MIC3203 Datasheet, PDF (12/21 Pages) Micrel Semiconductor – High-Brightness LED Driver Controller with High-Side Current Sense

Micrel, Inc.
MIC3203
Inductor
According to the above equation, choose the inductor to
make the operating frequency no higher than 1.5MHz.
The following Tables give a reference inductor value and
corresponding frequency for a given LED current. For
space-sensitive applications, smaller inductor with higher
switching frequency could be used but efficiency of the
regular will be reduced.
Given an inductor value, the size of the inductor can be
determined by its RMS and peak current rating.
ΔI_{L = 2 ×} ^V_CS(MAX) ^{- V}_{CS(MIN) = 0.18}
I_L
V_CS(MAX) + V_CS(MIN)
I_L(RMS) =
I_L2
+
1
12
ΔI_L2
≈ I_L
Table 2. Inductor for VIN = 12V, 1 LED
RCS (Ω) I_LED (A)
L (µH)
F_SW (kHz)
1.33
0.15
220
474
0.56
0.35
100
439
0.4
0.5
68
461
0.28
0.7
47
467
0.2
1.0
33
475
0.13
1.5
22
463
0.1
2.0
15
522
0.08
2.5
12
522
0.068
3.0
10
533
Table 3. Inductor for VIN = 24V, 4 LEDs
RCS (Ω) I_LED (A)
L (µH)
F_SW (kHz)
1.33
0.15
470
474
0.56
0.35
220
426
0.4
0.5
150
447
0.28
0.7
100
470
0.2
1.0
68
493
0.13
1.5
47
463
0.1
2.0
33
507
0.08
2.5
27
496
0.068
3.0
22
517
Table 4. Inductor for VIN = 36V, 8 LEDs
RCS (Ω) I_LED (A)
L (µH)
F_SW (kHz)
1.33
0.15
470
495
0.56
0.35
220
446
0.4
0.5
150
467
0.28
0.7
100
490
0.2
1.0
68
515
0.13
1.5
47
485
0.1
2.0
33
530
0.08
2.5
27
519
0.068
3.0
22
541
1
I_L(PK) = I_L + ₂ ΔI_L = 1.09I_L
where:
I_L is inductor average current.
Select an inductor with saturation current rating at least 30%
higher than the peak current.
MOSFET
MOSFET selection depends upon the maximum input
voltage, output LED current and switching frequency.
The selected MOSFET should have 30% margin on
maximum voltage rating for high reliability requirements.
The MOSFET channel resistance R_DSON is selected such
that it helps to get the required efficiency at the required LED
currents as well as meets the cost requirement.
Logic level MOSFETs are preferred as the drive voltage is
limited to 5V.
The MOSFET power loss has to be calculated for proper
operation. The power loss consists of conduction loss and
switching loss. The conduction loss can be found by:
P_LOSS(CON) = I_{R2 MS(FET)} × R_DSON
I_RMS(FET) = I_LED × D
D = ^{VTOTAL _ LED}
V_IN
March 2010
12
M9999-032910-A