aoz1950 Alpha & Omega Semiconductor, aoz1950 Datasheet - Page 9

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

Application Information
An example of the AOZ1950 application circuit is shown
on page 1. The description below details the component
selection.
LED Current Setting
The feedback voltage (FB) is set to 300mV. The total
output LED current is set by a current sense resistor in
series with the LED string. RSET can be calculated using
EQ.1
Input Capacitor
The input capacitor is connected to VIN and GND pins of
the AOZ1950 to filter and maintain a steady input DC
voltage. The voltage rating of input capacitor must be
greater than maximum input voltage plus ripple voltage.
The RMS current rating should be greater than the
inductor ripple current:
The input capacitor value should be 4.7
normal operation. The capacitor can be electrolytic,
tantalum or ceramic. The input capacitor should be place
as close as possible to the IC; if not possible, a 0.1
decoupling ceramic capacitor between VIN pin and GND
in close proximity.
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:
I
I
LED
I
Lpeak
Rev. 1.0 January 2010
Lripples
=
=
------------- -
R
V
R
SET
I
=
FB
SET
IN
60
30
20
15
12
10
V
----------------------------- -
+
OUT
(Ω)
ΔI
------- -
2
(Eq. 1)
L
L
–
V
(Eq. 3)
IN
×
T
OFFPULSE
I
LED
μ
F or higher for
10
15
20
25
30
5
(Eq. 2)
(mA)
www.aosmd.com
μ
F
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 22
for 6 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
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:
ΔV
LED
f
ΔV
OUT
=
O
CO
O
is the load current or LED current,
-------------------------------- -
V
is output capacitor value and
=
=
OUT
is the Equivalent Series Resistor of output capacitor.
I
I
L
V
LED
×
IN
×
⎛
⎜
⎝
----------------------------- -
t
OFF
1
f
×
×
–
⎛
⎜
⎜
⎜
⎜
⎜
⎝
C
-------- -
V
V
-------------- -
V
V
IN
OUT
O
OUT
(Eq. 5)
IN
×
ESR
⎞
⎟
⎠
(Eq. 6)
CO
+
μ
⎛
⎜
⎝
----------------------------- -
1
H is recommended
f C
×
–
-------------- -
V
V
OUT
OUT
AOZ1950
IN
Page 9 of 14
⎞
⎟
⎠
⎞
⎟
⎟
⎟
⎟
⎟
⎠
(Eq. 4)