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MIC3232YMM Datasheet(PDF) 16 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) 16 Page - Micrel Semiconductor

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Micrel, Inc.
MIC3230/1/2
January 2009
16
M9999-011409-A
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=
12
2
_
2
_
_
PP
L
AVE
IN
RMS
FET
I
I
D
I
The switching losses occur during the switching
transitions of the FET. The transition times, ttransition, are
ere are
the times when the FET is turning off and on. Th
two transition times per period, T. It is important not to
confuse T (the period) with the transition time, ttransition.
Eq. (24)
Fsw
T
1
=
Eq. (25)
SW
transition
OUT
AVE
FET
SWITCH
FET
F
t
V
I
P
×
×
×
=
max
_
max
_
max
_
_
max
_
_
max
_
transition
t
:
To find
Eq. (26)
Igatedrv
Qg
ttransition
≈
max
_
is the total gate c rge of the external
MOSFET provided by the MOSFET manufacturer and
charge at a
where Qg
ha
the Qg
ould chosen at a VGS≈10V. This is not an
exact value, but is more of an estimate of
max
_
transition
t
.
The FET manufacturers’ provide a gate
specified VGS voltage:
sh
GS
G
In
Q
C _
=
FET
V
@
This is the FET’s input capacitance. Select a FET with
RDS(on) and QG such that the external power is below
harge=68nC (typical)
The
perature. As the
erature
at 125°C is:
ation 23:
about 0.7W for a SO-8 or about 1W for a PowerPak
(FET package). The Vishay Siliconix Si7148DP in a
PowerPak SO-8 package is one good choice. The
internal gate driver in the MIC3230/1/2 is 2A. From the
Si7148DP data sheet:
RDS(on)_25°C=0.0145Ω
Total gate C
) is a function of tem
(
(
temp
R
on
DS
)
in the FET increases so does the
temp
RDS(on).
To find
)
(
)
(
temp
R
on
DS
use Equation 27, or simply
o
o
double the
) for
)
125
(
)
(
C
R
on
DS
.
Eq. (27)
(on
DS
R
25
(
)
(
C
R
on
DS
)
007
.
1
(
)
25
(
)
(
)
25
(
)
)
(
o
o
−
×
=
Temp
on
DS
C
R
temp
The
(
RDS
)
(
) temp
on
Ω
≈
−
∗
m
C
RDSon
30
)
7
)
125
(
)
25
125
(
o
o
×
=
00
.
1
(
0145
.
0
From Equ
mW
m
P
COND
FET
62
30
64
.
1
2
_
=
Ω
×
=
From Equation 26:
ns
A
nC
Igatedrv
Qg
ttransition
34
2
68
=
=
≈
A
AVE
64
.
1
max
_
_
=
IFET
V
VOUT
28
max
_
=
From Equation 25:
Watts
kHz
ns
V
A
78
.
0
500
34
28
64
.
1
P
SWITCH
FET
max
_
_
=
×
×
×
=
From Equation 22
W
W
mW
84
.
0
78
.
0
62
=
+
PFET =
This about the limit for a part on a circuit board without
having to use any additional heat sinks.
is best used here because of the lower
d the low reverse recovery time. The
Rectifier Diode
A Schottky Diode
forward voltage an
voltage stress on the diode is the max VOUT and
therefore a diode with a higher rating than max VOUT
should be used. An 80% de-rating is recommended
here as well.
Eq (28)
⎟
⎟
⎠
⎞
⎛
2
_
diode
I
I
⎜
⎜
⎝
+
−
=
12
)
1
(
_
2
max
_
_
max
_
PP
L
AVE
IN
RMS
I
D
Eq. (29)
W
P
I
V
P
diode
RMS
diode
SCHOTTKY
diode
81
.
0
max
_
_
≈
×
≈
MIC3230 power losses
IC3230are:
is the total gate charge of the
The power losses in the M
Eq.(30)
F
V
Q
P
gate
gate
MIC
×
×
=
3230
Vin
IQ ×
+
where
gat
Q
external
e
T.
r
ati
MOSFE
gate
V
is the gate drive voltage of the
MIC3230. F is the switching frequency.
Q
I
is the
quiescent cu rent of the MIC3230 found in th
lectrical
characteriz on table.
mA
IQ
2
.
3
=
.
VIN is the voltage at
the VIN pin of the MIC3230. From Eq.(30)
mA
kHz
nF
PMIC
14
2
.
3
500
12
68
3230
e e
W
45
.
0
=
×
+
×
×
=



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