MIC3205 Datasheet, PDF (13/23 Pages) Micrel Semiconductor – High-Brightness LED Driver Controller with Fixed-Frequency Hysteretic Control

Micrel, Inc.
MIC3205
t_r is the MOSFET ON-time
L is the inductor
For falling inductor current (MOSFET is off):
tf

L  IL
V L_FALL
Eq. 5
where:
V_{L_FALL} = V_D + I_LED × R_CS  V_LED
V_D is the freewheeling diode forward drop
t_f is the MOSFET OFF-time
R_CS (Ω)
0.56
0.56
0.28
0.28
0.2
0.2
0.1
0.1
I_LED (A)
0.35
0.35
0.7
0.7
1.0
1.0
2.0
2.0
V_IN (V)
5
12
5
12
5
12
5
12
L (µH)
22
68
10
33
6.8
22
3.6
10
∆V_HYS
(mV)
64.1
57.7
70.5
59.4
72.6
62.4
68.5
68.6
Table 2. Inductor for F_SW = 400 kHz, V_D = 0.4V, 1 LED
Operating frequency and time period are given by:
FSW

1
T
Eq. 6
T  tr  tf
Eq. 7
Using Equations 3, 4, 5, 6, and 7:
VHYS

(VIN - ILED  RCS
-
VLED)  (VD  ILED  RCS
( VIN  VD)  L  FSW

VLED)  RCS
Eq. 8
The value of ∆V_HYS 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 ∆V_HYS value
have been chosen.
Thus, inductor L is given by:
L

(VIN
-
ILED

RCS - VLED)  (VD  ILED  RCS
( VIN  VD)  VHYS  FSW

VLED) 
RCS
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 ∆V_HYS in the 55mV to
75mV range, we get the following inductor values:
R_CS (Ω)
0.56
0.56
0.28
0.28
0.2
0.2
0.1
0.1
I_LED (A)
0.35
0.35
0.7
0.7
1.0
1.0
2.0
2.0
V_IN (V)
24
36
24
36
24
36
24
36
L (µH)
150
220
68
100
47
68
22
33
∆V_HYS
(mV)
55.8
56.8
61.6
62.5
62.4
64.3
66.6
66.2
Table 3. Inductor for F_SW = 400 kHz, V_D = 0.4V, 4 LED
R_CS (Ω)
0.56
0.56
0.28
0.28
0.2
0.2
0.1
0.1
I_LED (A)
0.35
0.35
0.7
0.7
1.0
1.0
2.0
2.0
V_IN (V)
36
40
36
40
36
40
36
40
L (µH)
150
220
68
100
47
68
22
33
∆V_HYS
(mV)
58.4
54.3
64.4
59.6
65.2
61.4
69.6
63.3
Table 4. Inductor for F_SW = 400 kHz, V_D = 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,
IL  ILED
Eq. 10
From Equations 1, 3, and 10:
IL
IL

VHYS
200m
Eq. 11
October 2012
13
M9999-102312-A