aoz1038di Alpha & Omega Semiconductor, aoz1038di Datasheet - Page 8

• Manufacturer Part Number: aoz1038di
• Description: Ezbucktm 6 A Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

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.
Loop Compensation
The AOZ1038 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 also 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 the frequency domain. The pole is dominant can be
calculated by:
The zero is a ESR zero due to the output capacitor and
its ESR. It is can be calculated by:
where;
C
R
ESR
The compensation design functions to shape the
converter control loop transfer to provide the desired gain
and phase. Several different types of compensation
networks can be used for the AOZ1038. In 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 AOZ1038, FB pin and COMP pin are the inverting
input and the output of the internal error amplifier. A
series R and C compensation network connected to
COMP provides one pole and one zero. The pole is:
I
f
f
CO_RMS
P2
O
L
Z1
f
Rev. 1.2 February 2011
P1
is load resistor value, and
is the output filter capacitor,
CO
=
=
=
is the equivalent series resistance of output capacitor.
------------------------------------------ -
2π
------------------------------------------------
2π C
---------------------------------- -
2π
=
×
×
×
C
----------
ΔI
G
C
C
O
12
1
EA
O
L
×
1
×
×
G
ESR
R
VEA
L
CO
www.aosmd.com
where;
G
150 x 10
G
C
The zero given by the external compensation network,
capacitor C
To design the compensation circuit, a target crossover
frequency f
system crossover frequency is where the control loop
has unity gain. The crossover is the also called the
converter bandwidth. Generally a higher bandwidth
results in faster response to load transient. However, the
bandwidth should not be too high because of system
stability concern. When designing the compensation
loop, converter stability under all line and load condition
must be considered.
Usually, it is recommended to set the bandwidth to be
equal or less than 1/10 of the switching frequency. The
AOZ1038 operates at a frequency range from 400 kHz to
500 kHz. It is recommended to choose a crossover
frequency equal or less than 40 kHz.
The strategy for choosing R
cross over frequency with R
compensator zero with C
frequency, f
where;
f
f
V
G
150 x 10
G
8 A/V.
The compensation capacitor C
make a zero. This zero is put somewhere close to the
dominate pole f
crossover frequency. C
f
C
C
f
C
R
C
FB
EA
VEA
C
Z2
EA
CS
is desired crossover frequency. For best performance,
is set to be about 1/10 of switching frequency,
C
C
is compensation capacitor in Figure 1.
=
is 0.8 V,
is the error amplifier transconductance, which is
is the error amplifier transconductance, which is
is the current sense circuit transconductance, which is
=
=
is the error amplifier voltage gain, which is 500 V/V, and
=
40kHz
-6
-6
-----------------------------------
2π
-----------------------------------
2π R
f
C
A/V,
A/V, and
×
C
C
×
×
C
, to calculate R
--------- -
V
for closed loop must be selected. The
and resistor R
C
V
1.5
1
FB
C
O
p1
C
×
×
but lower than 1/5 of the selected
×
R
f
P1
----------------------------- -
G
C
2π
EA
C
C
can is selected by:
×
×
. Using selected crossover
C
C
C
G
C
C
, is located at:
:
C
and C
and then set the
CS
C
and resistor R
C
is to set the
AOZ1038
Page 8 of 14
C
together