aoz1960 Alpha & Omega Semiconductor, aoz1960 Datasheet - Page 14

• Manufacturer Part Number: aoz1960
• Description: Multi-string Boost White Led Driver
• Manufacturer: Alpha & Omega Semiconductor

Inductor
The inductor is used to supply higher output voltage
when the NMOS switch is off. For a given input and
output voltage, the inductance and switching frequency
determine the inductor ripple current, defined as,
The peak inductor current is:
Higher boost inductance will yield lower inductor ripple
current but this will require an inductor with higher
saturation current rating at the highest operating
temperature. Lower ripple current helps to reduce
inductor core losses. It also reduces RMS current
through inductor, switch and freewheeling diode, which
results in less conduction loss. The peak to peak ripple
current of the inductor should be between 30% to 50% of
input current. An inductor value of 10μH is recommended
for 10 to 12 LED operation.
Output Capacitor
The output ripple voltage specification is key in the
selection of the output capacitor. In a boost converter, the
output ripple voltage is determined by load current, input
voltage, output voltage, switching frequency, output
capacitor value and ESR. It can be calculated by the
equation below: The voltage rating of the output
capacitor must be higher than the intended output boost
voltage plus the output ripple voltage. Therefore some
de-rating is required for ensure long term reliability.
where,
I
C
ESR
ΔI
LED
I
f
ΔV
Lpeak
OUT
Rev. 1.0 August 2010
=
L
CO
O
is the load current or LED current,
=
-------------------------------- -
V
is output capacitor value, and
=
OUT
is the Equivalent Series Resistor of output capacitor.
V
----------------------------- -
=
OUT
I
V
LED
I
IN
IN
×
L
t
–
+
OFF
×
V
ΔI
------- -
⎛
⎜
⎜
⎜
⎜
⎜
⎝
2
IN
-------- -
V
V
L
IN
O
(Eq. 5)
×
×
I
(Eq. 3)
OFFPULSE
ESR
CO
+
⎛
⎜
⎝
----------------------------- -
1
f
(Eq. 2)
×
–
C
-------------- -
V
V
OUT
OUT
IN
⎞
⎟
⎠
⎞
⎟
⎟
⎟
⎟
⎟
⎠
(Eq. 4)
www.aosmd.com
When low ESR ceramic output capacitors are used, the
impedance of the capacitor at the switching frequency
dominates. Output ripple is mainly caused by capacitor
value and load current with the frequency, input and
output voltage. The output ripple voltage calculation can
be simplified to:
An output capacitor with the value of 10μF is usually
sufficient to meet most 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
AOZ1960 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 AOZ1960’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,
D
ΔV
R
f
RHPZ
P
OUT
O
(
I
LOAD
=
(
I
=
LOAD
I
L
D
-------------------------------------------------------------------------- -
)
×
P
=
⎛
⎜
⎝
----------------------------- -
(
)
1
f
I
1 D I
×
LOAD
–
=
–
C
-------------- -
V
V
----------------------------------- -
V
OUT
OUT
OUT
IN
(
)
2
2π L
LOAD
I
LOAD
⎞
⎟
⎠
×
(
×
R
I
(Eq. 6)
LOAD
OUT
)
(Eq. 8)
(
)
I
LOAD
(Eq. 9)
)
o
AOZ1960
Page 14 of 20
in the
(Eq. 7)