MIC3223 Datasheet, PDF (15/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
MIC3223
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 6 LEDs at 350mA with a ripple of about 20%. We are
designing for 80% efficiency at a switching frequency of
1MHz.
Select R_ADJ
Having chosen the LED drive current to be 350mA in this
example, the current can be set by choosing the R_ADJ
resistor from Equation 1:
R _ADJ
=
0.2V
0.35A
=
0.57Ω
Use the next lowest standard value 0.56Ω.
I_LED = 0.36A
The power dissipation in this resistor is:
P_RADJ = ILED ² × R _ADJ = 71mW
Use a resistor rated at quarter watt or higher.
Operating Duty Cycle
The operating duty cycle can be calculated using Equation
four provided below:
Eq. (4)
( ) _{D =} V_OUT − V_IN + V_DIODE
V_OUT + V_DIODE
V_DIODE is the V_f of the output diode D1 in the Typical
Application. It is recommended to use a schottky diode
because it has a lower V_f than a junction 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.
( ) Dnom = VOUT(nom) − VIN(nom) + VDIODE
V_OUT(nom) + V_DIODE
( ) Dmax = VOUT(max) − VIN(min) + VDIODE
V_OUT(max) + V_DIODE
( ) Dmin = VOUT(min) − VIN(max) + VDIODE
V_OUT(min) + V_DIODE
Dnom = ^{(21 − 12 − 0.5)} = 0.44
21 + 0.5
(21- 12 + 0.5)
Dnom = _{21+ 0.5} = 0.44
Therefore D_nom = 44%, D_max = 72% and D_min = 15%.
Inductor Selection
First calculate the RMS input current (nominal, min and
max) for the system given the operating conditions listed in
the design example table. The minimum value of the RMS
input current is necessary to ensure proper operation.
Using Equation 5, the following values have been
calculated:
I_{IN_RMS(max)}
=
V_OUT(max) × I_OUT(max)
eff × V_IN(min)
= 1.54A _(RMS)
Eq (5)
I_{IN_RMS(nom)}
=
V_OUT(nom) × I_OUT(nom)
eff × V_IN(nom)
= 0.74A _(RMS)
I_{IN_RMS(min)}
=
V_OUT(min) × I_OUT(min)
eff × V_IN(max)
= 0.46A _(RMS)
I_OUT is the same as I_LED.
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)} = 0.4 × I_{IN_RMS(nom)} = 0.4 × 0.74 = 0.30A_P-P
It can be difficult to find large inductor values with high
saturation currents in a surface mount package. Due to
this, the percentage of the ripple current may be limited by
the available inductor. It is recommended to operate in the
continuous conduction mode. The selection of L described
here is for continuous conduction mode.
Eq. (6)
L = ^{VIN × D}
IIN_PP × FSW
Using the nominal values, we get:
12V × 0.44
L=
= 18μH
0.3A × 1MHz
Select the next higher standard inductor value of 22µH.
Going back and calculating the actual ripple current gives:
I_{IN_PP(max)}
= ^{VIN(min) × Dmax}
L × F_SW
=
8V × 0.72
22μH × 1MHz
=
0.26A _PP
The average input current is different than the RMS input
current because of the ripple current. If the ripple current is
low, then the average input current nearly equals the RMS
input current. In the case where the average input current
is different than the RMS, equation 7 shows the following:
Eq. (7)
( ) I_{IN_AVE(max)} =
I_{IN_RMS(max)}
2
− ^{(IIN_PP )2}
12
I_{IN_AVE(max)} =
(1.54)² − ^(0.24)2 ≈ 1.54A
12
The Maximum Peak input current I_{L_PK} can found using
Equation 8:
Eq. (8)
I_{L_PK(max)} = I_{IN_AVE(max)} + 0.5 ×I_{L_PP(max)} = 1.67A
The saturation current (I_SAT) at the highest operating
temperature of the inductor must be rated higher than this.
The power dissipated in the inductor is:
January 2010
15
M9999-011510-A