mic3230 Micrel Semiconductor, mic3230 Datasheet - Page 13

• Manufacturer Part Number: mic3230
• Description: Constant Current Boost Controller For Driving High Power Leds
• Manufacturer: Micrel Semiconductor

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Design Example
In this example, we will be designing a boost LED driver
operating off a 12V input. This design has been created
to drive six LEDs at 350mA with a ripple of about 12%.
We are designing for 80% efficiency at a switching
frequency of 500kHz.
Select R
To operate at a switching frequency of 500kHz, the R
resistor must be chosen using Equation 3.
Use the closest standard value resistor of 16.5kΩ.
Select R
Having chosen the LED drive current to be 350mA in this
example, the current can be set by choosing the RADJ
resistor from Equation 1:
The power dissipation in this resistor is:
Use a resistor rated at ¼ watt or higher. Choose the
closest value from a resistor manufacture.
Micrel, Inc.
January 2009
FS
ADJ
ENABLE
PWMD
16.5k
R
RFS
P
VIN
FS
4.7µF/50v
(
R
( ) (
R
k
ADJ
CCOMP
Ω
ADJ
CIN
10nF
=
)
=
=
7526
I
. 0
. 0
2
10µF
Analog ground
10V
*
C3
500
25
35
R
)
V
. 1
A
ADJ
R2
100k
035
=
. 0
=
=
71
87
16
Ω
mW
VIN
EN
PWMD
SYNC
FS
COMP
VDD
AGND
6 .
k
Ω
Figure 7. Design Example Schematic
MIC3230/31
EPAD
47µH
L
FS
PGND
IADJ
OVP
DRV
13
IS
V
FB
Operating Duty Cycle
The operating duty cycle can be calculated using
Equation 12 provided below:
Eq. (12)
These can be calculated for the nominal (typical)
operating conditions, but should also be understood for
the minimum and maximum system conditions as listed
below.
Therefore D
Inductor Selection
First, it is necessary to calculate the RMS input current
(nominal, min and max) for the system given the
operating conditions listed in the design example table.
This minimum value of the RMS input current is
necessary to ensure proper operation. Using Equation
9, the following values have been calculated:
= 0.25V
Power ground
RSLC
51
RADJ
1/4W
I
IN
_
D
D
RMS
D
nom
min
max
NOM
_
Q1
RCS
1/2W
=
max
=
=
=56% D
(
D
(
(
Vout
Vout
Vout
D1
=
=
V
(
Vout
OUT
nom
min
max
Vout
Vout
Vout
eff
R8
100k
_
−
MAX
R9
4.33k
−
−
Vout
max
−
eff
×
eff
eff
eff
nom
max
min
V
= 78% and D
×
IN
×
×
×
×
I
+
Vin
OUT
_
Vin
+
Vin
+
+
Vin
V
min
V
V
V
diode
max
schottky
nom
schottky
schottky
min
_
+
max
V
COUT
4.7µF
100V
+
+
diode
+
V
V
V
=
schottky
schottky
M9999-011409-A
schottky
. 1
)
MIN
64
MIC3230/1/2
A
= 33%
LED 1
LED X
_
)
)
)
rms
VOUT
ILED Return