AOZ1110QI Alpha & Omega Semiconductor Inc, AOZ1110QI Datasheet - Page 10

• Manufacturer Part Number: AOZ1110QI
• Description: IC BUCK SYNC ADJ 4A 24QFN
• Manufacturer: Alpha & Omega Semiconductor Inc
• Series: EZBuck™r
• Type: Step-Down (Buck), PWM - Current Moder

Specifications of AOZ1110QI

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 V ~ 5.5 V
• Current - Output: 4A
• Frequency - Switching: 500kHz, 1MHz
• Voltage - Input: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-WFQFN Exposed Pad
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 785-1257-2

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:
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.
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:
ΔI
I
ΔV
Lpeak
Rev. 1.0 October 2010
L
O
=
=
---------- -
f
=
V
×
ΔI
O
I
L
O
L
×
×
+
⎛
⎜
⎝
⎛
⎝
ΔI
------- -
1
ESR
2
–
L
-------- -
V
V
IN
O
CO
⎞
⎟
⎠
+
-------------------------
8
×
f C
1
×
O
⎞
⎠
www.aosmd.com
where;
C
and ESR
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:
If the impedance of ESR at switching frequency
dominates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum are recommended
to be used as output capacitors.
In a buck converter, output capacitor current is
continuous. The RMS current of output capacitor is
decided by the peak to peak inductor ripple current. It can
be calculated by:
Usually, the ripple current rating of the output capacitor is
a smaller issue because of the low current stress. When
the buck inductor is selected to be very small and
inductor ripple current is high, output capacitor could be
overstressed.
Loop Compensation
The AOZ1110QI employs peak current mode control for
easy use and fast transient response. Peak current mode
control eliminates the double pole effect of the output
L&C filter. It greatly simplifies the compensation loop
design.
With peak current mode control, the buck power stage
can be simplified to be a one-pole and one-zero system
in frequency domain. The pole is dominant pole can be
calculated by:
I
f
ΔV
ΔV
CO_RMS
O
p1
is output capacitor value,
O
O
=
=
=
---------------------------------- -
2π
CO
ΔI
ΔI
is the Equivalent Series Resistor of output
×
=
L
L
C
×
×
----------
1
ΔI
O
ESR
⎛
⎝
12
-------------------------
8
L
×
×
R
f
CO
L
1
×
C
O
⎞
⎠
Page 10 of 16
AOZ1110