MIC3230_11 Datasheet, PDF (13/19 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
VIN
CIN
4.7µF/50v
ENABLE
PWMD
Synch to other MIC3230
R2
100k
RFS
16.5k
CCOMP
10nF
C3
10µF
10V
Analog ground
L
47µH
D1
VIN
EN
OVP
PWMD
SYNC
FS
COMP
VDD
AGND
MIC3230/31
DRV
EPAD
RSLC
51
IS
IADJ ^{VFB = 0.25V}
PGND
RADJ
1/4W
Q1
RCS
1/2W
Power ground
Figure 7. Design Example Schematic
R8
100k
R9
4.33k
MIC3230/1/2
VOUT
COUT
4.7µF
100V
LED 1
LED X
ILED Return
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_FS
To operate at a switching frequency of 500kHz, the R_FS
resistor must be chosen using Equation 3.
R_FS (kΩ) =
(7526)^1.035
500
= 16.6kΩ
Use the closest standard value resistor of 16.5kΩ.
Select R_ADJ
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:
R _ADJ
=
0.25V
0.35A
= 0.71Ω
The power dissipation in this resistor is:
P(R_ADJ ) = I ² * R_ADJ = 87mW
Use a resistor rated at ¼ watt or higher. Choose the
closest value from a resistor manufacture.
Operating Duty Cycle
The operating duty cycle can be calculated using
Equation 12 provided below:
Eq. (12)
D = ^{(Vout − eff ×Vin + Vdiode )}
Vout + V_diode
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.
D_nom
=
(Vout_nom − eff × Vin_nom + V_schottky
Vout_nom + V_schottky
)
D_max
=
(Vout_max − eff × Vin_min + V_schottky
Vout_max + V_schottky
)
D_min
=
(Vout_min − eff ×Vin_max + V_schottky )
Vout_min + V_schottky
Therefore D_NOM =56% D_MAX = 78% and D_MIN = 33%
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:
I_{IN _ RMS _ max}
= ^{VOUT _ max × IOUT _ max}
eff ×V_{IN _ min}
= 1.64A _ rms
I_{IN _ RMS _ nom}
= ^{VOUT _ nom × IOUT _ nom}
eff ×V_{IN _ nom}
= 0.78A _ rms
I_{IN _ RMS _ min}
= ^{VOUT _ min × IOUT _ min}
eff ×V_{IN _ max}
= 0.48A _ rms
Iout is the same as ILED
Selecting the inductor current (peak-to-peak), I_{L_PP}, to be
between 20% to 50% of I_{IN_RMS_nom}, in this case 40%, we
obtain:
I_{in _ PP _ nom} = 0.4I_{in _ rms _ nom} = 0.4 * 0.78 = 0.31A_{P −P}
March 2011
13
M9999-030311-D