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MIC3232 Datasheet(PDF) 16 Page - Micrel Semiconductor

Part # MIC3232
Description  Constant Current Boost Controller for Driving High Power LEDs
PDF  19 Pages
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Manufacturer  MICREL [Micrel Semiconductor]
Direct Link  http://www.micrel.com
Logo MICREL - Micrel Semiconductor

MIC3232 Datasheet(HTML) 16 Page - Micrel Semiconductor

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Micrel, Inc.
MIC3230/1/2
November 2009
16
M9999-113009-B
provided by the MOSFET manufacturer and the Qg
should 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 charge at a
specified VGS voltage:
GS
G
FET
In
V
Q
C
@
_
=
This is the FET’s input capacitance. Select a FET with
RDS(on) and QG such that the external power is below
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 Charge=68nC (typical)
The
)
(
)
(
temp
R
on
DS
is a function of temperature. As the
temperature in the FET increases so does the RDS(on).
To find
)
(
)
(
temp
R
on
DS
use Equation 27, or simply
double the
)
25
(
)
(
C
R
on
DS
o
for
)
125
(
)
(
C
R
on
DS
o
.
Eq. (27)
)
007
.
1
(
)
25
(
)
(
)
25
(
)
(
)
(
o
o
−
×
=
Temp
on
DS
on
DS
C
R
temp
R
The
)
(
)
(
temp
R
on
DS
at 125°C is:
Ω
≈
×
=
−
∗
m
C
RDSon
30
)
007
.
1
(
0145
.
0
)
125
(
)
25
125
(
o
o
From Equation 23:
mW
m
P
COND
FET
62
30
64
.
1
2
_
=
Ω
×
=
From Equation 26:
ns
A
nC
Igatedrv
Qg
ttransition
34
2
68
=
=
≈
A
I
AVE
FET
64
.
1
max
_
_
=
V
VOUT
28
max
_
=
From Equation 25:
Watts
kHz
ns
V
A
P
SWITCH
FET
78
.
0
500
34
28
64
.
1
max
_
_
=
×
×
×
=
From Equation 22
W
W
mW
PFET
84
.
0
78
.
0
62
=
+
=
This is about the limit for a part on a circuit board without
having to use any additional heat sinks.
Rectifier Diode
A Schottky Diode is best used here because of the lower
forward voltage and the low reverse recovery time. The
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)
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−
=
12
)
1
(
2
_
2
max
_
_
max
_
_
PP
L
AVE
IN
RMS
diode
I
I
D
I
Eq. (29)
W
P
I
V
P
diode
RMS
diode
SCHOTTKY
diode
81
.
0
max
_
_
≈
×
≈
MIC3230 Power Losses
The power losses in the MIC3230are:
Eq.(30)
Vin
I
F
V
Q
P
Q
gate
gate
MIC
×
+
×
×
=
3230
where
gate
Q
is the total gate charge of the external
MOSFET.
gate
V
is the gate drive voltage of the MIC3230.
F
is the switching frequency.
Q
I
is the quiescent current of
the MIC3230 found in the electrical characterization table.
mA
IQ
2
.
3
=
.
VIN is the voltage at the VIN pin of the MIC3230.
From Eq.(30)
W
mA
kHz
nF
PMIC
45
.
0
14
2
.
3
500
12
68
3230
=
×
+
×
×
=
OVP
-Over voltage protection
Set OVP higher than the maximum output voltage by at least
one volt. To find the resistor divider values for OVP use
Equation 3 and set the OVP=30V and R8=100kΩ:
Ω
=
−
×
Ω
=
k
k
R
33
.
4
245
.
1
30
245
.
1
100
9
PCB Layout
1. All typologies of DC-to-DC converters have a reverse
recovery current (RRC) of the flyback or (freewheeling)
diode. Even a Schottky diode, which is advertised as having
zero RRC, it really is not zero. The RRC of the freewheeling
diode in a boost converter is even greater than in the Buck
converter. This is because the output voltage is higher than
the input voltage and the diode has to charge up to –VOUT
during each on-time pulse and then discharge to VF during
the off-time.
2. Even though the RRC is very short (tens of nanoseconds)
the peak currents are high (multiple amperes). The high
RRC causes a voltage drop on the ground trace of the PCB
and if the converter control IC is referenced to this voltage
drop, the output regulation will suffer.
3. It is important to connect the IC’s reference to the same
point as the output capacitors to avoid the voltage drop
caused by RRC. This is also called a star connection or
single point grounding.
4. Feedback trace: The high impedance traces of the FB
should be short.



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