aoz1022di-01 Alpha & Omega Semiconductor, aoz1022di-01 Datasheet - Page 10

• Manufacturer Part Number: aoz1022di-01
• Description: Ezbucktm 3a Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

tor ripple current is high, the output capacitor could be
overstressed.
Loop Compensation
The AOZ1022 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 the dominant pole can
be calculated by:
The zero is an ESR zero due to output capacitor and its
ESR. It is can be calculated by:
where;
C
R
ESR
The compensation design is actually to shape the
converter control loop transfer function to get the desired
gain and phase. Several different types of compensation
network can be used for the AOZ1022. 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 AOZ1022, FB pin and COMP pin are the inverting
input and the output of internal 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
f
f
f
O
L
p2
EA
VEA
C
p1
Z1
Rev.3.0 November 2011
is load resistor value, and
is the output filter capacitor,
is compensation capacitor in Figure 1.
CO
is the error amplifier transconductance, which is 200 x 10
=
=
=
is the error amplifier voltage; and
is the equivalent series resistance of output capacitor.
------------------------------------------ -
2 C
---------------------------------- -
2
------------------------------------------------
2 C



C
G
1
C
O
O
EA


1

G
R
ESR
VEA
L
CO
www.aosmd.com
-6
The zero given by the external compensation network,
capacitor C
To design the compensation circuit, a target crossover
frequency f
crossover frequency is where control loop has unity gain.
The crossover is the also called the converter bandwidth.
Generally a higher bandwidth means faster response to
load transient. However, the bandwidth should not be too
high because of system stability concern. When design-
ing 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 switching frequency. The
AOZ1022 operates at a frequency range from 350kHz
to 600kHz. It is recommended to choose a crossover
frequency equal or less than 40kHz.
The strategy for choosing R
cross over frequency with R
zero with C
to calculate R
where;
where f
f
V
G
A/V, and
G
A/V
The compensation capacitor C
make a zero. This zero is put somewhere close to the
dominate pole f
crossover frequency. C
The above equation can be simplified to:
f
C
C
f
C
R
C
FB
Z2
EA
CS
is set to be about 1/10 of switching frequency,
C
C
C
=
is 0.8V,
is the error amplifier transconductance, which is 200 x 10
is the current sense circuit transconductance, which is 6.86
=
=
=
=
40kHz
C
C
---------------------
-----------------------------------
2
---------------------------------- -
2
f
is desired crossover frequency. For best performance,
C
O
R

C
C
C



. Using selected crossover frequency, f
C
for close loop must be selected. The system
--------- -
V
and resistor R
C
C
V
1.5
R
R
1
FB
:
C
O
p1
C
L


but lower than 1/5 of selected

R
f
----------------------------- -
G
p1
C
2
EA
C
can is selected by:


C
C
C
G
C
, is located at:
O
and C
and set the compensator
CS
C
and resistor R
AOZ1022DI-01
C
is to set the
Page 10 of 15
C
together
C
,
-6