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MIC3205 Datasheet(PDF) 13 Page - Micrel Semiconductor

Part # MIC3205
Description  High-Brightness LED Driver Controller with Fixed-Frequency Hysteretic Control
PDF  23 Pages
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Manufacturer  MICREL [Micrel Semiconductor]
Direct Link  http://www.micrel.com
Logo MICREL - Micrel Semiconductor

MIC3205 Datasheet(HTML) 13 Page - Micrel Semiconductor

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Micrel, Inc.
MIC3205
October 2012
13
M9999-102312-A
tr is the MOSFET ON-time
L is the inductor
For falling inductor current (MOSFET is off):
L_FALL
L
f
V
I
L
t
Eq. 5
where:
VL_FALL = VD + ILED × RCS  VLED
VD is the freewheeling diode forward drop
tf is the MOSFET OFF-time
Operating frequency and time period are given by:
T
1
FSW
Eq. 6
Eq. 7
f
r
t
t
T
Using Equations 3, 4, 5, 6, and 7:
SW
D
IN
CS
LED
CS
LED
D
LED
CS
LED
IN
HYS
F
L
)
V
V
(
R
)
V
R
I
(V
)
V
-
R
I
-
(V
V
Eq. 8
The value of ∆VHYS calculated in this way must be
between 40mV and 100mV to ensure frequency
regulation.
Inductor
According to the above equations, the inductor value can
be calculated once average LED current, operating
frequency and an appropriate hysteresis ∆VHYS value
have been chosen.
Thus, inductor L is given by:
SW
HYS
D
IN
CS
LED
CS
LED
D
LED
CS
LED
IN
F
V
)
V
V
(
R
)
V
R
I
(V
)
V
-
R
I
-
(V
L
Eq. 9
Table 2, Table 3, and Table 4 give reference inductor
values for an operating frequency of 400 kHz, for a given
LED current, freewheeling diode forward drop, and
number of LEDs. By selecting ∆VHYS in the 55mV to
75mV range, we get the following inductor values:
RCS (Ω)
ILED (A)
VIN (V)
L (µH)
∆VHYS
(mV)
0.56
0.35
5
22
64.1
0.56
0.35
12
68
57.7
0.28
0.7
5
10
70.5
0.28
0.7
12
33
59.4
0.2
1.0
5
6.8
72.6
0.2
1.0
12
22
62.4
0.1
2.0
5
3.6
68.5
0.1
2.0
12
10
68.6
Table 2. Inductor for FSW = 400 kHz, VD = 0.4V, 1 LED
RCS (Ω)
ILED (A)
VIN (V)
L (µH)
∆VHYS
(mV)
0.56
0.35
24
150
55.8
0.56
0.35
36
220
56.8
0.28
0.7
24
68
61.6
0.28
0.7
36
100
62.5
0.2
1.0
24
47
62.4
0.2
1.0
36
68
64.3
0.1
2.0
24
22
66.6
0.1
2.0
36
33
66.2
Table 3. Inductor for FSW = 400 kHz, VD = 0.4V, 4 LED
RCS (Ω)
ILED (A)
VIN (V)
L (µH)
∆VHYS
(mV)
0.56
0.35
36
150
58.4
0.56
0.35
40
220
54.3
0.28
0.7
36
68
64.4
0.28
0.7
40
100
59.6
0.2
1.0
36
47
65.2
0.2
1.0
40
68
61.4
0.1
2.0
36
22
69.6
0.1
2.0
40
33
63.3
Table 4. Inductor for FSW = 400 kHz, VD = 0.4V, 8 LED
Given an inductor value, the size of the inductor can be
determined by its RMS and peak current rating.
Because LEDs are in series with the inductor,
LED
L
I
I
Eq. 10
From Equations 1, 3, and 10:
200m
V
I
I
HYS
L
L
Eq. 11



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