AOZ1083 Ver la hoja de datos (PDF) - Alpha and Omega Semiconductor
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AOZ1083 Datasheet PDF : 12 Pages
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AOZ1083
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:
ΔVO
=
ΔIL
×
⎛
⎝
ES
RCO
+
-8----×-----f--1-×-----C-----O--⎠⎞
where,
CO is output capacitor value, and
ESRCO is the equivalent series resistance of the output
capacitor.
When a low ESR ceramic capacitor is used as the 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:
ΔVO
=
ΔIL
×
------------1-------------
8 × f × CO
If the impedance of ESR at switching frequency
dominates, the output ripple voltage is mainly decided by
the capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
ΔVO = ΔIL × ESRCO
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum capacitors or
aluminum electrolytic capacitors may also 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:
ICO_RMS = --Δ----I--L--
12
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, the output capacitor could
be overstressed.
Schottky Diode Selection
The external freewheeling diode supplies the current to
the inductor when the high side NMOS switch is off. To
reduce the losses due to the forward voltage drop and
recovery of diode, Schottky diode is recommended to
use. The maximum reverse voltage rating of the chosen
Schottky diode should be greater than the maximum
input voltage, and the current rating should be greater
than the maximum load current.
Thermal Management and Layout
Considerations
In the AOZ1083 buck regulator circuit, high pulsing
current flows through two circuit loops. The first loop
starts from the input capacitors, to the VIN pin, to the
LX pin, to the filter inductor, to the output capacitor and
load, and then returns to the input capacitor through
ground. Current flows in the first loop when the high side
switch is on. The second loop starts from the inductor,
to the output capacitor and load, to the anode of Schottky
diode, to the cathode of Schottky diode. Current flows in
the second loop when the low side diode is on.
In PCB layout, minimizing the area of the two loops
reduces the noise of the circuit and improves efficiency.
A ground plane is strongly recommended to connect
input capacitor, output capacitor, and PGND pin of the
AOZ1083.
In the AOZ1083 buck regulator circuit, the major power
dissipating components are the AOZ1083, the Schottky
diode and output inductor. The total power dissipation of
the converter circuit can be measured by input power
minus output power:
Ptotal_loss = (VIN × IIN) – (VO × VIN)
The power dissipation in the Schottky diode can be
approximated as:
Pdiode_loss = IO × (1 – D) × VFW_Schottky
where,
VFW_Schottky is the Schottky diode forward voltage drop.
The power dissipation of the inductor can be
approximately calculated by output current and DCR of
the inductor.
Pinductor_loss = IO2 × Rinductor × 1.1
Rev. 1.0 July 2011
www.aosmd.com
Page 8 of 12
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