aoz1242 Alpha & Omega Semiconductor, aoz1242 Datasheet - Page 10

• Manufacturer Part Number: aoz1242
• Description: Ezbucktm 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

The compensation design is actually to shape the
converter close loop transfer function to get desired gain
and phase. Several different types of compensation
network can be used for AOZ1242. For most cases, a
series capacitor and resistor network connected to the
COMP pin sets the pole-zero and is adequate for a stable
high-bandwidth control loop.
In the AOZ1242, FB pin and COMP pin are the inverting
input and the output of internal transconductance error
amplifier. A series R and C compensation network
connected to COMP provides one pole and one zero.
The pole is:
where;
G
A/V,
G
C
The zero given by the external compensation network,
capacitor C
Figure 1), is located at:
To design the compensation circuit, a target crossover
frequency f
crossover frequency is where control loop has unity gain.
The crossover frequency is also called the converter
bandwidth. Generally a higher bandwidth means faster
response to load transient. However, the bandwidth
should not be too high due to system stability concern.
When designing the compensation loop, converter stabil-
ity under all line and load condition must be considered.
Usually, it is recommended to set the bandwidth to be
less than 1/10 of switching frequency. It is recommended
to choose a crossover frequency less than 30kHz.
The strategy for choosing R
over frequency with R
with C
calculate R
where;
f
f
C
f
f
R
C
p2
EA
VEA
C
Z2
Rev. 1.4 November 2010
is desired crossover frequency,
C
is the compensation capacitor
=
is the error amplifier transconductance, which is 200 x 10
=
=
is the error amplifier voltage, and
=
C
30kHz
. Using selected crossover frequency, f
------------------------------------------ -
2π C
-----------------------------------
2π
f
C
×
C
C
×
C
×
:
for close loop must be selected. The system
--------- -
V
(C
C
V
G
1
FB
C
C
O
5
EA
×
in Figure 1) and resistor R
×
×
G
R
----------------------------- -
G
C
VEA
C
2π
EA
and set the compensator zero
×
×
C
C
G
O
and C
CS
C
is to set the cross
C
(R
C
1
, to
in
www.aosmd.com
-6
V
G
A/V, and
G
5.64 A/V
The compensation capacitor C
make a zero. This zero is put somewhere close to the
dominate pole f
crossover frequency. C
The equation above can also be simplified to:
An easy-to-use application software which helps to
design and simulate the compensation loop can be found
at www.aosmd.com.
Thermal Management and Layout
Consideration
In the AOZ1242 buck regulator circuit, high pulsing cur-
rent flows through two circuit loops. The first loop starts
from the input capacitors, to the V
the filter inductor, to the output capacitor and load, and
then return to the input capacitor through ground. Current
flows in the first loop when the high side switch is on. The
second loop starts from inductor, to the output capacitors
and load, to the GND pin of the AOZ1242, to the LX pins
of the AZO1242. Current flows in the second loop when
the low side diode is on.
In PCB layout, minimizing the two loops area reduces the
noise of this circuit and improves efficiency. A ground
plane is recommended to connect input capacitor, output
capacitor, and GND pin of the AOZ1242.
In the AOZ1242 buck regulator circuit, the three major
power dissipating components are the AOZ1242, exter-
nal diode and output inductor. The total power dissipation
of converter circuit can be measured by input power
minus output power.
The power dissipation of inductor can be approximately
calculated by output current and DCR of the inductor.
C
C
P
P
FB
EA
CS
inductor_loss
C
C
total_loss
is 0.8V,
is the error amplifier transconductance, which is 200x10
is the current sense circuit transconductance, which is
=
=
---------------------------------- -
2π
C
---------------------
O
R
×
×
C
=
1.5
R
R
C
p1
V
L
=
×
IN
but lower than 1/5 of selected
I
O
f
p1
×
2
I
×
C
IN
R
can is selected by:
–
inductor
V
O
C
×
and resistor R
IN
I
O
×
pin, to the LX pins, to
1.1
AOZ1242
Page 10 of 18
C
together
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