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TD1484A Datasheet(PDF) 11 Page - TECHCODE SEMICONDUCTOR, INC. |
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TD1484A Datasheet(HTML) 11 Page - TECHCODE SEMICONDUCTOR, INC. |
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11 / 15 page  October, 20, 2010. America Techcode Semiconductor, INC. www.techcodesemi.com 11 Techcode® 3.2A 23V Synchronous Rectified Step-Down Converte TD1484A DATASHEET Function Description Component Selection Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin.The voltage divider divides the output voltage down to the feedback voltage by the ratio: Where VFB is the feedback voltage and VOUT is the output voltage.Thus the output voltage is: R2 can be as high as 100kΩ, but a typical value is 10kΩ. Using the typical value for R2, R1 is determined by: For example, for a 3.3V output voltage, R2 is 10kΩ, and R1 is 26.1kΩ. Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However,the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by: Where VOUT is the output voltage, VIN is the input voltage, fS is the switching frequency, and ΔIL is the peak-to-peak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: Where ILOAD is the load current. The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirements. Optional Schottky Diode During the transition between high-side switch and low-side switch, the body diode of the lowside power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 1 lists example Schottky diodes and their Manufacturers. Part Number Voltage/Current Vendor B130 30V, 1A Diodes, Inc. SK13 30V, 1A Diodes, Inc. MBRS130 30V, 1A International Rectifier Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose X5R or X7R dielectrics when using ceramic capacitors. |
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