AOZ3011PI
Rev. 0.6 August 2012
www.aosmd.com
Page 9 of 14
Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be
considered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck
converter circuit, output ripple voltage is determined by
inductor value , s witc hin g fr eq ue n cy, output capacitor
value and ESR. It can be calculated by the equation
below:
where,
C
O
is output capacitor value, and
ESR
CO
is the equivalent series resistance of the output
capacitor.
When a low ESR ceramic cap aci tor is u sed as the output
capacitor, the impedance of the cap acitor at the switching
frequency dominates. Output ripple is mainly caused by
capacitor value and inductor ripple current. The output
ripple voltage calculation ca n be simp lified to :
If the impedance of ESR at switching frequency
dominates, the ou tput r ipple volt age is mainly de cided 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 capacitors are
recommended as output capacitors.
In a buck converter, output capacitor current is
continuous. The RMS current of output capacitor is
decided by the peak to peak ind uctor ripple current. It can
be calculated by:
Usually, the ripple current r ating of the ou tput cap acitor 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 AOZ3011PI employs peak current mode control for
ease of use and fast transient response. Peak current
mode control eliminates the double pole effect of the
output L&C filter. It also gr eatly sim plif ies 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 the output capacitor and
its ESR. It is can be calculated by:
where;
C
O
is the output filter capacitor,
R
L
is load resistor value, and
ESR
CO
is the equivalent series resistance of output capacitor.
The compensation design shapes the converter control
loop transfer function for the desired gain and phase.
Several dif ferent type s of compensation ne tworks can be
used with the AOZ3011PI. For most cases, a series
capacitor and resistor network connected to the
COMP pin set s the pole-zero and is adequate for a stable
high-bandwidth control loop.
In the AOZ3011PI, FB and COMP 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:
where;
G
EA
is the error amplifier transconductance, which is 200 x 10
-6
A/V,
G
VEA
is the error amplifier voltage gain, which is 500 V/V, and
C
C
is the compensation capacitor in Figure 1.
VOILESRCO 1
8fC
O
-------------------------
+
=
VOIL1
8fC
O
-------------------------
=
fP11
2CORL
-----------------------------------
=
fZ11
2COESRCO
------------------------------------------------
=
fP2GEA
2CCGVEA
-------------------------------------------
=
Not Recommended For New Designs