mic3230 Micrel Semiconductor, mic3230 Datasheet - Page 15

• Manufacturer Part Number: mic3230
• Description: Constant Current Boost Controller For Driving High Power Leds
• Manufacturer: Micrel Semiconductor

and for R
value for R
current limit was 179mΩ.
chosen a lower value which results in a higher current
limit. If we use a higher standard value the current limit
will have a lower value. The designer does not have the
same choices for small valued resistors as with larger
valued resistors. The choices differ from resistor
manufacturers. If too large a current sense resistor is
selected, the maximum output power may not be able to
be achieved at low input line voltage levels. Make sure
the inductor will not saturate at the actual current limit
Perform a check at I
Maximum Power dissipated in R
Eq. (17)
Eq. (18)
Use a 1/2 Watt resistor for R
Output Capacitor
In this LED driver application, the ILED ripple current is a
more important factor compared to that of the output
ripple voltage (although the two are directly related). To
find the C
calculation:
For an output ripple
Eq. (19)
Find the equivalent ac resistance
datasheet of the LED. This is the inverse slope of the
ILED vs. V
Eq. (20)
If the LEDs are connected in series, multiply
V
I
R
In this example use
in
Micrel, Inc.
January 2009
IS
LED
_
_
actual
PIN
_ ac
I
=
R
Limit
SLC
CS
250
OUT
=
F
_
I
C
CS
R
RMS
.
curve i.e.:
0
. If
CS
out
μ
ILED
1 .
for a required ILED ripple use the following
. The calculated value of R
_
A
RMS
P
Ω
=
I
R
×
P
=
_
in
R
max
CS
(
R
. 0
. 0
_
ripple
ILED
LED
by the total number of LEDs. In this
CS
actual
78
=
78
=
IN
ILED
I
R
. 1
=2.34Apk.
FET
=
_
⎛
⎜
⎜
⎝
)
. 1
ac
LED
×
25
ripple
I
=
ILED
R
64
511
_
Limit
=
CS
RMS
ripple
0
2
2
_ ac
2 .
_
×
Ω
Δ
is too high than use a higher
RMS
+
( *
CS
nom
_
.
×
Δ
In this example, we have
15
ILED
+
max
. 0
R
.
. 0
V
2
=
. 2
=
12
26
adj
CS
=
F
×
35
*
34
0
=
20% of
2
. 0
D
R
R
1 .
is;
A
+
⎞
⎟
⎟
⎠
D
nom
CS
=
LED
Ω
31
⎛
⎜
⎜
⎝
×
R
=
70
I
watt
IN
150
. 1
LED
for each LED.
_
_
*
mA
ac
44
AVE
T
m
ILED
_ total
A
from the
CS
Ω
_
_
max
=
rms
for a 1.9A
nom
. 0
)
2
+
45
I
L
V
_
12
PP
2
⎞
⎟
⎟
⎠
15
example of 6 LED
Use the next highest standard value, which is 4.7uF.
There is a trade off between the output ripple and th
rising edge of the PWMD pulse.
between PWM dimming pulses, the converter stops
pulsing and C
C
Dimming pluses. At the next PWMD pulse C
be charged up to the full output voltage V
desired LED current flows.
Input Capacitor
The input current
performance, ceramic capacitors should be used
because of their low equivalent series resistance (ESR).
The input ripple current is equal to the ripple in the
inductor plus the ripple voltage across the input
capacitor, which is the ESR of C
ripple.
generated by the converter as well as any voltage spikes
generated by the inductance of the input line. For a
required V
Eq. (21)
This is the minimum value that should be used. The
input capacitor should also be rated for the maximum
RMS input current.
spikes or any overshoot, a larger value of input
capacitance may be required and it is recommended that
ceramic capacitors be used. In this design example a
value of 4.7µF ceramic capacitor was selected.
MOSFET Selection
In this design exampl
voltage maximum of 30V. It is recommended to use an
80% de-rating value on switching FETs, so a minimum
of a 38V FET should be selected.
example, a 75V FET has been selected.
The switching FET power losses are the sum of the
conduction loss and the switching loss:
Eq. (22)
The conduct ion loss of the FET is when the FET is
turned on. The conduction power loss of the FET is
found by the following equation:
Eq. (23)
C
OUT
IN
C
=
will discharge depends on the time between PWM
out
8
×
The input capacitor will also bypass the EMI
V
=
IN_RIPPLE
IN
P
ILED
P
_
FET
I
OUT
FET
R
IN
RIPPLE
LED
_
ripple
_
will start to discharge. The amount that
:
s, we obtain the following:
ILED
PP
=
COND
is shown in Figure 5. For superior
_ total
P
e, the FET has to hold off an output
FET
×
To protect the IC from inductive
( *
nom
F
R
SW
=
_
=
adj
COND
I
FET
*
6
D
×
=
+
nom
0
8
_
R
1 .
×
+
RMS
LED
Ω
50
P
*
IN
FET
=
T
mV
(
2
_
. 0
0
times the inductor
×
total
6 .
This is because
28
_
M9999-011409-A
R
×
Ω
SWITCH
DSON
In this design
A
500
)
OUT
MIC3230/1/2
)
=
4
kHz
OUT
before the
1 .
, where
uF
has to
=
1
4 .
e
μ
F