aoz1094 Alpha & Omega Semiconductor, aoz1094 Datasheet - Page 11

• Manufacturer Part Number: aoz1094
• Description: Ezbucktm 5a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck con-
verter circuit, output ripple voltage is determined by
inductor value, switching frequency, output capacitor
value and ESR. It can be calculated by the equation
below:
where,
C
ESR
capacitor.
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the
switching frequency dominates. Output ripple is mainly
caused by capacitor value and inductor ripple current.
The output ripple voltage calculation can be simplified to:
If the impedance of ESR at switching frequency
dominates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum or aluminum
electrolytic capacitors are recommended to be used as
output capacitors.
In a buck converter, output capacitor current is contin-
uous. The RMS current of 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, output capacitor could be
overstressed.
Schottky Diode Selection
The external freewheeling diode supplies the current to
the inductor when the high side PMOS switch is off. To
reduce the losses due to the forward voltage drop and
I
ΔV
ΔV
ΔV
CO_RMS
O
Rev. 1.3 October 2010
is output capacitor value, and
CO
O
O
O
=
is the equivalent series resistance of the output
=
=
ΔI
ΔI
ΔI
=
L
L
L
×
----------
×
×
ΔI
12
ESR
⎛
⎝
-------------------------
8
L
ESR
×
f C
1
×
CO
CO
O
+
-------------------------
8
×
f C
1
×
O
⎞
⎠
www.aosmd.com
recovery of diode, Schottky diode is recommended to
use. The maximum reverse voltage rating of the chosen
Schottky diode should be greater than the maximum
input voltage, and the current rating should be greater
than the maximum load current.
Loop Compensation
The AOZ1094 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 and 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 close loop transfer function to get desired gain
and phase. Several different types of compensation
network can be used for the AOZ1094. 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 AOZ1094, 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
f
f
f
P2
P1
O
L
EA
VEA
C
Z1
is load resistor value, and
is compensation capacitor.
is the output filter 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 C
------------------------------------------------
2π
×
×
×
C
C
G
1
C
O
O
EA
×
1
×
×
G
R
ESR
L
VEA
CO
AOZ1094
Page 11 of 19
-6