aoz1025di Alpha & Omega Semiconductor, aoz1025di Datasheet - Page 10

• Manufacturer Part Number: aoz1025di
• Description: Ezbucktm 8a Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Loop Compensation
The AOZ1025D 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 dominant pole can be
calculated by:
The zero is a 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 desired
gain and phase. Several different types of compensation
network can be used for the AOZ1025D. 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 AOZ1025D, 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
The zero given by the external compensation network,
capacitor C
f
f
f
f
P2
P1
O
L
EA
VEA
C
Z2
Z1
Rev. 1.3 September 2009
is load resistor value, and
is the output filter capacitor,
is the compensation capacitor.
CO
is the error amplifier transconductance, which is 200 x 10
=
=
=
=
is the error amplifier voltage gain, which is 500 V/V, and
is the equivalent series resistance of output capacitor.
------------------------------------------ -
2π
-----------------------------------
2π
---------------------------------- -
2π
------------------------------------------------
2π C
×
C
×
×
×
and resistor R
C
C
C
G
1
1
C
C
O
O
EA
×
×
1
×
×
G
R
R
ESR
C
L
VEA
CO
C
, is located at:
www.aosmd.com
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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
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 switching frequency. The
AOZ1025D operates at a fixed 500kHz switching
frequency. It is recommended to choose a crossover
frequency equal or less than 40kHz.
The strategy for choosing R
over frequency with R
with C
late R
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 previous equation can also be simplified to:
f
C
C
C
C
R
C
FB
EA
CS
is the desired crossover frequency. For best performance,
is set to be about 1/10 of the 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 10.8
=
=
=
C
C
40kHz
:
. Using selected crossover frequency, f
C
---------------------
-----------------------------------
2π R
f
C
O
R
×
C
×
×
C
for close loop must be selected. The system
--------- -
V
V
1.5
R
FB
O
p1
C
L
×
but lower than 1/5 of selected
×
f
P1
----------------------------- -
G
C
2π
EA
C
and set the compensator zero
can is selected by:
×
×
C
C
G
O
and C
CS
C
and resistor R
C
is to set the cross
AOZ1025D
Page 10 of 15
C
C
, to calcu-
together
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