aoz1281di Alpha & Omega Semiconductor, aoz1281di Datasheet - Page 9

• Manufacturer Part Number: aoz1281di
• Description: Ezbucktm 1.8 A Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum capacitor or
aluminum electrolytic capacitor may also be used as
output capacitors.
In a buck converter, output capacitor current is
continuous. The RMS current of the 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, 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 the diode, a Schottky diode is recommended.
The maximum reverse voltage rating of the 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
Consideration
In the AOZ1281 buck regulator circuit, high pulsing
current flows through two circuit loops. The first loop
starts from the input capacitors, to the VIN pins, 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 capacitors 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 will
reduce the noise of this circuit and improves efficiency.
A ground plane is strongly recommended to connect the
input capacitor, the output capacitor, and the GND pin of
the AOZ1281.
In the AOZ1281 buck regulator circuit, the major power
dissipating components are the AOZ1281, the Schottky
diode and the output inductor. The total power dissipation
of the converter circuit can be measured by input power
minus output power.
I
P
CO_RMS
Rev. 1.0 June 2011
total_loss
=
=
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ΔI
(
12
V
L
IN
×
I
IN
)
–
(
V
O
×
V
IN
)
www.aosmd.com
The power dissipation in the Schottky diode can be
approximated as:
where,
V
The power dissipation of the inductor can be
approximately calculated by output current and DCR of
the inductor.
The actual junction temperature can be calculated with
power dissipation in the AOZ1281 and thermal
impedance from junction to ambient.
The maximum junction temperature of AOZ1281 is
150 ºC, which limits the maximum load current capability.
The thermal performance of the AOZ1281 is strongly
affected by the PCB layout. Extra care should be taken
during the design process to ensure that the IC will
operate under the recommended environmental
conditions.
Several layout tips are listed below for the best electrical
and thermal performance.
1. The input capacitor should be connected as close as
2. The inductor should be placed as close as possible
3. Keep the connection of the schottky diode between
4. Place the feedback resistors and compensation
5. Keep sensitive signal traces away from the LX pin.
6. Pour a maximized copper area to the VIN pins, the
7. Pour a copper plane on all unused board areas and
P
T
P
+
FW_Schottky
diode_loss
=
junction
inductor_loss
T
possible to the VIN pins and the GND pin.
to the LX pin and the output capacitor.
the LX pin and the GND pin as short and wide
as possible.
components as close to the chip as possible.
LX pin and especially the GND pin to help thermal
dissipation.
connect the plane to stable DC nodes, like VIN,
GND or VOUT.
(
amb
P
total_loss
is the Schottky diode forward voltage drop.
=
I
O
=
–
P
I
×
O
diode_loss
2
(
1 D
×
–
R
inductor
)
×
–
V
P
FW_Schottky
inductor_loss
×
1.1
AOZ1281
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