aoz1950 Alpha & Omega Semiconductor, aoz1950 Datasheet - Page 10

• Manufacturer Part Number: aoz1950
• Description: Boost Regulator White Led Driver
• Manufacturer: Alpha & Omega Semiconductor

An output capacitor with the value in the range of
2.2
applications requirements.
Output Boost Diode
A low forward drop Schottky with fast transit time should
be selected to improve converter efficiency.
Its current rating should be higher than the peak current
that commutates during its conduction interval. In
addition, the rated breakdown voltage should be higher
than the application boost voltage plus some margin of
transient overshoot.
Loop Compensation
AOZ1950 employs constant off time control and does not
require slope compensation. The right half plane zero
that is often problematic in all most boost converters has
no significant effect in the AOZ1950’s frequency
response. The RHP zero has the effect of a zero in the
gain causing +20dB/decade on the roll off, but has the
effect of a pole in the phase, subtracting 90
phase. The RHP zero can cause instability issues if the
bandwidth is higher, therefore it is recommended to lower
the bandwidth by one half frequency of the RHP zero.
The RHPZ can be calculated as follows:
where,
The cross over frequency in typical applications should
be a fifth of the frequency of the RHPZ location
The objective of compensation is to shape the gain and
phase of the converter’s closed loop transfer function
ultimately to achieve stability. The compensation pin of
AOZ1950 is serves as the output of the voltage
transconductance error amplifier. In a typical application,
a series capacitor and resistor network connected to the
COMP pin creates the pole-zero compensation network
enabling a very stable high-bandwidth control loop.
D
R
f
f
CROSS
RHPZ
Rev. 1.0 January 2010
P
OUT
μ
(
F to 4.7
I
LOAD
(
I
=
LOAD
=
D
-------------------------------------------------------------------------- -
)
μ
f
--------------- -
P
F is usually sufficient to meet most
RHPZ
=
(
)
5
I
1 D I
LOAD
=
–
V
----------------------------------- -
OUT
(Eq. 10)
(
)
2π
2
LOAD
I
LOAD
×
(
×
R
I
LOAD
L
OUT
)
(Eq. 8)
(
)
I
LOAD
(Eq. 9)
)
o
in the
(Eq. 7)
www.aosmd.com
The closed loop transfer function
The corresponding pole is:
The corresponding zero is:
where,
G
150·10
R
C
R
A compensation resistor and capacitor value of 1.5kΩ
and a 22nF along with a filter capacitor of 1nF from
COMP to GND is recommended for a typical 12 LED
application.
Circuit Layout and Thermal management
To minimize unwanted noise and voltage transients,
careful PCB layout must be exercised to reduce the main
current loop areas. This optimization helps to reduce the
switching noise associated with current commutation in
the circuit and also helps to improve the efficiency of the
converter. In addition to reducing the main power loops it
is also important to keep sensitive nodes such as
compensation and enable pins in quieter ground areas
away from the main power ground connection.
In the AOZ1950 boost LED driver circuit, there are two
main switching loops that pulsate current flow when the
NMOS turns on and off. The first loop starts when the
NMOS switches on, the input current or inductor current
ramps positively through the inductor and NMOS device.
During this interval, the output boost diode is reversed
biased with its anode pulled low to ground. Since the
NMOS is internal in the AOZ1950 much of the parasitic
inductance is made from the power return to the ground
connection of the input capacitors.
f
H w
f
P1
Z
EA
COMP
COMP
M
( )
=
is the error amplifier transconductance, which is
= 1MΩ,
=
--------------------------------------------------------------- -
R
-------------------------------------
R
-6
=
-------------------------------------------------- -
2π R
is compensation capacitor,
is compensation capacitor
COMP
COMP
A/V,
G
------------------------------------------------------------
×
M
×
EA
+
×
R
⎛
⎝
R
R
1
1
EA
×
1
EA
EA
+
C
×
s w
----------- -
COMP
w
×
( )
⎛
⎝
P1
C
1
COMP
+
⎞
⎠
s w
----------- -
w
( )
(Eq. 12)
z
⎞
⎠
(Eq. 13)
(Eq. 11)
AOZ1950
Page 10 of 14