mic3230 Micrel Semiconductor, mic3230 Datasheet - Page 16

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

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