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MIC3232YMM Datasheet(PDF) 14 Page - Micrel Semiconductor

Part # MIC3232YMM
Description  Constant Current Boost Controller for Driving High Power LEDs
PDF  20 Pages
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Manufacturer  MICREL [Micrel Semiconductor]
Direct Link  http://www.micrel.com
Logo MICREL - Micrel Semiconductor

MIC3232YMM Datasheet(HTML) 14 Page - Micrel Semiconductor

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Micrel, Inc.
MIC3230/1/2
January 2009
14
M9999-011409-A
rms
A
V
eff
I
V
I
nom
IN
nom
OUT
nom
OUT
nom
RMS
IN
_
78
.
0
_
_
_
_
_
=
×
×
=
rms
A
V
eff
I
V
I
IN
OUT
OUT
RMS
IN
_
48
.
0
max
_
min
_
min
_
min
_
_
=
×
×
=
Iout is the same as ILED
Selecting the inductor current (peak-to-peak), IL_PP, to be
between 20% to 50% of IIN_RMS_nom, in this case 40%, we
obtain:
P
P
nom
rms
in
nom
PP
in
A
I
I
−
=
=
=
31
.
0
78
.
0
*
4
.
0
4
.
0
_
_
_
_
(see the current waveforms in Figure 5).
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. (13)
PP
in
IN
I
T
D
V
L
_
×
×
=
Using the nominal values, we get:
H
A
s
V
L
μ
μ
43
31
.
0
2
56
.
0
12
=
×
×
=
Select the next higher standard inductor value of 47µH.
Going back and calculating the actual ripple current
gives:
Eq. (13a)
PP
nom
nom
IN
PP
in
A
uh
us
v
L
T
D
V
I
29
.
0
47
2
56
.
0
12
_
_
=
×
×
=
×
×
=
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 10 shows the
following:
Eq. (13b)
() (
)
12
2
_
2
max
_
_
max
_
_
PP
IN
RMS
IN
AVE
IN
I
I
I
−
=
() (
)
A
I
AVE
IN
64
.
1
12
/
29
.
0
64
.
1
2
2
max
_
_
≈
−
=
The Maximum Peak input current IL_PK can found using
equation 11:
A
I
I
I
PP
L
AVE
IN
PK
L
78
.
1
5
.
0
max
_
_
max
_
_
max
_
_
=
×
+
=
The saturation current (ISAT) at the highest operating
temperature of the inductor must be rated higher than
this.
The power dissipated in the inductor is:
Eq. (13c)
DCR
I
P
RMS
in
INDUCTOR
×
=
2
max
_
_
Current Limit and Slope Compensation
Having calculated the IL_pk above, We can set the current
limit 20% above this maximum value:
A
A
I
Limit
pk
L
9
.
1
6
.
1
2
.
1
_
=
×
=
The internal current limit comparator reference is set at
0.45V, therefore when
, the IC enters
current limit.
45
.
0
_
=
PIN
IS
V
Eq. (14)
(
)
PK
A
Vcs
V
PK
+
=
45
.
0
Where
is the peak of the
waveform and
is the peak of the Vcs waveform
PK
A
V
A
V
PK
Vcs
Eq. (14a)
CS
pk
L
SLC
RAMP
R
I
D
R
I
Limit
×
+
×
×
=
_
45
.
0
To calculate the value of the slope compensation
resistance, RSLC, we can use Equation 5:
(
)
SW
CS
IN
OUT
SLC
F
A
L
R
V
V
R
MIN
MAX
×
×
×
−
=
μ
250
First we must calculate RCS, which is given below in
Equation 15:
Eq. (15)
()
Limit
pk
L
SW
MIN
MAX
CS
I
F
L
D
VIN
VOUT
R
_
max
45
.
0
+
×
×
−
=
Therefore;
() (
)
Ω
=
+
×
×
−
=
m
A
kHz
H
v
v
RCS
179
9
.
1
500
47
50
.
0
8
28
45
.
0
μ
Using a standard value 150mΩ resistor for RCS, we
obtain the following for RSLC:
(
)
Ω
=
×
×
Ω
×
−
=
511
500
250
47
150
8
28
kHz
A
H
m
RSLC
μ
μ
Use the next higher standard value if this not a standard
value. In this example 511Ω is a standard value.
Check: Because we must use a standard value for Rcs
and RSLC;
may be set at a different level (if the
calculated value isn’t a standard value) and we must
calculate
the
actual
value
(remember
is the same as
).
Limit
pk
L
I _
Limit
pk
L
I _
Limit
pk
in _
Limit
pk
L
I _
I
Rearranging Equation 14a to solve for
:
Limit
pk
L
I _
CS
SLC
RAMP
pk
in
R
D
R
I
I
Limit
)
45
.
0
(
_
×
×
−
=
A
ua
I
Limit
actual
in
34
.
2
150
.
)
75
.
0
511
250
45
.
0
(
_
=
×
×
−
=
This is higher than the initial
limit because we have to use standard values for RCS
A
I
PK
L
9
.
1
2
.
1
max
_
_
=
×



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