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MAX15004 Datasheet(PDF) 20 Page  Maxim Integrated Products 

MAX15004 Datasheet(HTML) 20 Page  Maxim Integrated Products 
20 / 27 page An additional small LC filter may be necessary to sup press the remaining lowenergy highfrequency spikes. The LC filter also helps attenuate the switching frequen cy ripple. Care must be taken to avoid any compensa tion problems due to the insertion of the additional LC filter. Design the LC filter with a corner frequency at more than a decade higher than the estimated closedloop, unitygain bandwidth to minimize its effect on the phase margin. Use 1μF to 10μF lowESR ceramic capacitors and calculate the inductance using following equation: where fC = estimated converter closedloop unitygain frequency. SEPIC Converter The MAX15004A/B/MAX15005A/B can be configured for SEPIC conversion when the output voltage must be lower and higher than the input voltage when the input voltage varies through the operating range. The duty cycle equation: indicates that the output voltage is lower than the input for a duty cycle lower than 0.5 while VOUT is higher than the input at a duty cycle higher than 0.5. The inherent advantage of the SEPIC topology over the boost converter is a complete isolation of the output from the source during a fault at the output. For the MAX15004/MAX15005, the SEPIC converter output can be fed back to VCC (Figure 6), so that the controller can function even during coldcrank input voltage ( ≤ 2.5V). Use a Schottky diode (DVIN) in the VIN path to avoid backfeeding the input source. A currentlimiting resistor (RVCC) is also needed from the output to VCC depend ing upon the converter output voltage. The total VCC current sink must be limited to 25mA. See the Selecting VCC Resistor (RVCC) section to calculate the optimum value of the VCC resistor. The SEPIC converter design includes sizing of induc tors, a MOSFET, series capacitance, and the rectifier diode. The inductance is determined by the allowable ripple current through all the components mentioned above. Lower ripple current means lower peak and RMS currents and lower losses. The higher inductance value needed for a lower ripple current means a largersized inductor, which is a more expensive solution. The induc tors L1 and L2 can be independent, however, winding them on the same core reduces the ripple currents. Calculate the maximum duty cycle using the following equation and choose the RT and CT values accordingly for a given switching frequency (see the Oscillator Frequency/External Synchronization section). where VD is the forward voltage of the Schottky diode, VCS (0.305V) is the currentsense threshold of the MAX15004/MAX15005, and VDS is the voltage drop across the switching MOSFET during the ontime. Inductor Selection in SEPIC Converter Use the following equations to calculate the inductance values. Assume both L1 and L2 are equal and that the inductor ripple current ( ΔIL) is equal to 20% of the input current at nominal input voltage to calculate the induc tance value. where fOUT is the converter switching frequency and η is the targeted system efficiency. Use the coupled inductors MSDseries from Coilcraft or PF0553series from Pulse Engineering, Inc. Make sure the inductor saturating current rating (ISAT) is 30% higher than the peak inductor current calculated using the following equation. Use the currentsense resistor calculated based on the ILPK value from the equation below (see the Current Limit section). I ID D II LPK OUT MAX MAX MAX OUT MAX L = × × ++ ⎡ ⎣ − − − () 1 η Δ ⎢⎢ ⎤ ⎦ ⎥ LL L VD fI I IN MIN MAX OUT L L == = × ×× ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ = − 1 2 2 02 Δ Δ . ××× × ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ − − ID D OUT MAX MAX MAX () 1 η D VV VV V V V MAX OUT D IN MIN OUT D DS CS = + ++ + ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ − − () V V D D O IN = − 1 L fc C ≤ ×× × 1 410 32 4.5V to 40V Input Automotive Flyback/Boost/SEPIC PowerSupply Controllers 20 ______________________________________________________________________________________ 
