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MIC24420 Datasheet(PDF) 16 Page  Micrel Semiconductor 

MIC24420 Datasheet(HTML) 16 Page  Micrel Semiconductor 
16 / 34 page Micrel, Inc. MIC24420/MIC24421 June 2012 16 M9999062012C Application Information Component Selection Inductor The value of inductance is determined by the peakto peak inductor current. Higher values of inductance reduce the inductor current ripple at the expense of a larger inductor. Smaller inductance values allow faster response to output current transients but increase the output ripple voltage and require more output capacitance. The inductor value and saturation current are also controlled by the method of overcurrent limit used (see explanation in the previous section). The minimum value of inductance for the MIC24420/MIC24421 is 10µH/22µH. The peaktopeak ripple current may be calculated using the formula below. L f V η ) V V (η V I S IN(max) OUT IN(max) OUT PP ⋅ ⋅ ⋅ − ⋅ ⋅ = PP OUT PK I 0.5 I I × + = Where: IPP is the peaktopeak inductor ripple current L is the value of inductance fS is the switching frequency of the regulator η is the efficiency of the power supply Efficiency values from the Functional Characteristics section can be use for these calculations. The peak inductor current in each channel is equal to the average output current plus one half of the peak to peak inductor ripple current. The RMS inductor current is used to calculate the I 2R losses in the inductor. 2 OUT PP OUT INDUCTOR I I 3 1 1 I I RMS ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + ⋅ = Maximizing efficiency requires the proper selection of core material and minimizing the winding resistance. The high frequency operation of the MIC24420/MIC24421 requires the use of ferrite materials. Lower cost iron powder cores may be used but the increase in core loss will reduce the efficiency of the power supply. This is especially noticeable at low output power. The inductor winding resistance decreases efficiency at the higher output current levels. The winding resistance must be minimized although this usually comes at the expense of a larger inductor. The power dissipated in the inductor equals the sum of the core and copper losses. Core loss information is usually available from the magnetics vendor. Input Capacitor A 10μF ceramic is suggested on each of the VIN pins for bypassing. X5R or X7R dielectrics are recommended for the input capacitor. Y5V dielectrics should not be used. Besides losing most of their capacitance over temperature, they also become resistive at high frequencies, which reduce their ability to filter out high frequency noise. Output Capacitor The MIC24420/MIC24421 regulator is designed for ceramic output capacitors although tantalum and Aluminum Electrolytic may also be used. Output ripple voltage is determined by the magnitude of inductor current ripple, the output capacitor’s ESR and the value of output capacitance. When using ceramic output capacitors, the primary contributor to output ripple is the value of capacitance. Output ripple using ceramic capacitors may be calculated using the equation below: S OUT PP OUT f 2 ΔV 8 I C ⋅ ⋅ ⋅ ≥ Where: ΔVOUT is the peaktopeak output voltage ripple IPP is the peaktopeak ripple current as see by the capacitors fS is the switching frequency (1MHz nominal). When using tantalum or aluminum electrolytic capacitors, both the capacitance and ESR contribute to output ripple. The total ripple is calculated below: []2 ESR PP 2 S OUT PP OUT R I f 2 C 8 I ΔV ⋅ + ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⋅ ⋅ ⋅ = The output capacitor RMS current is calculated below: 12 I I PP COUTRMS = The power dissipated in the output capacitors can be calculated by the equation below: ( ) ESR 2 COUT DISS R I P RMS COUT ⋅ = Soft start capacitor considerations: Where a large amount of capacitance is present at the output of the regulator, a fast rising output voltage can, in extreme circumstances (since I=Cdv/dt), cause current limit to operate and prevent startup. In order to avoid this situation, the following equation can be used to ensure tR (output rise time) is set correctly. 
