MIC3202 Datasheet, PDF (11/19 Pages) Micrel Semiconductor – High-Brightness LED Driver with Integrated MOSFET and High-Side Current Sense

Micrel, Inc.
MIC3202
Inductor
According to the above equation, choose the inductor to
make the operating frequency no higher than 1.0MHz.
Tables 2, 3, and 4 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 the efficiency of
the regular will be reduced.
RCS (Ω)
1.33
0.56
0.4
0.28
0.2
I_LED (A)
0.15
0.35
0.5
0.7
1.0
L (µH)
220
100
68
47
33
F_SW (kHz)
474
439
461
467
475
Table 2. Inductor for V_IN = 12V, 1 LED
RCS (Ω)
1.33
0.56
0.4
0.28
0.2
I_LED (A)
0.15
0.35
0.5
0.7
1.0
L (µH)
470
220
150
100
68
F_SW (kHz)
474
426
447
470
493
Table 3. Inductor for V_IN = 24V, 4 LEDs
RCS (Ω)
1.33
0.56
0.4
0.28
0.2
I_LED (A)
0.15
0.35
0.5
0.7
1.0
L (µH)
470
220
150
100
68
F_SW (kHz)
495
446
467
490
515
Table 4. Inductor for V_IN = 36V, 8 LEDs
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
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.
Free-Wheeling Diode
The free-wheeling diode should have the reverse voltage
rating to accommodate the maximum input voltage. The
forward voltage drop should be small to get the lowest
conduction dissipation for high efficiency. The forward current
rating has to be at least equal to LED current. A Schottky
diode is recommended for highest efficiency.
Input Capacitor
The ceramic input capacitor is selected by voltage rating and
ripple current rating. To determine the input current ripple
rating, the RMS value of the input capacitor can be found by:
I_CIN(RMS) = I_LED × D × (1- D)
The power loss in the input capacitor is:
P_LOSS(CIN) = I² _CIN(RMS) × C_INESR
The input capacitor current rating can be considered as
I_LED/2 under the worst condition D = 50%.
LED Ripple Current
The LED current is the same as inductor current. If LED ripple
current needs to be reduced then place a 4.7µF/50V ceramic
capacitor across LED.
Frequency Dithering
The MIC3202 is designed to reduce EMI by dithering the
switching frequency ±12% in order to spread the noise
spectrum over a wider range. This lowers the EMI noise
peaks generated by the switching regulator.
Switching regulators generate noise by their nature and they
are the main EMI source to interference with nearby circuits. If
the switching frequency of a regulator is modulated via
frequency dithering, the energy of the EMI is spread among
many frequencies instead of concentrated at fundamental
switching frequency and its harmonics. The MIC3202
modulates the V_CS(MAX) with amplitude ±6mV by a pseudo
random generator to generate the ±12% of the switching
frequency dithering to reduce the EMI noise peaks.
September 2010
11
M9999-091710-A