aoz1905 Alpha & Omega Semiconductor, aoz1905 Datasheet - Page 9

• Manufacturer Part Number: aoz1905
• Description: Ezboost 2a General Purpose Regulator
• Manufacturer: Alpha & Omega Semiconductor

input and output voltage. The output ripple voltage
calculation can be simplified to:
Output capacitor with the range of 4.7µF to 22µF ceramic
capacitor usually can meet most applications.
Diode
The output rectifier diode freewheels the inductor current
to output when the internal MOSFET is off. To reduce
losses due to diode forward voltage and reverse recov-
ery, Schottky diode is preferred in AOZ1905. The reverse
voltage of selected diode should be higher than output
voltage, the average current rating should be higher than
the maximum load current and the peak current rating
should be greater than the peak current of inductor:
I
Loop Compensation
The AOZ1905 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 boost power stage
can be simplified to be a one-pole, one left plane zero
and one right half plane (RHP) 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 RHP zero has the effect of a zero in the gain causing
an imposed +20dB/decade on the roll off, but has the
effect of a pole in the phase, subtracting 90° in the phase.
f
f
Lpeak
P 1
Z 1
O
L
V
Rev. 1.4 May 2008
is load resistor value, and
is the output filter capacitor,
O
CO
=
=
=
is the equivalent series resistance of output capacitor.
----------------------------------- -
2
------------------------------------------------- -
2
=
I
L
I
IN
C
C
------------------------------ -
1
+
1
O
O
f
-------- -
–
1
2
I
--------------- -
V
L
R
ESR
V
C
OUT
L
IN
O
CO
www.aosmd.com
The RHP zero can be calculated by:
The RHP zero obviously can cause the instable issue if
the bandwidth is higher. It is recommended to design the
bandwidth to lower than the one half frequency of RHP
zero.
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 AOZ1905. 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 AOZ1905, FB pin and COMP pin are the inverting
input and the output of internal transconductance error
amplifier. A series R and C compensation network con-
nected 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
Figure 1), is located at:
Choosing the suitable C
and bandwidth.
Thermal Management and Layout
Consideration
In the AOZ1905 boost regulator circuit, high pulsing
current flows through two circuit loops. The first loop
starts from the input capacitors, to the filter inductor, to
the LX pin, to the internal NMOS switch, to the ground
and back to the input capacitor, when the switch turns on.
The second loop starts from input capacitor, to the filter
inductor, to the LX pin to the external diode, to the ground
and back to the input capacitor, when the switch is off.
f
f
f
Z 2
Z 2
P 2
EA
VEA
C
is compensation capacitor.
is the error amplifier transconductance, which is 200 x 10
=
=
=
is the error amplifier voltage gain, which is 340 V/V, and
------------------------------------ -
2
------------------------------------------- -
2
------------------------------------------ -
2
C
(C
C
C
L I
V
G
1
C
C
3
I N
EA
in Figure 1) and resistor R
O
2
G
R
C
VEA
V
C
O
and R
C
by trading-off stability
AOZ1905
C
Page 9 of 16
(R
3
in
-6