MIC3203 Datasheet, PDF (13/21 Pages) Micrel Semiconductor – High-Brightness LED Driver Controller with High-Side Current Sense

Micrel, Inc.
MIC3203
The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
P_LOSS(TRAN)
=
V_IN × I_LED × F_SW
I_DRV
× (Q_gs2
+ Q_gd )
I_DRV
=
V_DRV
R_GATE
where:
R_GATE is total MOSFET resistance, Q_gs2 and Q_gd can be
found in a MOSFET manufacturer datasheet.
The total power loss is:
P_LOSS(TOT) = P_LOSS(CON) + P_LOSS(TRAN)
The MOSFET junction temperature is given by:
T_J = P_LOSS(TOT) × R_θJA + T_A
The input capacitor current rating can be considered as
I_LED/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 V_CS(MAX) with amplitude ±6mV by a pseudo
random generator to generate the ±12% of the switching
frequency dithering to reduce the EMI noise peaks.
The T_J 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:
I_CIN(RMS) = I_LED × D × (1- D)
The power loss in the input capacitor is:
P_LOSS(CIN) = I² _CIN(RMS) × C_INESR
March 2010
13
M9999-032910-A