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MIC3203 데이터시트(PDF) 13 Page - Micrel Semiconductor

부품명 MIC3203
상세설명  High-Brightness LED Driver Controller with High-Side Current Sense
PDF  21 Pages
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제조업체  MICREL [Micrel Semiconductor]
홈페이지  http://www.micrel.com
Logo MICREL - Micrel Semiconductor

MIC3203 데이터시트(HTML) 13 Page - Micrel Semiconductor

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Micrel, Inc.
MIC3203
March 2010
13
M9999-032910-A
The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
GATE
DRV
DRV
gd
2
gs
DRV
SW
LED
IN
)
TRAN
(
LOSS
R
V
=
I
)
Q
+
Q
(
×
I
F
×
I
×
V
=
P
where:
RGATE is total MOSFET resistance, Qgs2 and Qgd can be
found in a MOSFET manufacturer datasheet.
The total power loss is:
)
TRAN
(
LOSS
)
CON
(
LOSS
)
TOT
(
LOSS
P
+
P
=
P
The MOSFET junction temperature is given by:
A
JA
θ
)
TOT
(
LOSS
J
T
+
R
×
P
=
T
The TJ must not exceed maximum junction temperature
under any conditions.
Freewheeling Diode
The free wheeling diode should have the reverse voltage
rating to accommodate the maximum input voltage. The
forward voltage drop should be small to get the lowest
conduction dissipation for high efficiency. The forward
current rating has to be at least equal to LED current. A
Schottky diode is recommended for highest efficiency.
Input Capacitor
The ceramic input capacitor is selected by voltage rating
and ripple current rating. To determine the input current
ripple rating, the RMS value of the input capacitor can be
found by:
D)
-
(1
D
LED
CIN(RMS)
I
I
×
×
=
The power loss in the input capacitor is:
ESR
IN
CIN(RMS)
2
LOSS(CIN)
C
I
P
×
=
The input capacitor current rating can be considered as
ILED/2 under the worst condition D = 50%.
LED Ripple Current
The LED current is the same as inductor current. If LED ripple
current needs to be reduced then place a 4.7µF/50V ceramic
capacitor across LED.
Frequency Dithering
The MIC3203 is designed to reduce EMI by dithering the
switching frequency
±12% in order to spread the frequency
spectrum over a wider range. This lowers the EMI noise
peaks generated by the switching regulator.
Switching regulators generate noise by their nature and they
are the main EMI source to interference with nearby circuits. If
the switching frequency of a regulator is modulated via
frequency dithering, the energy of the EMI is spread among
many frequencies instead of concentrated at fundamental
switching frequency and its harmonics. The MIC3203
modulates the VCS(MAX) with amplitude ±6mV by a pseudo
random generator to generate the
±12% of the switching
frequency dithering to reduce the EMI noise peaks.



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