aoz1605 Alpha & Omega Semiconductor, aoz1605 Datasheet - Page 9

• Manufacturer Part Number: aoz1605
• Description: 500 Ma / 3 Mhz Ezbuck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Inductor Selection
There are two main considerations when choosing an
inductor; the inductor should not saturate, and the
inductor current ripple should be small enough to achieve
the desire output voltage ripple. A 1 μH inductor with a
saturation current of at least 1 A is recommended for the
AOZ1605 full load application. For maximum efficiency,
the inductor’s resistance (DCR) should be as low as
possible. For 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 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 need 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.
Input Capacitor
The input capacitor must be connected to the V
PGND pin of AOZ1605 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. For greater capacitor
performance, the working capacitance voltage should be
twice Vin.
ΔI
I
Lpeak
Rev. 1.0 March 2011
L
=
---------- -
f
=
V
×
O
I
L
O
×
+
⎛
⎜
⎝
ΔI
------- -
1
2
–
L
-------- -
V
V
IN
O
⎞
⎟
⎠
IN
pin and
www.aosmd.com
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 1 below. It can be seen that when V
half of V
worst current stress on C
For reliable operation and best performance, the input
capacitors must have current rating higher than I
at worst operating conditions. Ceramic capacitors are
preferred for input capacitors because of their low ESR
and high current rating. 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 certain
amount of life time. Further de-rating may be necessary
in practical design.
-------- -
V
ΔV
V
I
CIN_RMS
IN
I
O
CIN_RMS
IN
=
I
O
=
Figure 3. I
IN
m
(m)
, C
---------------- -
f
×
0.5
0.4
0.3
0.2
0.1
I
=
IN
O
C
0
0
is under the worst current stress. The
IN
I
O
CIN
×
×
⎛
⎜
⎝
vs. Voltage Conversion Ratio
1
-------- - 1
V
V
–
IN
O
IN
-------- -
V
V
⎛
⎜
⎝
IN
O
is 0.5 x I
⎞
⎟
⎠
–
×
0.5
-------- -
V
V
m
-------- -
V
IN
V
O
IN
O
⎞
⎟
⎠
O
.
AOZ1605
Page 9 of 15
CIN_RMS
1
O
is