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MIC3231YML Datasheet(PDF) 13 Page - Micrel Semiconductor |
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MIC3231YML Datasheet(HTML) 13 Page - Micrel Semiconductor |
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13 / 20 page  Micrel, Inc. MIC3230/1/2 January 2009 13 M9999-011409-A L 47µH D1 R8 100k R9 4.33k COUT 4.7µF 100V R2 100k RADJ 1/4W RFS 16.5k CCOMP 10nF CIN 4.7µF/50v RSLC 51 VFB = 0.25V RCS 1/2W Analog ground Power ground VOUT VIN PWMD ENABLE Synch to other MIC3230 ILED Return LED 1 LED X Q1 COMP PWMD VDD AGND PGND EPAD IS OVP DRV VIN EN IADJ MIC3230/31 SYNC FS C3 10µF 10V Figure 7. Design Example Schematic Design Example In this example, we will be designing a boost LED driver operating off a 12V input. This design has been created to drive six LEDs at 350mA with a ripple of about 12%. We are designing for 80% efficiency at a switching frequency of 500kHz. Select RFS To operate at a switching frequency of 500kHz, the RFS resistor must be chosen using Equation 3. () () Ω = = Ω k k RFS 6 . 16 500 7526 035 . 1 Use the closest standard value resistor of 16.5kΩ. Select RADJ Having chosen the LED drive current to be 350mA in this example, the current can be set by choosing the RADJ resistor from Equation 1: Ω = = 71 . 0 35 . 0 25 . 0 A V RADJ The power dissipation in this resistor is: () mW R I R P ADJ ADJ 87 * 2 = = Use a resistor rated at ¼ watt or higher. Choose the closest value from a resistor manufacture. Operating Duty Cycle The operating duty cycle can be calculated using Equation 12 provided below: Eq. (12) diode diode V Vout V Vin eff Vout D + + × − = ) ( These can be calculated for the nominal (typical) operating conditions, but should also be understood for the minimum and maximum system conditions as listed below. schottky nom schottky nom nom nom V Vout V Vin eff Vout D + + × − = ) ( schottky schottky V Vout V Vin eff Vout D + + × − = max min max max ) ( schottky schottky V Vout V Vin eff Vout D + + × − = min max min min ) ( Therefore DNOM =56% DMAX = 78% and DMIN = 33% Inductor Selection First, it is necessary to calculate the RMS input current (nominal, min and max) for the system given the operating conditions listed in the design example table. This minimum value of the RMS input current is necessary to ensure proper operation. Using Equation 9, the following values have been calculated: rms A V eff I V I IN OUT OUT RMS IN _ 64 . 1 min _ max _ max _ max _ _ = × × = |
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