mic3203 Micrel Semiconductor, mic3203 Datasheet - Page 13

• Manufacturer Part Number: mic3203
• Description: High-brightness Driver Controller With High-side Current Sense
• Manufacturer: Micrel Semiconductor

The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
where:
R
found in a MOSFET manufacturer datasheet.
The total power loss is:
The MOSFET junction temperature is given by:
The T
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:
The power loss in the input capacitor is:
Micrel, Inc.
March 2010
GATE
J
is total MOSFET resistance, Q
must not exceed maximum junction temperature
I
P
I
P
P
T
DRV
CIN(RMS)
LOSS
LOSS
LOSS(CIN)
J
=
=
P
(
(
TRAN
TOT
LOSS
R
V
GATE
DRV
=
)
=
)
=
(
I
TOT
LED
=
I
P
2
LOSS
CIN(RMS)
V
)
IN
×
×
I ×
R
(
CON
D
I
LED
θ
DRV
JA
×
×
)
(1
×
+
+
C
-
F
T
P
IN
D)
SW
LOSS
A
ESR
×
gs2
(
(
TRAN
Q
and Q
gs
2
)
+
Q
gd
gd
can be
)
13
The input capacitor current rating can be considered as
I
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 V
random generator to generate the ± 12% of the switching
frequency dithering to reduce the EMI noise peaks.
LED
/2 under the worst condition D = 50%.
CS(MAX)
with amplitude ± 6mV by a pseudo
M9999-032910-A
MIC3203