MIC3203 datasheet(13/21 Pages) MICREL

13 / 21 page

Micrel, Inc.

MIC3203

March 2010

M9999-032910-A

The switching loss occurs during the MOSFET turn-on

and turn-off transition and can be found by:

GATE

DRV

LED

)

TRAN

(

LOSS

)

(

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

The MOSFET junction temperature is given by:

)

TOT

(

LOSS

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:

LED

CIN(RMS)

The power loss in the input capacitor is:

ESR

CIN(RMS)

LOSS(CIN)

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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