:

The relationship between the input capacitor RMS

current and voltage conversion ratio is calculated and

shown in Figure 2 below. 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 0.5 x IO.

Figure 2. ICIN vs. Voltage Conversion Ratio

For reliable operation and best performance, the input

capacitors must have a 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 current rating. Depending on the

application circuits, other low ESR tantalum capacitors

may be used. When selecting ceramic capacitors, X5R or

X7R type dielectric ceramic capacitors should be used

for their better temperature and voltage characteristics.

Note that the ripple current rating from capacitor

manufactures are based on a certain operating 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:

The peak inductor current is:

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 the inductor is designed to be

20% to 40% of output current.

When selecting the inductor, confirm it is able to handle

the peak current without saturation at the highest

operating temperature.

The inductor takes the highest current in a buck circuit.

The conduction loss on the 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. However,

they cost more than unshielded inductors. The choice

depends on EMI requirement, price and size.

V

fCIN



-----------------

VIN

---------

–







VIN

---------



ICIN_RMS

VIN

--------- 1

VIN

---------

–









VIN

---------

0.1

0.2

0.3

0.4

0.5

CIN_RMS(m)

I



-----------

VIN

---------

–









ILpeak

I

--------

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